NRG ENERGY INC
424B2, 1999-05-21
ELECTRIC SERVICES
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<PAGE>   1
 
PROSPECTUS SUPPLEMENT
(TO PROSPECTUS DATED APRIL 7, 1999)
 
[NRG LOGO]                        $300,000,000
 
                                NRG ENERGY, INC.
                          7 1/2% SENIOR NOTES DUE 2009
                               ------------------
 
     The notes will bear interest at the rate of 7 1/2% per year. Interest on
the notes is payable in arrears on June 1 and December 1 of each year, beginning
on December 1, 1999. The notes will mature on June 1, 2009. Interest on the
notes will accrue from the date of delivery. We may redeem some or all of the
notes at any time. The redemption prices are discussed under the caption
"Description of Notes -- Optional Redemption."
 
     The notes will be senior obligations of ours and will rank equally with all
of our unsecured senior indebtedness.
                               ------------------
 
      INVESTING IN THE NOTES INVOLVES CERTAIN RISKS. SEE "RISK FACTORS"
BEGINNING ON PAGE S-6.
                               ------------------
 
     Neither the Securities and Exchange Commission nor any state securities
commission has approved or disapproved these securities or determined if this
prospectus supplement or the related prospectus is truthful or complete. Any
representation to the contrary is a criminal offense.
                               ------------------
 
<TABLE>
<CAPTION>
                                                                PER SENIOR NOTE       TOTAL
                                                                ---------------    ------------
<S>                                                             <C>                <C>
Public Offering Price                                                   99.970%    $299,910,000
Underwriting Discount                                                    0.650%    $  1,950,000
Proceeds to NRG Energy, Inc. (before expenses)                          99.320%    $297,960,000
</TABLE>
 
                               ------------------
 
     The underwriters are offering the notes subject to various conditions. The
underwriters expect to deliver the notes to purchasers in book-entry form only,
through the facilities of The Depository Trust Company, on or about May 25,
1999.
                               ------------------
 
SALOMON SMITH BARNEY
                  ABN AMRO INCORPORATED
                                     BANC OF AMERICA SECURITIES LLC
                                                   MERRILL LYNCH & CO.
 
May 20, 1999
<PAGE>   2
 
     YOU SHOULD RELY ONLY ON THE INFORMATION CONTAINED IN OR INCORPORATED BY
REFERENCE IN THIS SUPPLEMENT AND THE ACCOMPANYING PROSPECTUS. WE HAVE NOT
AUTHORIZED ANYONE TO PROVIDE YOU WITH DIFFERENT INFORMATION. WE ARE NOT MAKING
AN OFFER OF THESE SECURITIES IN ANY STATE WHERE THE OFFER IS NOT PERMITTED. YOU
SHOULD NOT ASSUME THAT THE INFORMATION PROVIDED BY THIS SUPPLEMENT OR THE
ACCOMPANYING PROSPECTUS IS ACCURATE AS OF ANY DATE OTHER THAN THE DATE ON THE
FRONT OF THIS SUPPLEMENT.
                               ------------------
 
                               TABLE OF CONTENTS
 
<TABLE>
<CAPTION>
                                                              PAGE
                                                              ----
<S>                                                           <C>
                   PROSPECTUS SUPPLEMENT

Prospectus Supplement Summary...............................  S-1
Risk Factors................................................  S-6
Business....................................................  S-14
Use of Proceeds.............................................  S-22
Selected Consolidated Financial and Other Data..............  S-23
Earnings to Fixed Charges Ratio.............................  S-25
Description of Notes........................................  S-26
Underwriting................................................  S-29
Legal Matters...............................................  S-30
                     
                         PROSPECTUS

Where You Can Find More Information.........................     2
Forward Looking Statements..................................     2
The Company.................................................     3
Use of Proceeds.............................................     3
Earnings to Fixed Charges Ratio.............................     4
Description of Debt Securities..............................     4
Plan of Distribution........................................    13
Legal Matters...............................................    14
Experts.....................................................    14
</TABLE>
 
                                        i
<PAGE>   3
 
                         PROSPECTUS SUPPLEMENT SUMMARY
 
     The following summary is qualified in its entirety by and should be read
together with the more detailed information and financial statements included or
incorporated by reference in this supplement and the accompanying prospectus.
 
                                  THE COMPANY
 
     We are one of the leading participants in the independent power generation
industry. We are principally engaged in the acquisition, development and
operation of, and ownership of interests in, independent power production and
co-generation facilities, thermal energy production and transmission facilities,
landfill gas collection and generation facilities and resource recovery
facilities. The power generation facilities in which we currently have
interests, including those under construction, as of December 31, 1998 have a
total design capacity of 10,605 megawatts ("MW"), of which we have or will have
total or shared operational responsibility for 6,966 MW and net ownership of, or
leasehold interests in, 3,300 MW. In addition, we have substantial interests in
district heating and cooling systems and steam generation and transmission
operations. As of December 31, 1998, these thermal businesses had a steam
capacity of approximately 3,750 million British thermal units per hour
(mmBtu/hr.), of which our equity interest was 2,905 mmBtu/hr. Our refuse-derived
fuel plants processed more than 1,350,000 tons of municipal solid waste into
approximately 1,100,000 tons of refuse-derived fuel during 1998.
 
     We were established in 1989 and are a wholly-owned subsidiary of Northern
States Power Company ("NSP"). Our headquarters and principal executive offices
are located at 1221 Nicollet Mall, Suite 700, Minneapolis, Minnesota 55403. Our
telephone number is (612) 373-5300.
 
                                    STRATEGY
 
     We intend to continue to grow through a combination of acquisitions and
greenfield development of power generation, thermal energy production and
transmission facilities and related assets in the United States and abroad. We
believe that our facility operations and engineering expertise, fuel and
environmental strategies, labor and government relations expertise and legal and
financial skills give us a competitive advantage in the independent power
market. We also believe that our experience in meeting or exceeding applicable
environmental regulatory standards and our environmental compliance record will
give us an advantage as regulators continue to impose increasingly stringent
environmental requirements on the operation of power generation facilities. In
addition, we continue to have access to technical and administrative support
from NSP on a contract basis to augment our own expertise. We believe the
knowledge and expertise we have gained in the financial and legal restructuring
of our existing facilities, as well as our engineering expertise and reputation
with respect to environmental compliance and labor relations, can be effectively
employed in the development of both domestic and international greenfield
projects.
 
     In the United States, our near-term focus will be primarily on the
acquisition of existing power generation capacity and thermal energy production
and transmission facilities, particularly in situations in which our expertise
can be applied to improve the operating and financial performance of the
facilities. In the international market, we will continue to pursue development
and acquisition opportunities in those countries in which we believe that the
legal, political and economic environment is conducive to increased foreign
investment. We expect to acquire or develop most domestic and international
projects on a joint venture basis. Where appropriate, we will include a local or
host country partner or a partner with substantial experience in the area.
Although we exercise flexibility in structuring our investments in projects, our
goal is to own a 20% to 50% equity interest in, and have operating control or
influence over, the projects in which we invest.

                                       S-1
<PAGE>   4
 
                              RECENT DEVELOPMENTS
 
RECENT AND PENDING ACQUISITIONS
 
     We have completed or we expect to complete several acquisitions for
domestic projects located on both the East and West coasts as a result of
entering into several binding purchase agreements since October 1998. These
completed or pending acquisitions include the following assets:
 
EAST COAST
 
     - On April 26, 1999, we completed the purchase of the Somerset power
       station from Eastern Utilities Associates ("EUA") for approximately $55
       million. The Somerset station is located in Somerset, Massachusetts and
       consists of two coal-fired generating facilities supplying a total of 181
       MW and two aeroderivative combustion peaking units supplying a total of
       48 MW. We hold a 100% interest in the project and will own, operate and
       maintain the units.
 
     - In April 1999, we executed an acquisition agreement with Niagara Mohawk
       Power Corporation to purchase the 1,700 MW oil/gas fired Oswego
       generating station, located in Oswego, New York for approximately $91
       million. The purchase price reflects the agreement of Rochester Gas &
       Electric, 24% owner of one of the two 850 MW units, to sell its share of
       that unit. This transaction is expected to close in the fourth quarter of
       1999, pending regulatory approval.
 
     - In January 1999, we executed an acquisition agreement with Consolidated
       Edison Company of New York to acquire the Astoria gas turbines facility
       and the Arthur Kill generating station for $505 million. These
       facilities, located in New York, have a combined summer capacity rating
       of 1,456 MW. This transaction is expected to close in the third quarter
       of 1999, pending regulatory approval.
 
     - In December 1998, we executed acquisition agreements with Niagara Mohawk
       Power Corporation to purchase the Dunkirk and Huntley coal-fired
       generating facilities located near Buffalo, New York for $355 million.
       These facilities have a combined summer capacity rating of 1,360 MW. This
       transaction is expected to close in the second quarter of 1999, pending
       regulatory approval.
 
     We intend to aggregate these assets into a regional generating company, NRG
Northeast Generating LLC, and finance the acquisition of these assets on a
pooled basis with nonrecourse debt and our equity. A portion of our equity is
expected to come from the proceeds of this offering.
 
WEST COAST
 
     In December 1998, Dynegy Inc. and we signed acquisition agreements with San
Diego Gas & Electric to jointly acquire 1,218 MW of power generating facilities
for $356 million. These facilities are the 965 MW Encina Power Plant located
adjacent to the Pacific Ocean in Carlsbad, California and seventeen combustion
turbines totaling 253 MW located on seven different sites in San Diego County,
California. Dynegy Inc. and we will each own 50% of these facilities. We expect
these transactions to close in the second quarter of 1999, pending regulatory
approval.
 
     Dynegy Inc. and we intend to aggregate the Encina Power Plant, the San
Diego combustion turbines and other jointly owned assets into a regional
generating company, West Coast Power LLC, and finance the acquisition of the
assets on a pooled basis with nonrecourse debt as well as equity from both us
and Dynegy Inc. A portion of our equity is expected to come from the proceeds of
this offering.

                                       S-2
<PAGE>   5
 
PROPOSED MERGER OF NORTHERN STATES POWER COMPANY
 
     On March 24, 1999, NSP and New Century Energies, Inc., a Delaware
corporation ("NCE"), entered into an Agreement and Plan of Merger providing for
a strategic business combination of NCE and NSP. Pursuant to the merger
agreement, NCE will be merged with and into NSP with NSP as the surviving
corporation in the merger. Subject to the terms of the merger agreement, at the
time of the merger, each share of NCE common stock, other than certain shares to
be canceled, together with any associated purchase rights, will be converted
into the right to receive 1.55 shares of NSP common stock. The merger is
expected to be a tax-free stock-for-stock exchange for shareholders of both
companies and to be accounted for as a pooling of interests.
 
     Consummation of the merger is subject to certain closing conditions,
including, among others, approval by the shareholders of NSP and NCE, approval
or regulatory review by certain state utilities regulators, the Securities and
Exchange Commission under the Public Utility Holding Company Act of 1935, the
Federal Energy Regulatory Commission, the Nuclear Regulatory Commission, the
Federal Communications Commission and expiration or termination of the waiting
period applicable to the merger under the Hart-Scott-Rodino Antitrust
Improvements Act of 1976. Each of NCE and NSP have agreed to certain
undertakings and limitations regarding the conduct of their businesses prior to
the closing of the transaction, some of which impact us, but none of which
materially impact the conduct of our business. The merger is expected to take
from 12 to 18 months to complete.

                                       S-3
<PAGE>   6
 
                                  THE OFFERING
 
Securities Offered............   $300,000,000 principal amount of 7 1/2% Senior
                                 Notes due 2009.
 
Maturity Date.................   June 1, 2009.
 
Interest Payment Dates........   June 1 and December 1, commencing December 1,
                                 1999.
 
Ranking.......................   The notes will be senior unsecured obligations
                                 and will rank pari passu with all of our senior
                                 unsecured indebtedness. All existing and future
                                 liabilities of our subsidiaries and affiliates
                                 will be effectively senior to the notes.
 
Ratings.......................   The notes have been assigned ratings of "BBB-"
                                 by Standard & Poor's Ratings Group and "Baa3"
                                 by Moody's Investors Service, Inc.
 
Optional Redemption...........   We may redeem the notes at any time, and, if
                                 redeemed before June 1, 2009, we may be
                                 required to pay a redemption premium.
 
Sinking Fund..................   None.
 
Change of Control.............   Upon a "Change of Control" (as defined on page
                                 7), a holder of notes may require us to
                                 repurchase that holder's notes, in whole or in
                                 part, at 101% of the principal amount of the
                                 notes, plus accrued interest. A Change of
                                 Control will not be deemed to have occurred if,
                                 after giving effect thereto, the notes are
                                 rated BBB- or better by Standard & Poor's
                                 Ratings Group and Baa3 or better by Moody's
                                 Investors Service, Inc.
 
Use of Proceeds...............   The net proceeds from the sale of the notes is
                                 estimated to be approximately $298.0 million. A
                                 portion of the net proceeds of this offering
                                 will be used to finance our equity investment
                                 in connection with pending acquisitions. The
                                 remainder of the net proceeds will be used for
                                 general corporate purposes, which may include
                                 financing the development and construction of
                                 new facilities, additions to working capital,
                                 reductions of our indebtedness and our
                                 subsidiaries' indebtedness, financing of
                                 capital expenditures and potential
                                 acquisitions.
                                       S-4
<PAGE>   7
 
             SUMMARY AND CONSOLIDATED FINANCIAL AND OPERATING DATA
 
     The following summary historical financial data is derived from our audited
consolidated financial statements. All dollar amounts are set forth in
thousands.
 
CONSOLIDATED STATEMENTS OF INCOME DATA:
 
<TABLE>
<CAPTION>
                                                      YEAR ENDED DECEMBER 31,
                                       ------------------------------------------------------
                                         1994       1995       1996       1997        1998
                                         ----       ----       ----       ----        ----
<S>                                    <C>        <C>        <C>        <C>         <C>
Revenues from wholly-owned
  operations.........................  $ 63,970   $ 64,180   $ 71,649   $  92,052   $ 100,424
Equity in earnings of unconsolidated
  affiliates.........................    32,155     28,639     34,315      26,200      81,706
Operating costs and expenses.........   (63,529)   (75,465)   (84,188)   (100,143)   (125,118)
                                       --------   --------   --------   ---------   ---------
Operating income.....................    32,596     17,354     21,776      18,109      57,012
Other income (expense)(1)............      (586)    22,657     (7,453)    (19,618)    (40,934)
Income tax benefit (expense)(2)......    (2,472)    (8,810)     5,655      23,491      25,654
                                       --------   --------   --------   ---------   ---------
Net income...........................  $ 29,538   $ 31,201   $ 19,978   $  21,982   $  41,732
                                       ========   ========   ========   =========   =========
</TABLE>
 
CONSOLIDATED BALANCE SHEET AND OTHER DATA:
 
<TABLE>
<CAPTION>
                                                      YEAR ENDED DECEMBER 31,
                                      --------------------------------------------------------
                                        1994       1995       1996        1997         1998
                                        ----       ----       ----        ----         ----
<S>                                   <C>        <C>        <C>        <C>          <C>
Net property, plant and equipment...  $107,634   $111,919   $129,649   $  185,891   $  204,729
Net equity investments in
  projects..........................   164,863    221,129    365,749      694,655      800,924
Total assets........................   376,570    454,589    680,809    1,168,102    1,293,426
Long-term debt, including current
  maturities........................    93,339     90,034    212,141      620,855      626,476
Stockholder's equity................   234,722    319,764    421,914      450,698      579,332
Percentage of total debt to
  capitalization....................      28.5%      22.0%      33.5%        57.9%        52.0%
Our net power generating capacity
  (MW)..............................       992        999      1,326        2,637        3,300
</TABLE>
 
- -------------------------
(1) Includes pretax charges of $5.0 million, $5.0 million, $1.5 million, $9.0
    million and $26.7 million in the years 1994, 1995, 1996, 1997 and 1998,
    respectively, to write-down the carrying value of certain energy projects.
    These amounts also include the gain on sale of our interest in projects of
    $8.7 million in 1997 and $30.0 million in 1998.
 
(2) We have substantial tax credits which can be utilized by NSP. NSP pays us
    for these tax credits as they are used.
                                       S-5
<PAGE>   8
 
                                  RISK FACTORS
 
     Before purchasing the notes you should carefully consider the following
risk factors as well as the other information contained in this supplement, the
accompanying prospectus, and the information incorporated by reference in order
to evaluate an investment in the notes.
 
INVESTMENTS IN LESS THAN 100% OWNED PROJECTS LIMIT OUR CONTROL.
 
     Our ability to control the development, construction, acquisition or
operation of project investments and joint ventures is limited because most of
our current project investments consist of minority interests in project
affiliates (i.e., where we beneficially own 50% or less of the ownership
interests). A substantial portion of our future investments in projects may also
take the form of minority interests. As a result, the indenture for the notes
does not contain any limitations on our ability to make minority investments.
 
     Although we seek to exert a degree of influence with respect to the
management and operation of projects in which we are a minority investor by
negotiating to receive certain limited governance rights (such as rights to veto
significant actions or to obtain positions on management committees), we may not
always succeed in such negotiations. We may be dependent on our co-venturers to
construct and operate such projects. We cannot assure you that our co-venturers
will have the same level of experience, technical expertise, human resources
management and other attributes that we possess. Any co-venturer may have
conflicts of interest, including those relating to its status as a provider of
goods or services to the project. The approval of co-venturers also may be
required for us to receive distributions of funds from projects.
 
WE ARE UNCERTAIN ABOUT ACCESS TO CAPITAL FOR FUTURE PROJECTS.
 
     We require continued access to debt capital from outside sources on
acceptable terms in order to assure the success of future projects and
acquisitions. Our ability to arrange financing on a substantially non-recourse
basis and the costs of such capital are dependent on numerous factors, including
general economic and capital market conditions, credit availability from banks
and other financial institutions, investor confidence in us, our partners and in
the local independent power market, the success of current projects, the
perceived quality of new projects and provisions of tax and securities laws that
are conducive to raising capital in this manner. In order to access capital on a
substantially non-recourse basis in the future, we may have to make larger
equity investments in, or provide more financial support for, our project
subsidiaries. To date, the equity capital for our projects has been provided by
equity contributions from NSP and, to a lesser extent, internally-generated cash
flow from our projects as well as other borrowings. We cannot assure you that we
will be successful in structuring the financing for our projects on a
substantially non-recourse basis or that we will obtain sufficient additional
equity capital from NSP, project cash flow or additional borrowings to enable us
to fund the equity commitments required for future projects.
 
OUR HOLDING COMPANY STRUCTURE LIMITS OUR ABILITY TO SERVICE INDEBTEDNESS.
 
     The notes will be exclusively our obligations and not those of any of our
project subsidiaries or project affiliates. As a result, all existing and future
liabilities of our direct and indirect subsidiaries and affiliates will be
effectively senior to the notes. Because substantially all of our operations are
conducted by our project subsidiaries and project affiliates, our cash flow and
our ability to service our indebtedness, including our ability to pay the
interest on and principal of the notes when due, will be dependent upon our
receipt of cash dividends and distributions or other transfers from our project
and other subsidiaries. The debt agreements of our subsidiaries and project
affiliates generally restrict their ability to pay dividends, make distributions
or otherwise transfer funds to us. The restrictions in such
 
                                       S-6
<PAGE>   9
 
agreements generally require that, prior to and after giving effect to the
payment of dividends, distributions or other transfers:
 
     - such subsidiaries or project affiliates meet certain financial
       performance or coverage ratios,
 
     - no default or event of default shall have occurred, and
 
     - the subsidiary or project affiliate proposing to pay the dividend,
       distribution or other transfer must provide for the payment of other
       current or prospective obligations, including operating expenses, debt
       service and reserves.
 
     Our subsidiaries and project affiliates are separate and distinct legal
entities that have no obligation, contingent or otherwise, to pay any amounts
due pursuant to the notes or to make any funds available therefor, whether by
dividends, loans or other payments, and they do not guarantee the payment of
interest on, or principal of, the notes. We own a minority interest in most of
our projects, and therefore we are unable unilaterally to cause dividends or
distributions to be made to us from these operations.
 
     Any right we may have to receive assets of any of our subsidiaries or
project affiliates upon a liquidation or reorganization of such subsidiaries or
project affiliates (and the consequent right of holders of the notes to
participate in the distribution of, or to realize proceeds from, those assets)
will be effectively subordinated to the claims of any such subsidiary's or
project affiliate's creditors, including trade creditors and holders of debt
issued by such subsidiary or project affiliate.
 
     The indenture for the notes does not impose any limitation on the ability
of our subsidiaries or project affiliates to incur additional indebtedness or to
permit contractual restrictions on the distribution of cash to us. As part of
our global tax strategy, we intend to maintain our earnings from foreign
investments offshore, for reinvestment in other foreign projects. For this
reason, we intend to utilize the cash from our domestic operations including
principal and interest received from the repayment of loans made by us to our
foreign affiliates to make the payments with respect to the notes. We cannot
assure you that cash available from our domestic operations and the repayment of
the loans made to our foreign affiliates will be sufficient to make the payments
under the notes as and when due. If we elect to repatriate earnings from our
foreign operations to make these payments in case of such a shortfall, then we
may incur United States taxes, net of any available foreign tax credits, on the
repatriation of such foreign earnings. As a result of these additional taxes, we
cannot assure you that the foreign earnings would be sufficient to make the
payments on the notes as and when due.
 
WE HAVE A SUBSTANTIAL AMOUNT OF INDEBTEDNESS.
 
     As of December 31, 1998, we had total recourse debt of $513.0 million, with
an additional $113.4 million of non-recourse debt appearing on our balance
sheet. The percentage of our total recourse debt to capitalization was 47% as of
December 31, 1998. The indenture imposes no limitations on our ability to incur
additional indebtedness. Our substantial amount of debt and the debt of our
project subsidiaries and project affiliates presents the risk that we might not
generate sufficient cash to service our indebtedness, including the notes, and
that our leveraged capital structure could limit our ability to finance the
acquisition and development of additional projects, to compete effectively or to
operate successfully under adverse economic conditions.
 
     In addition, under certain of the instruments governing our debt, including
our credit facility and indentures governing our other outstanding public debt,
such debt may be accelerated upon certain events of default or if we undergo a
change of control. As a result, if any such event were to occur, we may not have
sufficient capital to fully pay holders of notes the amount due under the notes
or to redeem any notes tendered pursuant to the Change of Control Offer
described under "Description of Debt Securities -- Change of Control" in the
attached prospectus.

                                       S-7
<PAGE>   10
 
WE DEPEND ON AND ARE CONTROLLED BY NORTHERN STATES POWER COMPANY.
 
     NSP is our sole stockholder. Since our formation, NSP has provided all of
our equity funding for our business and operations. Our only other source of
funding has been our borrowings and internally-generated cash flow from our
existing projects and investments. We cannot assure you that NSP will contribute
additional equity capital to us in the future. In the absence of continued
equity contributions, we cannot assure you that we will have access to
sufficient capital to fund our obligations with respect to our existing projects
or to undertake new acquisition and development projects. In addition, as a
result of the pending NSP merger with NCE, we cannot assure you that NSP will
continue to act with respect to us as it has in the past. NSP is also not
required to continue to hold our stock.
 
     As our sole stockholder, NSP has the power to control the election of the
directors and all other matters submitted for stockholder approval and may be
deemed to have control over our management and affairs. Currently, there are no
outside directors on our board of directors. In circumstances involving a
conflict of interest between NSP, as the sole stockholder, on the one hand, and
our creditors on the other, we cannot assure you that NSP would not exercise its
power to control us in a manner that would benefit NSP to the detriment of the
holders of notes. NSP has policies in place, pursuant to applicable law, to
ensure that its ratepayers are protected from affiliate transactions that may be
adverse to the ratepayers' interests. The indenture imposes no limitations on
our ability to pay dividends or to make other payments to NSP or on our ability
to enter into transactions with NSP or our other affiliates.
 
     In addition, NSP is an important customer of, and supplier to, certain of
our businesses in the United States. We purchase steam production services from
NSP for our Rock-Tenn and Washco steam transmission lines and sell
refuse-derived fuel to NSP from our Newport resource recovery facility. We
provide management, operation and maintenance services for the Elk River
resource recovery facility and we dispose of the Elk River facility's
refuse-derived fuel ash at NSP's Becker ash landfill. The failure of NSP to
comply with its obligations to us under the agreements governing such sales and
services could have a material adverse effect on our revenues from these
projects.
 
WE DO SIGNIFICANT BUSINESS OUTSIDE THE UNITED STATES.
 
     A key component of our business strategy is the development or acquisition
of projects outside the United States. The economic and political conditions in
certain countries where we have interests or in which we are or could be
exploring development or acquisition opportunities present risks of delays in
permitting and licensing, construction delays and interruption of business, as
well as risks of war, expropriation, nationalization, renegotiation or
nullification of existing contracts and changes in law or tax policy, that are
greater than in the United States. The uncertainty of the legal environment in
certain foreign countries in which we may develop or acquire projects could make
it more difficult to obtain non-recourse project financing on suitable terms and
could impair our ability to enforce our rights under agreements relating to such
projects.
 
     Operations in foreign countries also can present currency exchange,
inflation, convertibility and repatriation risks. In certain countries in which
we may develop or acquire projects in the future, economic and monetary
conditions and other factors could affect our ability to convert our earnings to
United States dollars or other hard currencies or to move funds offshore from
such countries. Furthermore, the central bank of any such country may have the
authority in certain circumstances to suspend, restrict or otherwise impose
conditions on foreign exchange transactions or to approve distributions to
foreign investors. Although we generally seek to structure our power purchase
agreements and other project revenue agreements to provide for payments to be
made in, or indexed to, United States dollars or a currency freely convertible
into United States dollars, we cannot assure
 
                                       S-8
<PAGE>   11
 
you that we will be able to achieve this structure in all cases or that a power
purchaser or other customer will be able to obtain sufficient dollars or other
hard currency to pay such obligations.
 
     As part of privatizations or other acquisition opportunities, we may make
investments in ancillary businesses not directly related to power generation,
thermal energy production and transmission or resource recovery and in which our
management may not have had prior experience. In such cases, our policy is to
attract partners with the necessary expertise. However, we cannot assure you
that such persons will be available as co-venturers in every case. In addition,
as a condition to participating in privatizations and refurbishments of formerly
state-owned businesses, we may be required to undertake transitional obligations
relating to union contracts, employment levels and benefits obligations for
employees, which could prevent or delay the achievement of desirable operating
efficiencies and financial performance.
 
OUR ACQUISITIONS AND DEVELOPMENTS MAY NOT BE PROFITABLE.
 
     The development projects and acquisitions in which we may invest in the
future, including those described herein, may be large and complex, and we may
not be able to complete the development or acquisition of any such project.
Development projects and acquisitions require us to expend significant sums for
engineering, permitting, legal, financial advisory and other expenses in
preparation for competitive bids that we may not win or before it can be
determined whether a project is feasible, economically attractive or capable of
being financed. We cannot assure you that the projects that we pursue, and on
which we may spend significant sums, will prove to be desirable project
investments, or that we will be able to win any such competitive bids, obtain
new power purchase agreements, overcome any local opposition, or obtain the
necessary agreements, contracts, licenses, certifications and permits necessary
for the successful development of new projects and acquisition of interests in
existing projects. We may fail to acquire or develop projects despite having
incurred significant expenses. For instance, if we failed to complete either of
the pending asset acquisitions from Consolidated Edison Company of New York or
Niagara Mohawk Power Corporation, as discussed in the "Summary," such failure
could have a significant impact on our forecasted financial results. Even if we
are successful in the development or acquisition of an interest in a project, we
may require substantial additional debt or equity financing for such projects,
which additional financing may not be available on acceptable terms, if at all.
Most acquisition agreements and power purchase agreements permit the seller or
customer, respectively, to terminate the agreement or impose penalties if the
acquisition or operation of the project, as the case may be, is not achieved by
a specified date.
 
WE FACE STRONG COMPETITORS.
 
     The independent power industry is characterized by numerous strong and
capable competitors, some of which have more extensive developmental or
operating experience, more extensive experience in the acquisition and
development of power generation capacity, larger staffs and greater financial
resources than we do. Further, in recent years, the domestic independent power
industry has been characterized by strong and increasing competition which has
contributed to a reduction in priced offered by utilities for power produced by
independent power producers and has resulted in lower returns to project
investors.
 
     Many of our competitors also are seeking attractive opportunities, both in
the United States and abroad. This competition may adversely affect our ability
to make investments or acquisitions on terms favorable to us. many foreign and
domestic utilities are now engaging in "competitive bid" solicitations for new
capacity demands or acquisitions.
 
                                       S-9
<PAGE>   12
 
CONSTRUCTION AND START-UP OF FACILITIES COULD ENCOUNTER UNFORESEEN PROBLEMS.
 
     The construction, expansion or refurbishment of a power generation, thermal
energy production and transmission facility or resource recovery facility
involves many risks, including supply interruptions, work stoppages, labor
disputes, weather interferences, unforeseen engineering, environmental and
geological problems and unanticipated cost overruns. The commencement of
operation of such newly-constructed, expanded or refurbished facilities also
involves many risks, including the breakdown or failure of equipment or
processes and test performance below expected levels of output or efficiency.
New plants may employ recently developed and technologically complex equipment,
especially in the case of newer environmental emission control technology. While
insurance is maintained to protect against certain risks, warranties are
obtained from vendors for limited periods and contractors are obligated to meet
certain performance levels, the proceeds of such insurance, warranties or
performance guarantees may not be adequate to cover lost revenues, increased
expenses or liquidated damages payments. As a result, a project may operate at a
loss and be unable to fund principal and interest payments under its project
financing agreements, which may allow the affected lenders to accelerate such
debt.
 
     In addition, many power and thermal energy purchase agreements permit the
customer to terminate the agreement, retain security posted by the developer as
liquidated damages or change the payments to be made to the project subsidiary
or the project affiliate in the event certain milestones, such as commencing
commercial operation of the project, are not met by specified dates. In the
event such a termination right is exercised, a project may not commence
generating revenues, the default provisions in a financing agreement may be
triggered, rendering such debt immediately due and payable, and the project may
be rendered insolvent as a result.
 
OPERATING BELOW EXPECTED CAPACITY LEVELS COULD RESULT IN PENALTIES.
 
     The occurrence of one of the events which constitutes a risk of operating a
power generation facility may trigger default provisions under our financing
agreements. The risks associated with operating a power generation facility,
thermal energy production and transmission facility, resource recovery facility
or mining facility includes the breakdown or failure of generation equipment or
other equipment or processes, labor disputes, fuel interruption and operating
performance below expected levels. Operation below expected capacity levels may
result in lost revenues, increased expenses, higher maintenance costs and
penalties. As a result, a facility may be unable to perform its obligations
under its purchase agreements, triggering the default provisions in a financing
agreement, rendering such debt immediately due and payable, and the project may
be rendered insolvent as a result.
 
     Certain power purchase agreements of our project subsidiaries or project
affiliates permit the purchaser to terminate the agreement, modify the payments
required under the agreement, recover payments previously made under the
agreement or require such project subsidiaries or project affiliates to pay
liquidated damages under the agreement in certain circumstances. While insurance
is maintained to protect against certain risks, warranties are obtained from
vendors for limited periods and contractors are obligated to meet certain
performance levels, the proceeds of such insurance, warranties or performance
guarantees may not be adequate to cover lost revenues, increased expenses or
liquidated damages payments. As a result, default provisions in the project
subsidiary's or project affiliate's financing agreements may be triggered, which
might allow the affected lenders to accelerate such debt.
 
                                      S-10
<PAGE>   13
 
PAYMENTS UNDER POWER PURCHASE AGREEMENTS ARE SUBJECT TO VARIATION; WE HAVE
GUARANTEED DEBT OF SUBSIDIARIES.
 
     Payments under power purchase agreements for domestic projects that satisfy
the requirements for "qualifying facility" status under the Public Utility
Regulatory Policies Act ("PURPA") and that are based upon actual short-run, as
opposed to forecasted long-run, "avoided cost" or the cost that would otherwise
have been paid for power from the purchasing utility's highest-cost generating
facility, are subject to significant variations based upon a number of factors
outside of the control of the owners of such facilities, including weather,
economic conditions, and the particular operating profile and generating
capacity position of the purchasing utility.
 
     If payments under power purchase agreements are not sufficient, we may be
directly liable for the obligations of some of our project subsidiaries pursuant
to guarantees relating to certain of their equity and operating obligations. One
example is our guarantee of the obligations of our project subsidiary that
operates the Gladstone facility for up to AUS$25 million, indexed to the
Australian consumer price index ("ACPI") (US$15.9 million, based on exchange
rates and ACPI in effect at March 31, 1999), under the project subsidiary's
operating and maintenance agreement with the owners of the facility. If we were
required to satisfy all of these guarantees and other obligations, such event
would have a material adverse effect on our condition, financial and otherwise.
 
MOST OF OUR MORE RECENT PROJECTS OPERATE WITHOUT LONG-TERM POWER PURCHASE
AGREEMENTS.
 
     Most of the more recent projects in which we have acquired or are acquiring
an interest do not have long-term power purchase agreements. For example, Loy
Yang A does not have such agreements because under the new Australian regulatory
scheme, all generators must sell their output to a grid, where the price is
established by a neutral regulator based on the market prices during each
defined period. Facilities that sell their output into the market and operate
without long-term power purchase agreements in place are called "merchant
plants." The generation facilities that we have recently acquired or are
acquiring in California, New York and Massachusetts will be merchant plants,
although a portion of the output of certain of these plants in the first several
years of our ownership of them is contracted for, either under transition power
purchase contracts with their former owners or under bilateral contracts with
other wholesale customers. While these "merchant" projects introduce new risks
and uncertainties, and require careful advance analysis of the local power
markets, we believe that they are becoming increasingly accepted in the
independent power market.
 
WE DEPEND ON CERTAIN CUSTOMERS AND PROJECTS.
 
     Our power generation, thermal energy production and transmission or
resource recovery facilities typically rely on a single supplier for each of the
provision of fuel, water and other services required for operation of the
facility and on a single customer or a few customers to purchase all of the
facility's output, in each case under long-term agreements that provide the
support for any project debt used to finance such facilities. The failure of any
one customer or supplier to fulfill its contractual obligations to the facility
could have a material adverse effect on such facility's financial results. As a
result, the financial performance of such facilities is dependent on the
continued performance by customers and suppliers of their obligations under such
long-term agreements and, in particular, on the credit quality of the project's
customers. The Rock-Tenn project produced more than ten percent of our net
revenues for 1998.
 
WE ARE AFFECTED BY CHANGES IN STATE MUNICIPAL SOLID WASTE ("MSW") FLOW CONTROL
LAWS.
 
     Refuse-derived fuel projects, such as our Newport facility and NSP's Elk
River facility, which is operated by us, historically were assured an adequate
supply of MSW through state and local flow control legislation, which directed
that MSW be disposed of in certain facilities. In May 1994, the
 
                                      S-11
<PAGE>   14
 
United States Supreme Court held that MSW is a commodity in interstate commerce
and, accordingly, that flow control legislation that prohibited shipment of MSW
out of state is unconstitutional. Since this Supreme Court holding, the
refuse-derived fuel facilities owned or operated by us have faced increased
competition from landfills in surrounding states. As a result of such
competition, MSW delivered at our Newport facility decreased in 1995. In 1996,
however, due to our assistance and a reduction of tipping fees under contracts
entered into between haulers and the Ramsey and Washington Counties in
Minnesota, the downward trend in waste deliveries was reversed. MSW deliveries
have increased during the last three years, with 359,646 tons delivered in 1996,
409,838 tons delivered in 1997 and 446,040 tons delivered in 1998. However, in
the absence of valid flow control legislation, we cannot assure you that this
improved trend will continue.
 
WE ARE SUBJECT TO GOVERNMENTAL REGULATION.
 
     We and the projects in which we invest are subject to a number of complex
and stringent environmental and other laws and regulations affecting many
aspects of our present and future operations, including the disposal of various
forms of waste and the construction or permitting of new facilities. These laws
and regulations generally require us to obtain and comply with a wide variety of
licenses, permits and other approvals, and may in some cases be enforced by both
public officials and private individuals. We cannot assure you that existing
laws or regulations will not be revised or that new laws or regulations will not
be adopted or become applicable to us which could have an adverse impact on our
operations. There can be no assurance that we will be able to recover all or any
increased costs of compliance from our customers or that our business and
financial condition will not be materially and adversely affected by future
changes in environmental laws or regulations. In addition, regulatory compliance
for the construction of new facilities is a costly and time-consuming process,
and intricate and rapidly changing environmental regulations may require major
expenditures for permitting and create the risk of expensive delays or material
impairment of project value if projects cannot function as planned due to
changing regulatory requirements or local opposition.
 
     PURPA and the Public Utility Holding Company Act of 1935 ("PUHCA"), are two
of the laws (including the regulations thereunder) that affect our operations.
PURPA provides to qualifying facilities ("QFs") certain exemptions from federal
and state laws and regulations, including organizational, rate and financial
regulation. PUHCA regulates public utility holding companies and their
subsidiaries. We are not and will not be subject to regulation as a holding
company under PUHCA as long as the domestic power plants we own are QFs under
PURPA or are exempted as exempt wholesale generators ("EWGs"), and so long as
our foreign utility operations are exempted as EWGs or foreign utility companies
or are otherwise exempted under PUHCA. QF status is conditioned on meeting
certain criteria, and could be jeopardized, for example, by the loss of a steam
customer or reduction of steam purchases below the amount required by PURPA.
 
WE FACE ONGOING CHANGES IN THE U.S. UTILITY INDUSTRY.
 
     The U.S. electric utility industry is currently experiencing increasing
competitive pressures, primarily in wholesale markets, as a result of consumer
demands, technological advances, greater availability of natural gas and other
factors. The Federal Energy Regulatory Commission ("FERC") has proposed
regulatory changes to increase access to the nationwide transmission grid by
utility and non-utility purchasers and sellers of electricity. A number of
states are considering or implementing methods to introduce and promote retail
competition. Proposals have been introduced in Congress to repeal PURPA and
PUHCA, and the FERC has publicly indicated support for the PUHCA repeal effort.
Additionally, some utilities have brought litigation aimed at forcing the
renegotiation or termination of contracts requiring payments to owners of
qualifying facilities based upon past estimates of avoided cost that are now
substantially in excess of market prices. We cannot assure you
 
                                      S-12
<PAGE>   15
 
that, in the future, utilities, with the approval of state public utility
commissions, will not seek to abrogate their existing power purchase agreements.
 
     If the repeal of PURPA or PUHCA occurs, either separately or as part of
legislation designed to encourage the broader introduction of wholesale and
retail competition, the significant competitive advantages that independent
power producers currently enjoy over certain regulated utility companies would
be eliminated or sharply curtailed, and the ability of regulated utility
companies to compete more directly with independent power companies would be
increased. To the extent competitive pressures increase and the pricing and sale
of electricity assumes more characteristics of a commodity business, the
economics of domestic independent power generation projects may come under
increasing pressure, and the availability of long-term power purchase
agreements, which can serve as the basis for project financings, may decrease.
Deregulation may not only continue to fuel the current trend toward
consolidation among domestic utilities, but may also encourage the
disaggregation of vertically-integrated utilities into separate generation,
transmission and distribution businesses. As a result, additional significant
competitors could become active in the independent power industry. In addition,
independent power producers may find it increasingly difficult to negotiate
long-term power sales agreements with solvent utilities, which may affect the
profitability and financial stability of independent power projects.
 
THERE IS A NOT A PUBLIC MARKET FOR THE NOTES.
 
     The notes are a new issue of securities, and we do not intend to list them
on any securities exchange. The underwriters have advised us that they currently
intend to make a market in the notes, but the underwriters are not obligated to
do so and may discontinue any such market-making at any time. We cannot assure
you as to the liquidity of any market that may develop for the notes, your
ability to sell your notes or the price at which you would be able to sell your
notes.
 
                                      S-13
<PAGE>   16
 
                                    BUSINESS
 
INTRODUCTION
 
     We are one of the leading participants in the independent power generation
industry. We are principally engaged in the acquisition, development and
operation of, and ownership of interests in, independent power production and
co-generation facilities, thermal energy production and transmission facilities,
landfill gas collection and generation facilities and resource recovery
facilities. The power generation facilities in which we currently have
interests, including those under construction, as of December 31, 1998 have a
total design capacity of 10,605 MW, of which we have or will have total or
shared operational responsibility for 6,966 MW and net ownership of, or
leasehold interests in, 3,300 MW. In addition, we have substantial interests in
district heating and cooling systems and steam generation and transmission
operations. As of December 31, 1998, these thermal businesses had a steam
capacity of approximately 3,750 mmBtu/hr., of which our equity interest was
2,905 mmBtu/hr. Our refuse-derived fuel plants processed more than 1,350,000
tons of municipal solid waste into approximately 1,100,000 tons of
refuse-derived fuel during 1998.
 
STRATEGY
 
     We intend to continue to grow through a combination of acquisitions and
greenfield development of power generation, thermal energy production and
transmission facilities and related assets in the United States and abroad. We
believe that our facility operations and engineering expertise, fuel and
environmental strategies, labor and government relations expertise and legal and
financial skills give us a competitive advantage in the independent power
market. We also believe that our experience in meeting or exceeding applicable
environmental regulatory standards and our environmental compliance record will
give us an advantage as regulators continue to impose increasingly stringent
environmental requirements on the operation of power generation facilities. In
addition, we continue to have access to technical and administrative support
from NSP on a contract basis to augment our own expertise. We believe the
knowledge and expertise we have gained in the financial and legal restructuring
of our existing facilities, as well as our engineering expertise and reputation
with respect to environmental compliance and labor relations, can be effectively
employed in the development of both domestic and international greenfield
projects.
 
     In the United States, our near-term focus will be primarily on the
acquisition of existing power generation capacity and thermal energy production
and transmission facilities, particularly in situations in which our expertise
can be applied to improve the operating and financial performance of the
facilities. In connection with this focus, we study the opportunities that may
be created by the current restructuring of the domestic electric utility
industry, particularly the divestiture by some utility companies of their
generating assets. In connection with these utility company divestitures of
generating assets, although the assets themselves may be held by a purchasing
subsidiary, in many instances the sellers require that we remain directly liable
for certain indemnity, operating, provision of replacement power and/or debt
payment obligations. We intend to focus our domestic development activities
primarily on the acquisition or development of facilities in excess of 100 MW
and to pursue smaller projects when we have the opportunity to combine several
smaller projects into a larger transaction. We are also working with several
industrial companies to develop energy projects that would provide both
electricity and steam for their production facilities. In addition, to the
extent that the replacement of aging power generating capacity or growth in
demand creates the need for new power generation facilities in the United
States, we intend to pursue opportunities to participate in the development of
such facilities.
 
     In the international market, we will continue to pursue development and
acquisition opportunities in those countries in which we believe that the legal,
political and economic environment is conducive to increased foreign investment.
Once we have developed one project in a country, we use that as a
 
                                      S-14
<PAGE>   17
 
base to develop other projects in that same country or region, leveraging our
experience and knowledge to enhance our likelihood of success in the area. We
intend to continue to capitalize on opportunities created by the privatization
of existing government-owned generating capacity. In addition, due to the
significant existing demand for new power generating capacity in the
international market, we intend to engage in the development of international
greenfield projects. We intend to focus our international development activities
primarily on the acquisition or development of facilities with capacity in
excess of 100 MW and to pursue smaller projects when we have the opportunity to
combine several smaller projects into a larger transaction. We believe that the
global market will continue to provide attractive investment opportunities to us
as the countries that have initiated the privatization of their power generation
capacity and have solicited bids from private companies to purchase existing
facilities or to develop new capacity continue their privatization programs and
other countries begin similar privatization efforts.
 
     We expect to acquire or develop most projects on a joint venture basis.
Where appropriate, we will include a local or host country partner or a partner
with substantial experience in the area. By doing so, we expect to gain a number
of advantages, including technical expertise possessed by others, greater
knowledge of and experience with the political, economic, cultural and social
conditions and commercial practices of the region or country where the project
is being developed, and the ability to leverage our human and financial
resources. A local partner also may, among other things, assist in obtaining
financing from local capital markets as well as building political and community
support for the project. We expect such joint ventures will enable us to share
the risks associated with the acquisition and development of larger projects.
Joint acquisition and development of future projects also should further reduce
our financial risk by building a more diversified portfolio of projects.
 
     Although we exercise flexibility in structuring our investments in
projects, our goal is to own a 20% to 50% equity interest in, and have operating
control or influence over, the projects in which we invest. However, we may in
some instances be willing to modify these targets for a particular project if we
determine that strategic considerations and anticipated returns, when combined
with other factors, such as the ability to exercise "negative control" (i.e.,
the ability to control material project decisions by exercising a veto right) or
the ability to exercise oversight authority in the development or operation of a
project, justify an investment in that project. Alternatively, we may consider
investments or projects in which we are the sole or a majority owner or in which
we own less than a 20% equity interest. See "Risk Factors. -- Investments in
less than 100% owned projects limit our control."
 
     We intend to pursue the acquisition and development of natural gas-fired
power generation facilities where appropriate to complement our existing and
anticipated future investments in coal and other solid fuel-fired facilities. We
currently hold no interest in, and have no present intention of investing in,
any nuclear generating facility.
 
DESCRIPTION OF OUR PROJECTS
 
     We own interests in power generation and thermal generation projects and
other facilities described herein either directly or through project
subsidiaries or project affiliates. Each project is located on a site that is
owned or leased on a long-term basis by us, a project subsidiary or a project
affiliate. The ownership or leasehold interest generally is mortgaged to secure
project financing obligations, and, in certain instances, to secure the project
subsidiary's or project affiliate's obligations under its power purchase
agreement.
 
     PROJECT AGREEMENTS
 
     In the past, virtually all of our operating power generation facilities
have sold electricity under long-term power purchase agreements. A facility's
revenue from a power purchase agreement usually
 
                                      S-15
<PAGE>   18
 
consists of two components: energy payments and capacity payments. Energy
payments, which are intended to cover the variable costs of electric generation,
such as fuel costs and variable operation and maintenance expense, are normally
based on a facility's net electrical output measured in kilowatt hours, with
payment rates either fixed or indexed to the fuel costs of the power purchaser.
Capacity payments, which are generally intended to provide funds for the fixed
costs incurred by the project subsidiary or project affiliate, such as debt
service on the project financing and the equity return, are normally calculated
based on the net electrical output or the declared capacity of a facility and
its availability.
 
     Most of the more recent projects in which we have acquired or are acquiring
an interest do not have long-term power purchase agreements. For example, Loy
Yang A does not have such agreements because under the new Australian regulatory
scheme, all generators must sell their output to a grid, where the price is
established by a neutral regulator based on the market prices during each
defined period. The same will be true of Enfield, since the United Kingdom has
adopted a similar regulatory scheme. Similarly, the San Joaquin Valley Energy
Partners Facilities accepted a buy-out of their long-term contracts, so if they
recommence operations, it is anticipated that they will be merchant plants
(i.e., plants operating without long-term power purchase agreements in place,
selling their output into the market). The generation facilities which we have
recently acquired or are acquiring in California, New York and Massachusetts
will also be merchant plants, although a portion of the output of certain of
these plants in the first several years of our ownership of them is contracted
for, either under transition power purchase contracts with their former owners
or under bilateral contracts with other wholesale customers. In the case of the
Kladno project, where there is a long-term agreement, the energy price is tied
to the market price of electricity rather than to the costs incurred by the
project, so the contract does not provide the traditional level of certainty and
protection. In the case of the Loy Yang A project, Australian power pool prices
have been significantly lower than anticipated at the time of our purchase of
our interest in the Loy Yang A project, resulting in earnings much lower than
initially forecast. On the basis of historical Australian power pool prices,
absent project debt restructuring, the Loy Yang A project company will
experience difficulty in servicing its long-term debt obligations. This, in
turn, could trigger a senior debt default under the project's loan documents on
or about June 2002. While these "merchant" projects introduce new risks and
uncertainties, and require careful advance analysis of the local power markets,
we believe that they are becoming increasingly accepted in the independent power
market.
 
     NON-RECOURSE FINANCING
 
     As with our existing facilities, we expect to finance most of our future
projects with debt as well as equity. Leveraged financing permits the
development of projects with a limited equity base but also increases the risk
that a reduction in revenues could adversely affect a particular project's
ability to meet its debt or lease obligations.
 
     We have financed our principal power generation facilities, other than
Schkopau, primarily with non-recourse debt that is repaid solely from the
project's revenues and generally is secured by interests in the physical assets,
major project contracts and agreements, cash accounts and, in certain cases, the
ownership interest, in that project subsidiary. This type of financing is
referred to as "project financing." True project financing is not available for
all projects, including some assets purchased out of bankruptcy, some merchant
plants, some purchases of minority stock positions in publicly traded companies
and plants in certain countries that lack a sufficiently well-developed legal
system. Even in those instances, however, we may still be able to finance a
smaller portion of the total project cost with project financing, with the
remainder financed with debt that is either raised or supported at the corporate
rather than the project level.
 
     Project financing transactions generally are structured so that all
revenues of a project are deposited directly with a bank or other financial
institution acting as escrow or security deposit agent.
 
                                      S-16
<PAGE>   19
 
These funds then are payable in a specified order of priority set forth in the
financing documents to ensure that, to the extent available, they are used first
to pay operating expenses, senior debt service and taxes and to fund reserve
accounts. Thereafter, subject to satisfying debt service coverage ratios and
certain other conditions, available funds may be disbursed for management fees
or dividends or, where there are subordinated lenders, to the payment of
subordinated debt service.
 
     In the event of a foreclosure after a default, our project subsidiary or
project affiliate owning the facility would only retain an interest in the
assets, if any, remaining after all debts and obligations were paid. In
addition, the debt of each operating project may reduce the liquidity of our
equity interest in that project because the interest is typically subject both
to a pledge securing the project's debt and to transfer restrictions set forth
in the relevant financing agreements. Also, our ability to transfer or sell our
interest in certain projects is restricted by certain purchase options or rights
of first refusal in favor of our partners or the project's power and steam
purchasers and certain change of control restrictions in the project financing
documents.
 
     These project financing structures are designed to prevent the lenders from
looking to us or our other projects for repayment, that is, they are
"non-recourse" to us and our other project subsidiaries and project affiliates
not involved in the project, unless we or another project subsidiary or project
affiliate expressly agrees to undertake liability. We have agreed to undertake
limited financial support for certain of our project subsidiaries in the form of
certain limited obligations and contingent liabilities. These obligations and
contingent liabilities take the form of guarantees of certain specified
obligations, indemnities, capital infusions and agreements to pay certain debt
service deficiencies. To the extent we become liable under such guarantees and
other agreements in respect of a particular project, distributions received by
us from other projects may be used by us to satisfy these obligations. To the
extent of these obligations, creditors of a project financing may have recourse
to us.
 
     EXPOSURE TO CURRENCY FLUCTUATION
 
     As part of our strategy, we hold assets and liabilities denominated in
foreign currencies. We adjust the value of these holdings quarterly to reflect
fluctuations in the values of their respective currencies. This can, and has,
generated non-cash income and losses.
 
                                      S-17
<PAGE>   20
 
SUMMARY OF OUR PROJECTS
 
     Listed below are descriptions of our interests in facilities, operations or
projects under construction as of May 1, 1999:
 
          INDEPENDENT POWER PRODUCTION AND COGENERATION FACILITIES(1)
 
<TABLE>
<CAPTION>
                                     LATER OF
                                      DATE OF
                                    ACQUISITION                                OUR
                                    OR DATE OF                    PLANT     PERCENTAGE
                                    COMMERCIAL                   CAPACITY   OWNERSHIP
  NAME AND LOCATION OF FACILITY      OPERATION    PRIMARY FUEL   (MW)(2)     INTEREST               POWER PURCHASER
  -----------------------------     -----------   ------------   --------   ----------   -------------------------------------
<S>                                 <C>           <C>            <C>        <C>          <C>
INTERNATIONAL PROJECTS:
Loy Yang Power, Australia(3)......     1997              Coal     2,000       25.37      Victorian Pool
Gladstone Power Station,
  Australia.......................     1994              Coal     1,680       37.50      QTPTC; BSL
Collinsville, Australia...........     1998              Coal       189       50.00      QTPTC
Energy Developments Limited,
  Australia.......................     1997       LFG/Methane       262       33.97      Various
Kladno Czech Republic, existing
  project.........................     1994              Coal        28       44.26      STE/Industrials
Kladno Czech Republic, expansion
  project(4)......................     1999          Coal/Gas       345       44.50      STE/Industrials
Schkopau Power Station, Germany...     1996              Coal       960       20.95      VEAG
MIBRAG mbH(3), (Mumsdorf)
  Germany.........................     1994              Coal       110       33.33      WESAG
MIBRAG mbH(3), (Deuben) Germany...     1994              Coal        86       33.33      WESAG
MIBRAG mbH(3), (Wahlitz)
  Germany.........................     1994              Coal        37       33.33      WESAG
Enfield (London) UK...............     1999               Gas       396       25.00      U.K. Electricity Grid
COBEE, Bolivia....................     1996         Hydro/Gas       217(5)    48.30      Electropaz/ELF
Latin Power (Mamonal), Colombia...     1994               Gas        90        6.45      Proelectrica
Latin Power (Termovalle),
  Colombia........................     1998               Gas       199        4.88      EPSA
Latin Power (Termotasajero),
  Colombia........................     1998              Coal       150        7.93      Colombian Grid
Latin Power (ELCOSA), Honduras....     1994               Oil        80        7.65      Empresa Nacional de Energia Electrica
Latin Power (Dr. Bird), Jamaica...     1995            Diesel        74        8.78      Jamaica Public Service Company, Ltd.
Latin Power (Orzunil),
  Guatemala.......................     1999        Geothermal        24       12.25      INDE
Latin Power (Aguaytia), Peru......     1998               Gas       155        3.28      Central Peruvian Electricity Grid
Kingston Cogeneration, Canada.....     1997               Gas       110       25.00      Ontario Hydro
DOMESTIC PROJECTS:
Energy Investors Fund, 1 and 3....     1997           Various       436        3.70      Various
El Segundo Power, California......     1998               Gas     1,020       50.00      California ISO
Long Beach Generating,
  California......................     1998               Gas       530       50.00      California ISO
Crockett Cogeneration,
  California......................     1997               Gas       240       24.88      PG&E
Curtis-Palmer Hydro, New York.....     1997             Hydro        58        8.50      NIMO
Mt. Poso Cogeneration,
  California......................     1997              Coal        50       39.10      PG&E(6)
PowerSmith Cogeneration,
  Oklahoma........................     1997               Gas       110        8.75      OGE Energy
Turners Falls, Massachusetts......     1997              Coal        20        8.90      Unitil Power
Somerset, Massachusetts...........     1999              Coal       229(7)   100.00      EUA; NEPOOL
</TABLE>
 
                                      S-18
<PAGE>   21
 
<TABLE>
<CAPTION>
                                     LATER OF
                                      DATE OF
                                    ACQUISITION                                OUR
                                    OR DATE OF                    PLANT     PERCENTAGE
                                    COMMERCIAL                   CAPACITY   OWNERSHIP
  NAME AND LOCATION OF FACILITY      OPERATION    PRIMARY FUEL   (MW)(2)     INTEREST               POWER PURCHASER
  -----------------------------     -----------   ------------   --------   ----------   -------------------------------------
<S>                                 <C>           <C>            <C>        <C>          <C>
Cogeneration Corporation of
  America:
  CogenAmerica (Newark), New
    Jersey........................     1996               Gas        54       45.21      Jersey Central Power & Light
  CogenAmerica (Parlin), New
    Jersey........................     1996               Gas       122       45.21      Jersey Central Power & Light
  CogenAmerica (Grays Ferry),
    Pennsylvania..................     1996               Gas       150       15.07      PECO Energy Company
  CogenAmerica (Philadelphia),....     1996            Diesel        22       37.52      Philadelphia Municipal Authority
  CogenAmerica (Morris),
    Illinois......................     1998               Gas       117       45.21      Equistar Petro Chemicals, Inc.
  CogenAmerica (Pryor), Oklahoma..     1997               Gas       110       45.21      OGE Energy
San Joaquin Valley (Madera),
  California......................     1992           Biomass        23       45.00      NA(8)
San Joaquin Valley (Chowchilla
  II),............................     1992           Biomass        10       45.00      NA(8)
San Joaquin Valley (El Nido),.....     1992           Biomass        10       45.00      NA(8)
Jackson Valley Energy Partners,...     1991           Biomass        16       50.00      PG&E
Sunnyside Cogeneration Associates,
  Utah............................     1994              Coal        58       50.00      PacifiCorp
Artesia, California...............     1996               Gas        34        2.96      Southern California Edison
Cadillac Renewable Energy,
  Michigan........................     1997              Wood        38       50.00      Consumers Energy
</TABLE>
 
- -------------------------
(1) Does not include the small hydroelectric and landfill gas-fired power
    generation facilities owned by NEO with an aggregate capacity of 110 MW, of
    which NEO has net ownership of 58 MW. In addition, NEO has landfill gas
    projects under construction with an aggregate capacity of 28 MW, of which
    NEO has net ownership of 14 MW.
 
(2) Capacity shown is without deduction for internally consumed power.
 
(3) Each of Loy Yang and MIBRAG also owns coal mines which sell coal both to its
    respective power plant and to third parties.
 
(4) Includes 74 MW gas-fired facility placed in service in March 1999 and 271 MW
    coal-fired facility to be operational in late 1999.
 
(5) Includes the Huaji (29 MW) expansion project which is expected to be fully
    operational in 1999.
 
(6) Operations of the project are currently suspended pursuant to an agreement
    with this power purchaser.
 
(7) Includes 69 MW on deactivated reserve.
 
(8) Operations suspended following buy-out of power purchase contracts and
    pending negotiation of new power purchase agreements or sale of such
    facilities. PG&E has agreed to a buy-out of related power purchase
    agreements, but retains a right of first refusal with respect to output of
    facilities.
 
                                      S-19
<PAGE>   22
 
                     PENDING GENERATION ASSET ACQUISITIONS
 
<TABLE>
<CAPTION>
                                                                               OUR
                              LATER OF DATE OF                   PLANT      PERCENTAGE
   NAME AND LOCATION       ACQUISITION OR DATE OF    PRIMARY    CAPACITY    OWNERSHIP
      OF FACILITY           COMMERCIAL OPERATION      FUEL      (MW)(1)      INTEREST             POWER PURCHASER
- -----------------------    ----------------------    -------    --------    ----------    -------------------------------
<S>                        <C>                       <C>        <C>         <C>           <C>
Huntley, New York          Second Quarter 1999        Coal         760        100.00      NIMO; NYPP
Dunkirk, New York          Second Quarter 1999        Coal         600        100.00      NIMO; NYPP
Encina, California         Second Quarter 1999         Gas         253         50.00      Cal. Power Exchange; bilateral
                                                                                          contracts
Encina Combustion
  Turbines, California     Second Quarter 1999         Gas         965         50.00      Cal. Power Exchange; bilateral
                                                                                          contracts
Arthur Kill, New York      Third Quarter 1999          Gas         842        100.00      Con. Ed.; NYPP
Astoria Gas Turbines,
  New York                 Third Quarter 1999          Gas         614        100.00      Con. Ed.; NYPP
Oswego, New York           Fourth Quarter 1999         Oil       1,700        100.00      NIMO; NYPP
</TABLE>
 
             THERMAL ENERGY PRODUCTION AND TRANSMISSION FACILITIES
                        AND RESOURCE RECOVERY FACILITIES
 
<TABLE>
<CAPTION>
                                                                            OUR PERCENTAGE
                                   DATE OF                                    OWNERSHIP             THERMAL ENERGY
NAME AND LOCATION OF FACILITY    ACQUISITION          CAPACITY(2)              INTEREST         PURCHASER/MSW SUPPLIER
- -----------------------------    -----------    ------------------------    --------------    --------------------------
<S>                              <C>            <C>                         <C>               <C>
THERMAL ENERGY PRODUCTION
AND TRANSMISSION FACILITIES:
Minneapolis Energy Center           1993        Steam: 1,323 mmBtu/hr           100.00        Approximately 91 steam
(MEC), Minnesota                                (388 MW)                                      customers and 37 chilled
                                                Chilled water: 35,550                         water customers
                                                tons.
North American Thermal              1995        Pittsburgh: steam-240            49.40        Approximately 23 customers
  Systems (NATS),                               mmBtu/hr                                      in Pittsburgh and 158
  Pennsylvania &                                (70 MW) chilled water -                       customers in San Francisco
  California(3)                                 10,180 tons/
                                                San Francisco: steam-490
                                                mmBtu/hr.
                                                (144 MW)
San Diego Power & Cooling,          1997        Chilled Water: 5,250            100.00        Approximately 13 customers
  California                                    tons.
Rock-Tenn, Minnesota                1992        Steam:                          100.00        Rock-Tenn Company
                                                430 mmBtu/hr.
                                                (126 MW)
Washco, Minnesota                   1992        160 mmBtu/hr                    100.00        Andersen Corporation
                                                (47 MW)                                       Minnesota Correctional
                                                                                              Facility
Grand Forks Air Force Base,         1992        105 mmBtu/hr                    100.00        Grand Forks Air Force Base
  North Dakota                                  (31 MW)
Energy Center Kladno, Czech         1994        512 mmBtu/hr                     36.32        City of Kladno
  Republic(4)                                   (150 MW)
Camas Power Boiler,                 1997        200mmBtu/hr                     100.00        Fort James Corp.
  Washington                                    (59 MW)
RESOURCE RECOVERY FACILITIES:
Newport, Minnesota                  1993        MSW: 1,500 tons/day             100.00        Ramsey and Washington
                                                                                              Counties
</TABLE>
 
                                      S-20
<PAGE>   23
 
<TABLE>
<CAPTION>
                                                                            OUR PERCENTAGE
                                   DATE OF                                    OWNERSHIP             THERMAL ENERGY
NAME AND LOCATION OF FACILITY    ACQUISITION          CAPACITY(2)              INTEREST         PURCHASER/MSW SUPPLIER
- -----------------------------    -----------    ------------------------    --------------    --------------------------
<S>                              <C>            <C>                         <C>               <C>
Elk River, Minnesota                  (5)       MSW: 1,500 tons/day               0.00        Anoka, Hennepin, and
                                                                                              Sherburne Counties; Tri-
                                                                                              County Solid Waste
                                                                                              Management Commission

Penobscot Energy Recovery,          1997        MSW: 800 tons/day                28.71        Bangor Hydroelectric
  Maine                                                                                       Company

Maine Energy Recovery,              1997        MSW: 680 tons/day                16.25        CMP
  Maine
</TABLE>
 
- -------------------------
(1) Capacity shown is without deduction for internally consumed power.
 
(2) Thermal production and transmission capacity is based on 1,000 Btus per
    pound of steam production or transmission capacity. The unit mmBtu is equal
    to one million Btus.
 
(3) Includes 0.5% general partnership interests in each of PTLP and SFTLP. In
    January 1999, we signed a purchase agreement to acquire the remaining
    interest in the projects.
 
(4) Kladno also is included in the Independent Power Production and Cogeneration
    Facilities table on the preceding page.
 
(5) We operate the Elk River resource recovery facility on behalf of NSP.
 
                                      S-21
<PAGE>   24
 
                                USE OF PROCEEDS
 
     The net proceeds from the sale of the notes, estimated to be approximately
$298.0 million, will be used for general corporate purposes, which may include
financing the development and construction of new facilities, additions to
working capital, reductions of our indebtedness and our subsidiaries'
indebtedness, financing of capital expenditures and pending or potential
acquisitions. Funds not immediately required for such purposes may be invested
in short-term investment grade securities. Equity investments in pending
acquisitions which may be financed out of the proceeds of the notes include the
acquisition by our indirect subsidiaries, through separate transactions, of
certain generating assets from Niagara Mohawk Power Corporation, Consolidated
Edison Company of New York Inc., Eastern Utilities Associates, and San Diego Gas
& Electric Company.
 
                                      S-22
<PAGE>   25
 
                 SELECTED CONSOLIDATED FINANCIAL AND OTHER DATA
 
     The selected consolidated financial data set forth below as of December 31,
1994, 1995, 1996, 1997 and 1998 and for the years then ended, have been derived
from our audited consolidated financial statements. Certain financial
information for the year ended December 31, 1994 has been reclassified to
conform to the financial presentation for the year ended December 31, 1998. The
selected capacity data set forth below under the caption "Other Data" has been
supplied by us.
 
CONSOLIDATED STATEMENTS OF INCOME DATA:
 
<TABLE>
<CAPTION>
                                                            YEAR ENDED DECEMBER 31,
                                             ------------------------------------------------------
                                              1994       1995        1996        1997        1998
                                              ----       ----        ----        ----        ----
                                                                 (IN THOUSANDS)
<S>                                          <C>        <C>        <C>         <C>         <C>
OPERATING REVENUES
Revenues from wholly-owned operations....    $63,970    $64,180    $ 71,649    $ 92,052    $100,424
Equity in earnings of unconsolidated
  affiliates.............................     32,155     28,639      34,315      26,200      81,706
                                             -------    -------    --------    --------    --------
     Total operating revenues............     96,125     92,819     105,964     118,252     182,130
OPERATING COSTS AND EXPENSES
Cost of wholly-owned operations..........     34,861     32,535      36,562      46,717      52,413
Depreciation and amortization............      8,675      8,283       8,378      10,310      16,320
General, administrative, and
  development............................     19,993     34,647      39,248      43,116      56,385
                                             -------    -------    --------    --------    --------
       Total operating costs and
          expenses.......................     63,529     75,465      84,188     100,143     125,118
                                             -------    -------    --------    --------    --------
Operating income.........................     32,596     17,354      21,776      18,109      57,012
OTHER INCOME (EXPENSE)
Minority interest........................         --         --          --        (131)     (2,251)
Equity in gain from project termination
  settlements(1).........................      9,685     29,850          --          --          --
Other income, net(2).....................     (3,589)      (104)      7,977      11,502      11,630
Interest expense.........................     (6,682)    (7,089)    (15,430)    (30,989)    (50,313)
                                             -------    -------    --------    --------    --------
     Total other income (expense)........       (586)    22,657      (7,453)    (19,618)    (40,934)
                                             -------    -------    --------    --------    --------
Income (loss) before income taxes........     32,010     40,011      14,323      (1,509)     16,078
Income tax benefit (expense)(3)..........     (2,472)    (8,810)      5,655      23,491      25,654
                                             -------    -------    --------    --------    --------
Net income...............................    $29,538    $31,201    $ 19,978    $ 21,982    $ 41,732
                                             =======    =======    ========    ========    ========
</TABLE>
 
- -------------------------
(1) In 1994, we and our partner in the Michigan Cogeneration Partners Limited
    Partnership agreed to terminate a power sales contract with Consumers Power
    Company. The contract related to a 65 MW cogeneration facility being
    developed in Michigan. Due to the agreement to terminate the contract, we
    recorded a one-time pre-tax gain of $9.7 million in 1994. Equity in gain
    from project termination settlements in 1995 included a one-time pre-tax
    gain of $29.9 million related to the settlement and termination of the San
    Joaquin Valley power purchase agreements with PG&E.
 
(2) Includes pretax charges of $5.0 million, $5.0 million, $1.5 million, $9.0
    million and $26.7 million in the years 1994, 1995, 1996, 1997 and 1998,
    respectively, to write-down the carrying value of certain energy projects.
    These amounts also include the gain on sale of interest in projects of $8.7
    million in 1997 and $30.0 million in 1998.
 
(3) We are included in the consolidated federal income tax and state franchise
    tax returns of NSP. We calculate our tax position on a separate company
    basis under a tax sharing agreement with NSP and receive payment from NSP
    for tax benefits and we pay NSP for tax liabilities.
 
                                      S-23
<PAGE>   26
 
CONSOLIDATED BALANCE SHEET DATA:
 
<TABLE>
<CAPTION>
                                                             AS OF DECEMBER 31,
                                        ------------------------------------------------------------
                                          1994        1995        1996         1997          1998
                                          ----        ----        ----         ----          ----
                                                               (IN THOUSANDS)
<S>                                     <C>         <C>         <C>         <C>           <C>
Net property, plant and equipment...    $107,634    $111,919    $129,649    $  185,891    $  204,729
Net equity investments in
  projects..........................     164,863     221,129     365,749       694,655       800,924
Total assets........................     376,570     454,589     680,809     1,168,102     1,293,426
Long-term debt, including current
  maturities........................      93,339      90,034     212,141       620,855       626,476
Stockholder's equity................     234,722     319,764     421,914       450,698       579,332
</TABLE>
 
OTHER DATA:
 
<TABLE>
<CAPTION>
                                                                      AS OF AND FOR THE
                                                                   YEAR ENDED DECEMBER 31,
                                                          -----------------------------------------
                                                          1994     1995     1996     1997     1998
                                                          ----     ----     ----     ----     ----
<S>                                                       <C>      <C>      <C>      <C>      <C>
Our net power generating capacity (MW)................      992      999    1,326    2,637    3,300
Our net thermal energy generating capacity:
  mmBtus per hour.....................................    1,961    2,318    2,654    2,693    2,905
  MW..................................................      575      679      778      789      851
Consolidated interest expense coverage ratio(1).......     7.09x    7.81x    2.47x    1.28x    1.64x
Consolidated funds from operations to recourse
  interest expense coverage ratio(2)..................       (3)      (4)    1.75x    2.38x    1.77x
</TABLE>
 
- -------------------------
(1) This coverage ratio equals EBITDA (the sum of income (loss) before taxes,
    interest expense (net of capitalized interest) and depreciation and
    amortization expense) divided by interest expense.
 
(2) This coverage ratio equals the sum of funds from operations plus recourse
    interest expense divided by recourse interest expense. Funds from operations
    is calculated by subtracting working capital changes from cash provided
    (used) from operations.
 
(3) Not meaningful because there was no recourse interest expense in 1994.
 
(4) Due primarily to undistributed equity from unconsolidated affiliates, funds
    from operations did not cover recourse interest by $13.3 million.
 
                                      S-24
<PAGE>   27
 
                        EARNINGS TO FIXED CHARGES RATIO
 
     The following table sets forth the ratio of our earnings to our fixed
charges for the periods indicated:
 
<TABLE>
<CAPTION>
                                 YEAR ENDED DECEMBER 31,
                         ----------------------------------------    THREE MONTHS ENDED    THREE MONTHS ENDED
                         1994     1995     1996     1997     1998      MARCH 31, 1998        MARCH 31, 1999
                         ----     ----     ----     ----     ----    ------------------    ------------------
<S>                      <C>      <C>      <C>      <C>      <C>     <C>                   <C>
Ratio of earnings to
  fixed charges(1)...    2.98x    1.56x    1.75x    1.16x     (2)            (3)                   (4)
</TABLE>
 
- -------------------------
(1) The ratio of earnings to fixed charges is calculated by dividing earnings by
    fixed charges. For this purpose "earnings" means income (loss) before income
    taxes less undistributed equity in our share of operating earnings of
    unconsolidated affiliates less equity in gain from project termination
    settlements plus cash distributions from project termination settlements
    plus fixed charges. "Fixed charges" means interest expense plus interest
    capitalized plus amortization of debt issuance costs plus one-third of our
    annual rental expense which the Securities and Exchange Commission defines
    as a reasonable approximation of rental expense interest.
 
(2) Due primarily to interest expense and undistributed equity from
    unconsolidated affiliates, earnings did not cover fixed charges by $7.3
    million.
 
(3) Due primarily to undistributed equity from unconsolidated affiliates,
    earnings did not cover fixed charges by $14.9 million.
 
(4) Earnings did not cover fixed charges by $10.5 million.
 
                                      S-25
<PAGE>   28
 
                              DESCRIPTION OF NOTES
 
     This section summarizes the specific financial and legal terms of the notes
that are more generally described under "Description of Debt Securities"
beginning on page 4 of the prospectus that is attached to the back of this
supplement. If anything described in this section is inconsistent with the terms
described under "Description of Debt Securities" in the attached prospectus, the
terms described here prevail.
 
     - TITLE: 7 1/2% Senior Notes due 2009
 
     - TOTAL PRINCIPAL AMOUNT BEING ISSUED: $300,000,000
 
     - DUE DATE FOR PRINCIPAL: June 1, 2009
 
     - INTEREST RATE: 7 1/2% per annum
 
     - DATE INTEREST STARTS ACCRUING: May 25, 1999
 
     - INTEREST DUE DATES: Every June 1 and December 1
 
     - FIRST INTEREST DUE DATE: December 1, 1999
 
     - FORM OF NOTES: The notes will be issued as Global Securities, and may be
       withdrawn from the Depositary only in the limited situations described on
       pages 14 and 15 of the attached prospectus.
 
     - NAME OF DEPOSITARY: The Depository Trust Company ("DTC").
 
     - TRADING IN DTC: Indirect holders that trade their beneficial interests in
       the Global Securities through DTC must trade in DTC's same-day funds
       settlement system and pay in immediately available funds.
 
     - CHANGE OF CONTROL: Upon a Change of Control, a holder of notes may
       require us to repurchase that holder's notes, in whole or in part, at
       101% of the principal amount of the notes, plus accrued interest. A
       Change of Control will not be deemed to have occurred if, after giving
       effect thereto, the notes are rated BBB- or better by Standard & Poor's
       Ratings Group and Baa3 or better by Moody's Investors Service, Inc.
 
     - OPTIONAL REDEMPTION: We may, at our option, redeem some or all of the
       notes -- that is, repay them early -- at any time. If we redeem the notes
       before June 1, 2009, we must pay you whichever of the following two items
       is greater:
 
      - 100% of the principal amount of the notes to be redeemed.
 
      - a "make whole" amount, which will be calculated as described below.
 
     When we redeem the notes, we must also pay all interest that has accrued to
the redemption date on the redeemed notes. The notes will stop bearing interest
on the redemption date, even if you do not collect your money.
 
     - CALCULATION OF MAKE WHOLE AMOUNT: The "make whole" amount will equal the
       sum of the present values of the Remaining Scheduled Payments (as defined
       below) discounted, on a semiannual basis (assuming a 360-day year
       consisting of twelve 30-day months), at a rate equal to the Treasury Rate
       (as defined below) plus 25 basis points.
 
      "REMAINING SCHEDULED PAYMENTS" means the remaining scheduled payments of
      the principal and interest that would be due if such debt security were
      not redeemed. However, if the redemption date is not a scheduled interest
      payment date, the amount of the next succeeding
 
                                      S-26
<PAGE>   29
 
      scheduled interest payment on such debt security will be reduced by the
      amount of interest accrued on such debt security to such redemption date.
 
      "TREASURY RATE" means an annual rate equal to the semiannual equivalent
      yield to maturity of the Comparable Treasury Issue (as defined below),
      assuming a price for the Comparable Treasury Issue (expressed as a
      percentage of its principal amount) equal to the Comparable Treasury Price
      (as defined below) for the redemption date. The semiannual equivalent
      yield to maturity will be computed as of the third business day
      immediately preceding the redemption date.
 
      "COMPARABLE TREASURY ISSUE" means the United States Treasury security
      selected by Salomon Smith Barney or an affiliate as having a maturity
      comparable to the remaining term of the notes that would be utilized, at
      the time of selection and in accordance with customary financial practice,
      in pricing new issues of corporate notes of comparable maturity to the
      remaining term of the notes.
 
      "COMPARABLE TREASURY PRICE" means the average of three Reference Treasury
      Dealer Quotations (as defined below) obtained by the trustee for the
      redemption date.
 
      "REFERENCE TREASURY DEALERS" means Salomon Smith Barney (so long as it
      continues to be a primary U.S. Government securities dealer) and any two
      other primary U.S. Government securities dealers chosen by us. If Salomon
      Smith Barney ceases to be a primary U.S. Government securities dealer, we
      will appoint in its place another nationally recognized investment banking
      firm that is a primary U.S. Government securities dealer.
 
      "REFERENCE TREASURY DEALER QUOTATION" means the average, as determined by
      the trustee, of the bid and asked prices for the Comparable Treasury Issue
      (expressed in each case as a percentage of its principal amount) quoted in
      writing to the trustee by a Reference Treasury Dealer at 3:30 p.m., New
      York City time, on the third business day preceding the redemption date.
 
     - REDEMPTION NOTICE: We will give notice to DTC at least 30 days (but not
       more than 60 days) before we redeem the notes. If we redeem only some of
       the notes, DTC's practice is to choose by lot the amount to be redeemed
       from the notes held by each of its participating institutions. DTC will
       give notice to these participants, and these participants will give
       notice to any "Street Name" holders of any indirect interests in the
       notes according to arrangements among them; these notices may be subject
       to statutory or regulatory requirements. We will not be responsible for
       giving notice to anyone other than DTC.
 
     - SALE OF PROPERTIES OR ASSETS: Except for a sale of our assets
       substantially as an entirety, and other than assets we are required to
       sell to conform with governmental regulations, we may not sell or
       otherwise dispose of any assets (other than short-term, readily
       marketable investments purchased for cash management purposes with funds
       not representing the proceeds of other asset sales) if on a pro forma
       basis, the aggregate net book value of all such sales during the most
       recent 12-month period would exceed 10% of our Consolidated Net Tangible
       Assets (as defined below) computed as of the end of the most recent
       quarter preceding such sale; provided, however, that any such sales shall
       be disregarded for purposes of this 10% limitation if the proceeds are
       invested in assets in similar or related lines of our business and,
       provided further, that we may sell or otherwise dispose of assets in
       excess of such 10% if we retain the proceeds from such sales or
       dispositions, which are not reinvested as provided above, as cash or cash
       equivalents or we use the proceeds to purchase and retire the notes
       offered hereby, other debt securities of ours or are used to retire debt
       ranking pari passu with the notes.
 
                                      S-27
<PAGE>   30
 
     "CONSOLIDATED NET TANGIBLE ASSETS" means, as of the date of any
determination thereof, the total amount of all our assets determined on a
consolidated basis in accordance with GAAP as of such date less the sum of (a)
our consolidated current liabilities determined in accordance with GAAP and (b)
assets properly classified as intangible assets, in accordance with GAAP.
 
     - SINKING FUND: There is no sinking fund.
 
     - DEFEASANCE: We may choose to terminate some of our obligations under the
       notes as described under "Defeasance and Covenant Defeasance" on pages 10
       and 11 of the attached prospectus.
 
     - TRUSTEE: We will issue the notes under an Indenture with Norwest Bank
       Minnesota, National Association, as trustee, dated as of May 25, 1999.
 
                                      S-28
<PAGE>   31
 
                                  UNDERWRITING
 
     Subject to the terms and conditions stated in the underwriting agreement
dated the date hereof, each underwriter named below has severally agreed to
purchase, and we have agreed to sell to such underwriter, the principal amount
of notes set forth opposite the name of such underwriter:
 
<TABLE>
<CAPTION>
                       NAME                            PRINCIPAL AMOUNT OF NOTES
                       ----                            -------------------------
<S>                                                    <C>
Salomon Smith Barney Inc...........................          $165,000,000
ABN AMRO Incorporated..............................            45,000,000
Banc of America Securities LLC.....................            45,000,000
Merrill Lynch, Pierce, Fenner & Smith
              Incorporated.........................            45,000,000
                                                             ------------
     Total.........................................          $300,000,000
                                                             ============
</TABLE>
 
     The underwriting agreement provides that the obligations of the several
underwriters to purchase the notes included in this offering are subject to
approval of certain legal matters by counsel and to certain other conditions.
The underwriters are obligated to purchase all of the notes if they purchase any
of the notes.
 
     The underwriters, for whom Salomon Smith Barney Inc., ABN AMRO
Incorporated, Banc of America Securities LLC and Merrill Lynch, Pierce, Fenner &
Smith Incorporated are acting as representatives, propose to offer some of the
notes directly to the public at the public offering price set forth on the cover
page of this supplement and some of the notes to certain dealers at the public
offering price less a concession not in excess of 0.400% of the principal amount
of the notes. The underwriters may allow, and such dealers may reallow, a
discount not in excess of 0.250% of the principal amount of the notes on sales
to certain other dealers. After the initial offering of the notes to the public,
the public offering price and such concessions may be changed by the
representatives.
 
     The following table shows the underwriting discounts and commissions to be
paid to the underwriters by us in connection with this offering (expressed as a
percentage of the principal amount of the notes).
 
<TABLE>
<CAPTION>
                                                           PAID BY THE COMPANY
                                                           -------------------
<S>                                                        <C>
Per note...............................................          0.650%
</TABLE>
 
     In connection with the offering, Salomon Smith Barney Inc., on behalf of
the underwriters, may purchase and sell notes in the open market. These
transactions may include over-allotment, syndicate covering transactions and
stabilizing transactions. Over-allotment involves syndicate sales of notes in
excess of the principal amount of the notes to be purchased by the underwriters
in the offering, which creates a syndicate short position. Syndicate covering
transactions involves purchases of the notes in the open market after the
distribution has been completed in order to cover syndicate short positions.
Stabilizing transactions consist of certain bids or purchases of notes made for
the purpose of preventing or retarding a decline in the market price of the
notes while the offering is in progress.
 
     The underwriters may also impose a penalty bid. Penalty bids permit the
underwriters to reclaim a selling concession from a syndicate member when
Salomon Smith Barney Inc., in covering syndicate short positions or making
stabilizing purchases, repurchases notes originally sold by that syndicate
member.
 
     Any of these activities may cause the price of the notes to be higher than
the price that otherwise would exist in the open market in the absence of such
transactions. These transactions may be effected in the over-the-counter market
or otherwise and, if commenced, may be discontinued at any time.
 
                                      S-29
<PAGE>   32
 
     We estimate that our total expenses of this offering will be $330,000.
 
     The representatives have performed certain investment banking and advisory
serviced on our behalf from time to time for which they have received customary
fees and expenses. The representatives may, from time to time, engage in
transactions with and perform services on our behalf in the ordinary course of
their business.
 
     We have agreed to indemnify the underwriters against certain liabilities,
including liabilities under the Securities Act, or to contribute to payments the
underwriters may be required to make in respect of any of those liabilities.
 
                                 LEGAL MATTERS
 
     Gibson, Dunn & Crutcher LLP of Denver, Colorado has rendered an opinion
which was filed as an exhibit to the registration statement with respect to the
legality of the notes. Legal matters with respect to the notes will be passed
upon for the underwriters by Skadden, Arps, Slate, Meagher & Flom LLP, New York,
New York. Each of Gibson, Dunn & Crutcher LLP and Skadden, Arps, Slate, Meagher
and Flom LLP have from time to time represented us, and may in the future from
time to time represent us, in connection with various matters. See "Legal
Matters" in the accompanying prospectus.
 
                                      S-30
<PAGE>   33
 
PROSPECTUS
 
                                  $500,000,000
 
                                   [NRG LOGO]
 
                                NRG ENERGY, INC.
                                DEBT SECURITIES
 
     We may sell at various times up to U.S. $500,000,000 worth of notes,
debentures or other debt securities in one or more series with the same or
different terms. We may sell the debt securities through agents, dealers or
underwriters we designate from time to time. We will determine the specific
terms of the debt securities at the time we sell them and include the following
information in a prospectus supplement:
 
- - Total amount of securities offered
 
- - Maturity date of debt securities
 
- - Interest rates or method of calculating interest rates
 
- - Interest payment dates
 
- - Purchase price
 
- - Terms for repayment or redemption, if any
 
- - Currencies or currency units in which the debt securities are denominated or
  payable
- - Form of debt securities (registered, bearer or uncertificated)
 
- - Whether the debt securities will be represented initially by a single
  temporary or permanent global note
 
- - Agent, dealer or underwriter, if any
 
- - Commission an agent will receive or discount a dealer or underwriter will
  receive and an estimate of the net proceeds we will receive
 
- - Any other terms applicable to the debt securities
 
                               ------------------
 
     Neither the Securities and Exchange Commission nor any state securities
commission has approved or disapproved these securities or determined if this
prospectus is truthful or complete. Any representation to the contrary is a
criminal offense.
 
     This prospectus may not be used to sell securities unless accompanied by a
prospectus supplement.
 
                               ------------------
 
                  THE DATE OF THIS PROSPECTUS IS APRIL 7, 1999
<PAGE>   34
 
                      WHERE YOU CAN FIND MORE INFORMATION
 
     We file annual, quarterly and special reports, proxy statements and other
information with the Securities and Exchange Commission (the "SEC"). You may
read and copy any document we file at the SEC's public reference rooms in
Washington, D.C., New York, New York and Chicago, Illinois. Please call the SEC
at 1-800-732-0330 for further information on the public reference rooms. You may
also obtain copies of these materials from the public reference section of the
SEC at 450 Fifth Street, N.W., Washington, D.C. 20549, at prescribed rates. Our
SEC filings are also available to the public from the SEC's web site at
http://www.sec.gov.
 
     We have filed a registration statement on Form S-3 with the SEC under the
Securities Act of 1933. This prospectus does not contain all of the information
set forth in the registration statement. You should read the registration
statement for further information about us and the debt securities. You may
inspect the registration statement and its exhibits without charge at the office
of the SEC at 450 Fifth Street, N.W., in Washington, D.C. 20549, and you may
obtain copies from the SEC at prescribed rates.
 
     The SEC allows us to "incorporate by reference" the information that we
file with them, which means that we can disclose important information to you by
referring you to those documents. The information incorporated by reference is
considered to be part of this prospectus. The information filed by us with the
SEC in the future will update and supersede this information. We incorporate by
reference the documents listed below and any future filings made by us with the
SEC under Section 13(a), 13(c), 14 or 15(d) of the Securities Exchange Act of
1934 until we sell all the debt securities:
 
     1. Our Annual Report on Form 10-K for the fiscal year ended December 31,
        1998; and
 
     2. Our Quarterly Report on Form 10-Q for the quarterly period ended March
        31, 1999.
 
     You may request a copy of these filings, at no cost, by writing or calling
us at the following address or telephone number:
 
                               Investor Relations
                                NRG Energy, Inc.
                         1221 Nicollet Mall, Suite 700
                          Minneapolis, Minnesota 55403
                                 (612) 373-5300
 
     You should rely only on the information incorporated by reference or
provided in this prospectus or any prospectus supplement. We have not authorized
anyone else to provide you with different information. You should not assume
that the information in this prospectus or any prospectus supplement is accurate
as of any date other than the date on the front of these documents.
 
                           FORWARD-LOOKING STATEMENTS
 
     This prospectus contains or incorporates by reference statements that do
not directly or exclusively relate to historical facts. Such statements are
"forward-looking statements" within the meaning of the Private Securities
Litigation Reform Act of 1995. You can typically identify forward-looking
statements by the use of forward-looking words, such as "may," "will," "could,"
"project," "believe," "anticipate," "expect," "estimate," "continue,"
"potential," "plan," "forecasts," and the like. These statements represent our
intentions, plans, expectations and beliefs and are subject to risks,
uncertainties and other factors. Many of these factors are outside our control
and could cause
 
                                        2
<PAGE>   35
 
actual results to differ materially from such forward-looking statements. These
factors include, among others:
 
     - general economic conditions in countries in which we do business
 
     - industry restructuring, changes in the market price of electricity and
       industry capacity
 
     - demographic changes
 
     - technology changes
 
     - changes in electricity usage patterns and practices
 
     - changes in fuel pricing
 
     - our ability to become Year 2000 ready and the ability of our customers
       and suppliers to become Year 2000 ready
 
                                  THE COMPANY
 
GENERAL
 
     We are one of the leading participants in the independent power generation
industry. We are principally engaged in the acquisition, development and
operation of, and ownership of interests in, independent power production and
co-generation facilities, thermal energy production and transmission facilities
and resource recovery facilities. The power generation facilities in which we
currently have interests (including those under construction) as of December 31,
1998 have a total design capacity of 10,605 megawatts ("MW"), of which we have
or will have total or shared operational responsibility for 6,966 MW and net
ownership of, or leasehold interests in, 3,300 MW. In addition, we have
substantial interests in district heating and cooling systems and steam
generation and transmission operations. As of December 31, 1998, these thermal
businesses had a steam capacity of approximately 3,750 million British thermal
units per hour. Our refuse-derived fuel ("RDF") plants processed more than
1,350,000 tons of municipal solid waste into approximately 1,100,000 tons of RDF
during 1998.
 
     We have experienced significant growth in the last year, expanding from
2,650 MW of net ownership interests in power generation facilities (including
those under construction) as of December 31, 1997 to 3,300 MW of net ownership
interests as of December 31, 1998. This growth resulted primarily from a number
of domestic and international investments and acquisitions.
 
     Our headquarters and principal executive offices are located at 1221
Nicollet Mall, Suite 700, Minneapolis, Minnesota 55403. Our telephone number is
(612) 373-5300. We were established in 1989 and are a wholly-owned subsidiary of
Northern States Power Company ("NSP").
 
                                USE OF PROCEEDS
 
     Unless otherwise specified in the supplement which accompanies this
prospectus, we will use the net proceeds from the sale of the debt securities
for general corporate purposes, which may include financing the development and
construction of new facilities, additions to working capital, reductions of our
indebtedness and the indebtedness of our subsidiaries, financing of capital
expenditures and pending or potential acquisitions. We may invest funds not
immediately required for such purposes in short-term investment grade
securities. The amount and timing of sales of the debt securities will depend on
market conditions and the availability to us of other funds.
 
                                        3
<PAGE>   36
 
                        EARNINGS TO FIXED CHARGES RATIO
 
     The following table sets forth the ratio of our earnings to our fixed
charges for the periods indicated:
 
<TABLE>
<CAPTION>
                                                                   YEAR ENDED DECEMBER 31,
                                                           ----------------------------------------
                                                           1998    1997     1996     1995     1994
                                                           ----    -----    -----    -----    -----
<S>                                                        <C>     <C>      <C>      <C>      <C>
Ratio of earnings to fixed charges(1)..................    (2)     1.16x    1.75x    1.56x    2.98x
</TABLE>
 
- -------------------------
 
(1) The ratio of earnings to fixed charges is calculated by dividing earnings by
    fixed charges. For this purpose "earnings" means income (loss) before income
    taxes less undistributed equity in our share of operating earnings of
    unconsolidated affiliates less equity in gain from project termination
    settlements plus cash distributions from project termination settlements
    plus fixed charges. "Fixed charges" means interest expense plus interest
    capitalized plus amortization of debt issuance costs plus a reasonable
    approximation of the interest factor of rental expenses.
 
(2) Due primarily to interest expense and undistributed equity from
    unconsolidated affiliates, earnings did not cover fixed charges by $7.3
    million.
 
                         DESCRIPTION OF DEBT SECURITIES
 
     This prospectus describes the general terms and provisions of our debt
securities. When we offer to sell a particular series of debt securities, we
will describe the specific terms of the series in a supplement to this
prospectus. We will also indicate in the supplement whether the general terms
and provisions described in this prospectus apply to a particular series of debt
securities.
 
     The following summaries of certain provisions of the indenture and the debt
securities do not purport to be complete. Except to the extent set forth in the
supplement with respect to a particular issue of debt securities, the indenture
or indentures for the debt securities will be substantially similar. Unless
otherwise stated in the supplement, the trustee under the first indenture under
which debt securities will be issued will be Norwest Bank Minnesota, National
Association. (See "Concerning the Trustee.")
 
GENERAL
 
     The debt securities will be unsecured senior obligations of the Company, as
set forth in the accompanying supplement. Because we conduct substantially all
of our business through numerous subsidiaries and affiliates, all existing and
future liabilities of our direct and indirect subsidiaries and affiliates will
be effectively senior to the debt securities. The debt securities will not be
guaranteed by, or otherwise be obligations of, our project subsidiaries and
project affiliates, or our other direct and indirect subsidiaries and affiliates
or NSP.
 
     Reference is made to the accompanying supplement for the following terms of
and information relating to the debt securities (to the extent such terms are
applicable to such debt securities):
 
     - the specific designation, aggregate principal amount, purchase price and
       denomination;
 
     - the date of maturity;
 
     - the interest rate or rates (or the method by which such rate will be
       determined), if any;
 
     - the date from which interest will accrue and dates on which any such
       interest will be payable;
 
     - our rights to defer interest, if any;
 
                                        4
<PAGE>   37
 
     - the place or places where the principal of, premium, if any, and
       interest, if any, on the debt securities will be payable;
 
     - any redemption, repayment or sinking fund provisions;
 
     - our obligation, if any, to offer to purchase the debt securities in the
       event of a "Change of Control" (as defined below);
 
     - any applicable material federal income tax consequences; and
 
     - any other material specific terms of the debt securities, including any
       material additional events of default or covenants provided for with
       respect to the debt securities and any material terms that may be
       required by or advisable under applicable laws or regulations.
 
     The debt securities will bear interest at a fixed rate or a floating rate.
Debt securities bearing no interest or interest at a rate that at the time of
issuance is below the prevailing market rate may be sold or deemed to be sold at
a discount below their stated principal amount. With respect to any debt
securities as to which we have the right to defer interest, the holders of such
debt securities may be allocated interest income for federal and state income
tax purposes without receiving equivalent, or any, interest payments. Any
material federal income tax considerations applicable to any such discounted
debt securities or to certain debt securities issued at par that are treated as
having been issued at a discount for federal income tax purposes will be
described in the supplement.
 
GLOBAL DEBT SECURITIES
 
     If any debt securities are represented by one or more global securities,
the applicable supplement will describe the terms of the depositary arrangement
with respect to such global securities.
 
REDEMPTION
 
     Except as may otherwise be set forth in the accompanying supplement, the
indenture will provide that, we, at any time, may redeem the debt securities, in
whole or in part (if in part, by lot or by such other method as the trustee
shall deem fair or appropriate) at the redemption price of 100% of principal
amount of such debt securities, plus accrued interest on the principal amount,
if any, to the redemption date, plus the applicable "Make-Whole Premium" (as
discussed below).
 
     Except as may otherwise be set forth in the accompanying supplement, the
indenture will provide that, to determine the applicable Make-Whole Premium for
any debt security, an independent investment banking institution of national
standing that we select will compute, as of the third business day prior to the
redemption date, the sum of the present values of all of the remaining scheduled
payments of principal and interest from the redemption date to maturity on such
debt security computed on a semiannual basis by discounting such payments
(assuming a 360-day year consisting of twelve 30-day months) using a rate to be
set forth in the supplement. If the sum of these present values of the remaining
payments as computed above exceeds the aggregate unpaid principal amount of the
debt security that we will redeem plus any accrued but unpaid interest thereon,
the difference will be payable as a premium upon redemption of such debt
security. If the sum is equal to or less than such principal amount plus accrued
interest, we will pay no premium with respect to such debt security.
 
                                        5
<PAGE>   38
 
CERTAIN COVENANTS OF THE COMPANY
 
     AFFIRMATIVE COVENANTS
 
     In addition to such other covenants, if any, as may be described in the
accompanying supplement and except as may otherwise be set forth therein, the
indenture will require us, subject to certain limitations described therein, to,
among other things, do the following:
 
     - deliver to the trustee copies of all reports filed with the SEC;
 
     - deliver to the trustee annual officers' certificates with respect to our
       compliance with our obligations under the indenture;
 
     - maintain our corporate existence subject to the provisions described
       below relating to mergers and consolidations; and
 
     - pay our taxes when due except where we are contesting such taxes in good
       faith.
 
     The indenture may also, as set forth in the accompanying supplement,
restrict our business or operations or that of our subsidiaries or limit our
indebtedness.
 
     RESTRICTIONS ON LIENS
 
     Except as may otherwise be set forth in the accompanying supplement, the
indenture will provide that, so long as any of the debt securities are
outstanding, we will not pledge, mortgage, hypothecate or permit to exist any
mortgage, pledge or other lien upon any property at any time directly owned by
us to secure any indebtedness for money borrowed which is incurred, issued,
assumed or guaranteed by us ("Indebtedness"), without making effective
provisions whereby the debt securities shall be equally and ratably secured with
any and all such Indebtedness and with any other Indebtedness similarly entitled
to be equally and ratably secured; provided, however, that this restriction
shall not apply to or prevent the creation or existence of: (i) liens existing
at the original date of issuance of the debt securities; (ii) purchase money
liens which do not exceed the cost or value of the purchased property; (iii)
other liens not to exceed 10% of our "Consolidated Net Tangible Assets" (defined
below) and (iv) liens granted in connection with extending, renewing, replacing
or refinancing in whole or in part the Indebtedness (including, without
limitation, increasing the principal amount of such Indebtedness) secured by
liens described in the foregoing clauses (i) through (iii). Except as may
otherwise be provided in the accompanying supplement, "Consolidated Net Tangible
Assets" will be defined as the following: as of the date of any determination
thereof, the total amount of all our assets determined on a consolidated basis
in accordance with GAAP as of such date less the sum of (a) our consolidated
current liabilities determined in accordance with GAAP and (b) assets properly
classified as intangible assets, in accordance with GAAP.
 
     Except as may otherwise be set forth in the accompanying supplement, the
indenture will further provide that, in the event we propose to pledge, mortgage
or hypothecate any property at any time directly owned by us to secure any
Indebtedness, other than as permitted by clauses (i) through (iv) of the
previous paragraph, we will agree to give prior written notice thereof to the
trustee, who shall give notice to the holders of debt securities, and we will
further agree, prior to or simultaneously with such pledge, mortgage or
hypothecation, effectively to secure all the debt securities equally and ratably
with such Indebtedness.
 
     The foregoing covenant will not restrict the ability of our subsidiaries
and affiliates to pledge, mortgage, hypothecate or permit to exist any mortgage,
pledge or lien upon their assets, in connection with project financings or
otherwise.
 
                                        6
<PAGE>   39
 
     CHANGE OF CONTROL
 
     Except as may otherwise be set forth in the accompanying supplement, the
indenture will provide that, if a Change of Control occurs, we will be obligated
to offer to purchase all outstanding debt securities. We will conduct any offer
to purchase debt securities upon a Change of Control in compliance with
applicable regulations under the federal securities laws, including Exchange Act
Rule 14e-l. Any limitations on our financial ability to purchase debt securities
upon a Change of Control will be described in the accompanying supplement.
 
     Except as may otherwise be provided in the accompanying supplement, a
"Change of Control" will be defined in the indenture as any of the following:
 
     - NSP (or its successors) ceases to own a majority of our outstanding
       voting stock;
 
     - at any time following the occurrence of the event described immediately
       above, a person or group of persons (other than NSP) becomes the
       beneficial owner, directly or indirectly, or has the absolute power to
       direct the vote of more than 35% of our voting stock; or
 
     - during any one year period, individuals who at the beginning of such
       period constitute our board of directors cease to be a majority of the
       board of directors (unless approved by a majority of the current
       directors then in office who were either directors at the beginning of
       such period or who were previously so approved).
 
     A Change of Control shall be deemed not to have occurred if, following such
an event described above, the debt securities are rated BBB- or better by
Standard & Poor's Ratings Group and Baa3 or better by Moody's Investors Service,
Inc. Except as may otherwise be set forth in the accompanying supplement, our
failure to comply with the Change of Control covenant as to the debt securities
will be an "Event of Default" (as defined below) under the indenture. See
"Events of Default" below.
 
     Except as may be provided otherwise in the accompanying supplement, the
Change of Control provisions may not be waived by the trustee or the board of
directors, and any modification thereof must be approved by each holder of a
debt security. We cannot assure you that we would have sufficient liquidity to
effectuate any required repurchase of debt securities upon a Change of Control.
 
     Except as may be provided otherwise in the accompanying supplement, within
30 days following any Change of Control, we will be required to mail a notice to
each debt security holder (with a copy to the trustee) stating:
 
     - that a Change of Control has occurred and that such holder has the right
       to require us to repurchase such holder's debt securities (the "Change of
       Control Offer");
 
     - the circumstances and relevant facts regarding such Change of Control
       (including information with respect to pro forma historical income, cash
       flow and capitalization after giving effect to such Change of Control);
 
     - the repurchase date (which shall be a business day and be not earlier
       than 30 days or later than 60 days from the date such notice is mailed
       (the "Repurchase Date"));
 
     - that interest on any debt security tendered will continue to accrue;
 
     - that interest on any debt security accepted for payment pursuant to the
       Change of Control Offer shall cease to accrue after the Repurchase Date;
 
     - that debt security holders electing to have a debt security purchased
       pursuant to a Change of Control Offer will be required to surrender the
       debt security, with the form entitled "Option to Elect Purchase" on the
       reverse of the debt security completed, to the trustee at the address
       specified in the notice prior to the close of business on the Repurchase
       Date;
 
                                        7
<PAGE>   40
 
     - that debt security holders will be entitled to withdraw their election if
       the trustee receives, not later than the close of business on the third
       business day (or such shorter periods as may be required by applicable
       law) preceding the Repurchase Date, a telegram, telex, facsimile or
       letter setting forth the name of the debt security holder, the principal
       amount of debt securities the holder delivered for purchase and a
       statement that such debt security holder is withdrawing its election to
       have such debt securities purchased; and
 
     - that debt security holders that elect to have their debt securities
       purchased only in part will be issued new debt securities in a principal
       amount equal to the unpurchased portion of the debt securities
       surrendered.
 
     MERGER, CONSOLIDATION, SALE, LEASE OR CONVEYANCE
 
     Except as may otherwise be provided in the accompanying supplement, the
indenture will provide that we will not merge or consolidate with or into any
other person and we will not sell, lease or convey all or substantially all of
our assets to any person, unless we are the continuing corporation, or the
successor corporation or the person that acquires all or substantially all of
our assets is a corporation organized and existing under the laws of the United
States or a State thereof or the District of Columbia and expressly assumes all
of our obligations under the debt securities and the indenture, and, immediately
after such merger, consolidation, sale, lease or conveyance, such person or such
successor corporation is not in default in the performance of the covenants and
conditions in the indenture. The meaning of the term "all or substantially all
of the assets" has not been definitely established and is likely to be
interpreted by reference to applicable state law if and at the time the issue
arises and will be dependent on the facts and circumstances existing at the
time.
 
     Except as may be provided otherwise in the accompanying supplement, the
indenture will provide that, except for a sale of our assets substantially as an
entirety as provided above, and other than assets we are required to sell to
conform with governmental regulations, we may not sell or otherwise dispose of
any assets (other than short-term, readily marketable investments purchased for
cash management purposes with funds not representing the proceeds of other asset
sales) if on a pro forma basis, the aggregate net book value of all such sales
during the most recent 12-month period would exceed 10% of our Consolidated Net
Tangible Assets computed as of the end of the most recent quarter preceding such
sale; provided, however, that any such sales shall be disregarded for purposes
of this 10% limitation if the proceeds are invested in assets in similar or
related lines of our business and, provided further, that we may sell or
otherwise dispose of assets in excess of such 10% if we retain the proceeds from
such sales or dispositions, which are not reinvested as provided above, as cash
or cash equivalents or we use the proceeds to purchase and retire the debt
securities.
 
     REPORTING OBLIGATIONS
 
     Except as may be provided otherwise in the accompanying supplement, the
indenture will provide that we will furnish or cause to be furnished to holders
of debt securities copies of our annual reports and of the information,
documents and other reports that we are required to file with the SEC pursuant
to Section 13 or 15(d) of the Exchange Act within 15 days after we file them
with the SEC.
 
EVENTS OF DEFAULT
 
     Except as may be described in the accompanying supplement, an "Event of
Default" will be defined under the indenture as being:
 
          (a) our failure to pay any interest on any senior debt security when
     due, which failure continues for 30 days;
 
                                        8
<PAGE>   41
 
          (b) our failure to pay principal or premium (including in connection
     with a Change of Control) when due;
 
          (c) our failure to perform any other covenant relative to the debt
     securities or the indenture for a period of 30 days after the trustee gives
     us written notice or we receive written notice by the holders of at least
     25% in aggregate principal amount of the debt securities;
 
          (d) an event of default occurring under any of our instruments under
     which there may be issued, or by which there may be secured or evidenced,
     any indebtedness for money borrowed that has resulted in the acceleration
     of such indebtedness, or any default occurring in payment of any such
     indebtedness at final maturity (and after the expiration of any applicable
     grace periods), other than (i) indebtedness which is payable solely out of
     the property or assets of a partnership, joint venture or similar entity of
     which we or any of our subsidiaries or affiliates is a participant, or
     which is secured by a lien on the property or assets owned or held by such
     entity, without further recourse to us or (ii) indebtedness not exceeding
     $20,000,000;
 
          (e) one or more final judgments, decrees or orders of any court,
     tribunal, arbitrator, administrative or other governmental body or similar
     entity for the payment of money aggregating more than $20,000,000 shall be
     rendered against us (excluding the amount thereof covered by insurance) and
     shall remain undischarged, unvacated and unstayed for more than 90 days,
     except while being contested in good faith by appropriate proceedings; and
 
          (f) certain events of bankruptcy, insolvency or reorganization in
     respect of us.
 
     The indenture will provide that if an Event of Default (other than an Event
of Default due to certain events of bankruptcy, insolvency or reorganization)
has occurred and is continuing, either the trustee or the holders of not less
than 25% in principal amount of the debt securities outstanding under the
indenture, or such other amount as may be specified in the supplement, may then
declare the principal of all debt securities under that indenture and interest
accrued thereon to be due and payable immediately.
 
     Except to the extent otherwise stated in the accompanying supplement, the
indenture will contain a provision entitling the trustee, subject to the duty of
the trustee during a default to act with the required standard of care, to be
indemnified by the holders of debt securities before proceeding to exercise any
right or power under the indenture at the request of such holders. Subject to
such provisions in the indenture for the indemnification of the trustee and
certain other limitations, the holders of a majority in principal amount of the
debt securities then outstanding may direct the time, method and place of
conducting any proceeding for any remedy available to the trustee or exercising
any trust or power conferred on the trustee.
 
     Except to the extent otherwise stated in the accompanying supplement, the
indenture will provide that no holder of debt securities may institute any
action against us under the indenture (except actions for payment of overdue
principal or interest) unless:
 
     - such holder previously has given the trustee written notice of the
       default and continuance thereof;
 
     - the holders of not less than 25% in principal amount of the debt
       securities then outstanding have requested the trustee to institute such
       action and offered the trustee reasonable indemnity;
 
     - the trustee has not instituted such action within 60 days of the request;
       and
 
     - the trustee has not received direction inconsistent with such written
       request from the holders of a majority in principal amount of the debt
       securities then outstanding under the indenture.
 
                                        9
<PAGE>   42
 
DEFEASANCE AND COVENANT DEFEASANCE
 
     DEFEASANCE
 
     Except to the extent otherwise stated in the accompanying supplement, the
indenture will provide that we will be deemed to have paid and will be
discharged from any and all obligations in respect of the debt securities, on
the 123rd day after the deposit referred to below has been made, and the
provisions of the indenture will cease to be applicable with respect to the debt
securities (except for, among other matters, certain obligations to register the
transfer of or exchange of the debt securities, to replace stolen, lost or
mutilated debt securities, to maintain paying agencies and to hold funds for
payment in trust) if (A) we have deposited with the trustee, in trust, money
and/or U.S. Government Obligations (as defined in the indenture) that, through
the payment of interest and principal in respect thereof in accordance with
their terms, will provide money in an amount sufficient to pay the principal of,
premium, if any, and accrued interest on the debt securities, at the time such
payments are due in accordance with the terms of the indenture, (B) we have
delivered to the trustee (i) an opinion of counsel to the effect that debt
security holders will not recognize income, gain or loss for federal income tax
purposes as a result of our exercise of our option under the defeasance
provisions of the indenture and will be subject to federal income tax on the
same amount and in the same manner and at the same times as would have been the
case if such deposit, defeasance and discharge had not occurred, which opinion
of counsel must be based upon a ruling of the Internal Revenue Service to the
same effect or a change in applicable federal income tax law or related treasury
regulations after the date of the indenture and (ii) an opinion of counsel to
the effect that the defeasance trust does not constitute an "investment company"
within the meaning of the Investment Company Act of 1940 and after the passage
of 123 days following the deposit, the trust fund will not be subject to the
effect of Section 547 of the U.S. Bankruptcy Code or Section 15 of the New York
Debtor and Creditor Law, (C) immediately after giving effect to such deposit, no
Event of Default, or event that after the giving of notice or lapse of time or
both would become an Event of Default, shall have occurred and be continuing on
the date of such deposit or during the period ending on the 123rd day after the
date of such deposit, and such deposit shall not result in a breach or violation
of, or constitute a default under, any other agreement or instrument to which we
are a party or by which we are bound and (D) if at such time the debt securities
are listed on a national securities exchange, we have delivered to the trustee
an opinion of counsel to the effect that the debt securities will not be
delisted as a result of such deposit and discharge.
 
     DEFEASANCE OF CERTAIN COVENANTS AND CERTAIN EVENTS OF DEFAULT
 
     Except to the extent otherwise stated in the accompanying supplement, the
indenture for the debt securities will further provide that the provisions of
the indenture will cease to be applicable with respect to (i) the covenants
described under "Change of Control" and (ii) clause (c) under "Events of
Default" with respect to such covenants and clauses (d) and (e) under "Events of
Default" upon the deposit with the trustee, in trust, of money and/or U.S.
Government Obligations that through the payment of interest and principal in
respect thereof in accordance with their terms will provide money in an amount
sufficient to pay the principal of, premium, if any, and accrued interest on the
debt securities, the satisfaction of the conditions described in clauses
(B)(ii), (C) and (D) of the preceding paragraph and our delivery to the trustee
of an opinion of counsel to the effect that, among other things, the holders of
the debt securities will not recognize income, gain or loss for federal income
tax purposes as a result of such deposit and defeasance of certain covenants and
Events of Default and will be subject to federal income tax on the same amount
and in the same manner and at the same times as would have been the case if such
deposit and defeasance had not occurred.
 
                                       10
<PAGE>   43
 
     DEFEASANCE AND CERTAIN OTHER EVENTS OF DEFAULT
 
     Except to the extent otherwise stated in the accompanying supplement, the
indenture will provide that if we exercise our option to omit compliance with
certain covenants and provisions of the indenture with respect to the debt
securities as described in the immediately preceding paragraph and the debt
securities are declared due and payable because of the occurrence of an Event of
Default that remains applicable, the amount of money and/or U.S. Government
Obligations on deposit with the trustee will be sufficient to pay amounts due on
the debt securities at the time of their stated maturity, but may not be
sufficient to pay amounts due on the debt securities at the time of acceleration
resulting from such Event of Default. In such event, we shall remain liable for
such payments.
 
MODIFICATIONS TO THE INDENTURE
 
     Except as may otherwise be set forth in the accompanying supplement, the
indenture will contain provisions permitting us and the trustee, with the
consent of the holders of not less than a majority in principal amount of the
debt securities then outstanding, to modify the indenture or the rights of the
debt security holders, except that no such modification may, without the consent
of each debt security holder, (i) extend the final maturity of any of the debt
securities or reduce the principal amount thereof, or reduce the rate or extend
the time of payment of interest thereon, or reduce any amount payable on
redemption thereof, or impair or affect the right of any debt security holder to
institute suit for the payment thereof or make any change in the covenant
regarding a Change of Control or (ii) reduce the percentage of debt securities,
the consent of the holders of which is required for any such modification.
 
     Except as may otherwise be set forth in the accompanying supplement, the
indenture will provide that we and the trustee without the consent of any debt
security holder may amend the indenture and the debt securities for the purpose
of curing any ambiguity, or of curing, correcting or supplementing any defective
provision thereof, or in any manner which we and the trustee may determine is
not inconsistent with the debt securities and will not adversely affect the
interest of any debt security holder.
 
BOOK-ENTRY, DELIVERY AND FORM
 
     Except as may otherwise be set forth in the accompanying supplement, the
indenture will provide that the debt securities will initially be issued in the
form of one or more registered notes in global form (the "Global Notes"). Each
Global Note will be deposited on the date of the closing of the sale of the debt
securities with, or on behalf of, The Depository Trust Company ("DTC"), as
depositary, and registered in the name of Cede & Co., as DTC's nominee.
 
     DTC is a limited-purpose trust company created to hold securities for its
participants (the "Participants") and to facilitate the clearance and settlement
of transactions in those securities between Participants through electronic
book-entry changes in accounts of the Participants. The Participants include
securities brokers and dealers, banks, trust companies, clearing corporations
and certain other organizations. Access to DTC's system is also available to
other entities such as banks, brokers, dealers and trust companies that clear
through or maintain a custodial relationship with a Participant, either directly
or indirectly (collectively, the "Indirect Participants"). Persons who are not
Participants may beneficially own securities held by or on behalf of DTC only
through the Participants or the Indirect Participants. The ownership interest
and transfer of ownership interest of each actual purchase of each security held
by or on behalf of DTC are recorded on the records of the Participants and
Indirect Participants.
 
     We expect that pursuant to procedures established by DTC, (i) upon deposit
of the Global Notes, DTC will credit the accounts of Participants designated by
the underwriters with portions of

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<PAGE>   44
 
the principal amount of the Global Notes and (ii) ownership of such interests in
the Global Notes will be shown on, and the transfer of ownership thereof will be
effected only through, records maintained by DTC (with respect to the
Participants) or by the Participants and the Indirect Participants (with respect
to other owners of beneficial interests in the Global Notes).
 
     Investors in the Global Notes may hold their interests therein directly
through DTC if they are Participants in such system, or indirectly through
organizations which are Participants in such system. All interests in a Global
Note may be subject to the procedures and requirements of DTC. The laws of some
states require that certain persons take physical delivery in certificated form
of securities that they own. Consequently, the ability to transfer beneficial
interests in a Global Note to such persons will be limited to that extent.
Because DTC can act only on behalf of Participants, which in turn act on behalf
of Indirect Participants and certain banks, the ability of a person having
beneficial interests in a Global Note to pledge such interest to persons that do
not participate in the DTC system, or otherwise take actions in respect of such
interest, may be affected by the lack of a physical certificate evidencing such
interests. For certain other restrictions on the transferability of the debt
securities, see "-- Exchange of Book-Entry Debt Securities for Certificated Debt
Securities" below.
 
     Except as described below, owners of interests in the Global Notes will not
have debt securities registered in their name, will not receive physical
delivery of debt securities in certificated form and will not be considered the
registered owners of holders thereof under the indenture for any purpose.
 
     Payments in respect of the Global Notes registered in the name of DTC or
its nominee will be payable by the trustee to DTC in its capacity as the
registered holder under the indenture. Under the terms of the indenture, the
trustee will treat the persons in whose names the debt securities, including the
Global Notes, are registered as the owners thereof for the purpose of receiving
such payments and for any and all purposes whatsoever. Consequently, neither the
trustee nor any agent thereof has or will have any responsibility or liability
for (i) any aspect of DTC's records or any Participant's or Indirect
Participant's records relating to or payments made on account of beneficial
ownership interests in the Global Note or for maintaining, supervising or
reviewing any of DTC's records or any Participant's or Indirect Participant's
records relating to the beneficial ownership interests in the Global Note or
(ii) any other matter relating to the actions and practices of DTC or any of its
Participants or Indirect Participants. DTC's current practice, upon receipt of
any payment in respect of securities such as the debt securities, is to credit
the accounts of the relevant Participants with the payment on the payment date,
in amounts proportionate to their respective holdings in principal amount of
beneficial interests in the relevant security as shown on the records of DTC
unless DTC has reason to believe it will not receive payment on such payment
date. Payments by the Participants and the Indirect Participants to the
beneficial owners of Notes will be governed by standing instructions and
customary practices and will be the responsibility of the Participants or the
Indirect Participants and will not be the responsibility of DTC, the trustee or
us. Neither we nor the trustee will be liable for any delay by DTC or any of its
Participants in identifying the beneficial owners of the debt securities, and we
and the trustee may conclusively rely on and will be protected in relying on
instructions from DTC or its nominee for all purposes.
 
     Except as may otherwise be set forth in the accompanying supplement, DTC
will take any action permitted to be taken by a holder of the debt securities
only at the direction of one or more Participants to whose account with DTC
interests in the Global Notes are credited and only in respect of such portion
of the Notes as to which such Participant or Participants has or have given such
direction. However, if there is an Event of Default, DTC reserves the right to
exchange the Global Notes for debt securities in certificated form and to
distribute such debt securities to its Participants.
 
     The information in this section concerning DTC and its book-entry system
has been obtained from sources that we believe to be reliable, but we have not
independently determined the accuracy
 
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<PAGE>   45
 
thereof. We will not have any responsibility for the performance by DTC or its
Participants of their respective obligations under the rules and procedures
governing their operations.
 
EXCHANGE OF BOOK ENTRY DEBT SECURITIES FOR CERTIFICATED DEBT SECURITIES
 
     Except as may otherwise be set forth in the accompanying supplement, a
Global Note is exchangeable for debt securities in registered certificated form
if (i) DTC notifies us that it is unwilling or unable to continue as clearing
agency for the Global Note or has ceased to be a clearing agency registered
under the Exchange Act and we thereupon fail to appoint a successor clearing
agency within 90 days, (ii) we in our sole discretion elect to cause the
issuance of definitive certificated debt securities or (iii) there has occurred
and is continuing an Event of Default or any event which after notice or lapse
of time or both would be an Event of Default under the indenture. In addition,
beneficial interests in a Global Note may be exchanged for certificated debt
securities upon request but only upon at least 20 days, prior written notice
given to the trustee by or on behalf of DTC in accordance with customary
procedures. In all cases certificated debt securities delivered in exchange for
any Global Note or beneficial interest therein will be registered in the names,
and issued in denominations of $100,000 and integral multiples of $1,000 in
excess thereof, requested by or on behalf of the clearing agency (in accordance
with its customary procedures).
 
CONCERNING THE TRUSTEE
 
     Unless otherwise stated in the supplement, the trustee under the first
indenture under which debt securities will be issued will be Norwest Bank
Minnesota, National Association, and, unless stated in the applicable
supplement, (i) it or any other trustee may also be the trustee under any other
indenture for debt securities and (ii) any trustee or its affiliates may lend
money to us, including under our principal credit facility, and may from time to
time have lender or other business arrangements with us. The indenture will
contain certain limitations on the rights of the trustee, should it or its
affiliates then be our creditors, to obtain payment of claims in certain cases
or to realize on certain property received in respect of any such claim as
security or otherwise. The trustee and its affiliates will be permitted to
engage in other transactions; however, if they acquire any conflicting interest,
the conflict must be eliminated or the trustee must resign.
 
GOVERNING LAW
 
     Unless otherwise specified in the accompanying supplement, the indenture
and the debt securities will be governed by New York law.
 
                              PLAN OF DISTRIBUTION
 
     We may offer and sell the debt securities (i) through agents, (ii) through
underwriters, (iii) through dealers, (iv) directly to purchasers (through a
specific bidding or auction process or otherwise), or (v) through a combination
of any such methods of sale. The distribution of the debt securities may be
effected from time to time in one or more transactions at a fixed price or
prices, which may be changed, or at market prices prevailing at the time of
sale, at prices relating to such prevailing market prices or at negotiated
prices.
 
     Offers to purchase the debt securities may be solicited by agents
designated by us from time to time. Any such agent involved in the offer or sale
of the debt securities will be named, and any commissions payable by us to such
agent will be set forth, in the supplement. Unless otherwise indicated in the
supplement, any such agent will be acting on a best efforts basis for the period
of its appointment. Any such agent may be deemed to be an underwriter, as that
term is defined in the Securities Act, of the debt securities so offered and
sold.
 
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<PAGE>   46
 
     If an underwriter or underwriters are utilized in the sale of the debt
securities, we will execute an underwriting agreement with such underwriter or
underwriters at the time an agreement for such sale is reached. The names of the
specific managing underwriter or underwriters, as well as any other
underwriters, and the terms of the transactions, including compensation of the
underwriters and dealers, which may be in the form of discounts, concessions or
commissions, if any, will be set forth in the supplement, which will be used by
the underwriters to make resales of the debt securities.
 
     If a dealer is utilized in the sale of the debt securities, we or an
underwriter will sell such debt securities to the dealer, as principal. The
dealer may then resell such debt securities to the public at varying prices to
be determined by such dealer at the time of resale. The name of the dealer and
the terms of the transactions will be set forth in the supplement relating
thereto.
 
     Offers to purchase the debt securities may be solicited directly by us and
sales thereof may be made by us directly to institutional investors or others.
The terms of any such sales, including the terms of any bidding or auction
process, if utilized, will be described in the supplement relating thereto.
 
     We may enter into agreements with agents, underwriters and dealers under
which we may agree to indemnify then against certain liabilities, including
liabilities under the Securities Act, or to contribution by us to payments they
may be required to make in respect thereof. The terms and conditions of such
indemnification or contribution will be described in the applicable supplement.
Certain of the agents, underwriters or dealers, or their affiliates may be
customers of, engage in transactions with or perform services for us in the
ordinary course of business.
 
                                 LEGAL MATTERS
 
     Gibson, Dunn & Crutcher LLP of Denver, Colorado will issue an opinion to us
relating to the legality of the debt securities. If legal matters in connection
with offerings made by this prospectus are passed on by counsel for the
underwriters of an offering of the debt securities, that counsel will be named
in the prospectus supplement relating to that offering.
 
                                    EXPERTS
 
     The financial statements incorporated in this prospectus by reference to
the Annual Report on Form 10-K for the year ended December 31, 1998, have been
so incorporated in reliance on the report of PricewaterhouseCoopers LLP,
independent accountants, given on the authority of said firm as experts in
auditing and accounting.
 
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<PAGE>   47
 

================================================================================
 
                                  $300,000,000
 
                                NRG ENERGY, INC.
                          7 1/2% SENIOR NOTES DUE 2009
 
                                  [NRG LOGO]
 
                               ------------------
 
                             PROSPECTUS SUPPLEMENT
 
                                  MAY 20, 1999
 
                               ------------------
 
                              SALOMON SMITH BARNEY

                             ABN AMRO INCORPORATED

                         BANC OF AMERICA SECURITIES LLC

                              MERRILL LYNCH & CO.
================================================================================



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