UNITED STATES

SECURITIES AND EXCHANGE
COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

Item 1.

Security and Issuer

This statement relates to the common stock, $0.01 par value per share (“Issuer Common Stock”), of NRG Energy, Inc., a Delaware corporation (the “Issuer”).  The principal executive offices of the Issuer are located at 211 Carnegie Center, Princeton, NJ 08540.

Item 2.

Identity and Background

This statement is being filed jointly by KKR Millennium Fund (Energy) L.P., a Delaware limited partnership (“Millennium Energy”), KKR Associates Millennium (Energy) L.P., a Delaware limited partnership (“KKR Energy Associates”), KKR Millennium GP (Energy) LLC, a Delaware limited liability company (“KKR GP LLC”) and KKR Partners III, L.P. (Series I), a Delaware limited partnership (“KKR Partners” and together with Millennium Energy, KKR Energy Associates and KKR GP LLC, the “Reporting Persons”).

KKR Energy Associates is the sole general partner of Millennium Energy and KKR GP LLC is the sole general partner of KKR Energy Associates.  KKR GP LLC is a general partner of KKR Partners.  Each of Millennium Energy and KKR Partners is principally engaged in the business of investing in other companies.  Each of KKR Energy Associates and KKR GP LLC is principally engaged in the business of investing in other companies through partnerships and limited liability companies.

The address of the principal office of each of the Reporting Persons is 9 West 57th Street, New York, New York 10019.

Messrs. Henry R. Kravis and George R. Roberts are the managing members of KKR GP LLC. The other members of KKR GP LLC are Todd A. Fisher, Jacques Garaïalde, Perry Golkin, James H. Greene, Jr., Johannes P. Huth, Marc S. Lipschultz, Michael W. Michelson, Alexander Navab and Paul E. Raether.  Messrs. Kravis, Roberts, Fisher, Golkin, Greene, Lipschultz, Michelson, Navab and Raether are each United States citizens.  Mr. Garaïalde is a French citizen.  Mr. Huth is a German citizen.  The present principal occupation or employment of each is as a managing member or member of KKR & Co. L.L.C., a Delaware limited liability company which is the general partner of Kohlberg Kravis Roberts & Co. L.P., a private investment firm, the addresses of which are 9 West 57th Street, New York, New York 10019, 2800 Sand Hill Road, Suite 200, Menlo Park, California 94025, Stirling Square, 7 Carlton Garden, London, SW1Y 5AD, England, 24 rue Jean Goujon, 75008 Paris, and 25/F AIG Tower, 1 Connaught Road, Central, Hong Kong.  The business address of Messrs. Kravis, Golkin, Lipschultz, Navab and Raether is 9 West 57th Street, New York, New York 10019; the business address of Messrs. Roberts, Greene and Michelson is 2800 Sand Hill Road, Suite 200, Menlo Park, California 94025; the business address of Messrs. Fisher, Garaïalde and Huth is Stirling Square, 7 Carlton Garden, London, SW1Y 5AD, England.

During the last five years, none of the Reporting Persons nor, to the best knowledge of the Reporting Persons, any of the other persons named in this Item 2: (i) has been convicted in a criminal proceeding (excluding traffic violations or similar misdemeanors); or (ii) was a party to a civil proceeding of a judicial or administrative body of competent jurisdiction and as a result of such proceeding was or is subject to a judgment, decree or final order enjoining future violations of, or prohibiting or mandating activities subject to, federal or state securities laws or finding any violation with respect to such laws.

Item 3.

Source and Amount of Funds or Other Consideration

As more fully described in Item 4 hereof, on September 30, 2005, Millennium Energy and KKR Partners entered into an Acquisition Agreement (the “Acquisition Agreement”) with the Issuer, Texas Genco LLC, a Delaware limited liability company (“Texas Genco”) and the other direct and indirect owners of Texas Genco party thereto (together with Millennium Energy and KKR Partners, the “Sellers”).  Pursuant to the Acquisition Agreement, on February 2, 2006, Millennium Energy and KKR Partners sold all their ownership interests in Texas Genco (the “Units”) to the Issuer for consideration consisting of cash and shares of Issuer Common Stock.  The transactions contemplated by the Acquisition Agreement did not require the expenditure of any funds by the Reporting Persons to acquire Issuer Common Stock.

Item 4.

Purpose of Transaction

The Acquisition.  On February 2, 2006, pursuant to the Acquisition Agreement, the Issuer acquired all of the outstanding Units (the “Acquisition”), and Texas Genco became a wholly owned subsidiary of the Issuer.  In addition to cash consideration paid to the Sellers and Texas Genco optionholders in connection with the Acquisition, the Issuer issued shares of Issuer Common Stock, including a total of 8,631,454 shares of Issuer Common Stock issued to Millennium Energy and KKR Partners.

Investor Rights Agreement.  Pursuant to the Acquisition Agreement, on February 2, 2006, the Issuer entered into an Investor Rights Agreement with the Sellers, including Millennium Energy and KKR Partners, and former Texas Genco optionholders who received Issuer Common Stock in the Acquisition (together with the Sellers, the “Stockholders”).  The Investor Rights Agreement prohibits the Stockholders from selling any of the Issuer Common Stock received in the Acquisition for a period of 180 days after consummation of the Acquisition (the “Lock-up Period”), other than with the Issuer’s consent and in connection with certain permitted transfers to affiliates.  The agreement also prohibits the Stockholders from entering into any transaction designed to reduce any Stockholder’s risk relative to such Stockholder’s position as a holder of Issuer Common Stock during the Lock-up Period.  Pursuant to the Investor Rights Agreement, the Issuer has agreed to file a shelf registration statement covering resales of Issuer Common Stock issued to the Stockholders in the Acquisition, subject to certain limitations (the “Resale Shelf”) and to cause the Resale Shelf to become effective no later than the expiration of the Lock-up Period.  The Investor Rights Agreement also prohibits the Stockholders from acquiring additional voting securities of the Issuer without the Issuer’s consent, or otherwise attempting to acquire control of the Issuer for a period of two years after the consummation of the Acquisition.

The foregoing summaries of the Acquisition Agreement and Investor Rights Agreement do not purport to be complete and are qualified in their entirety by reference to the complete text of such agreements attached hereto as Exhibit 2 and Exhibit 3, respectively.

The Reporting Persons intend to seek to dispose of their shares of Issuer Common Stock (including, without limitation, distributing some or all of such shares to such Reporting Person’s members, partners, stockholders or beneficiaries, as applicable) from time to time, subject to market conditions, the existence (and possible waiver of) the Lock-up Period and other investment considerations.

In addition to the foregoing, each Reporting Person, subject to the limitations imposed by the Investor Rights Agreement, at any time and from time to time may directly or indirectly acquire additional shares of Issuer Common Stock or its associated rights or securities exercisable for or convertible into Issuer Common Stock, depending upon an ongoing evaluation of its investment in Issuer Common Stock and securities exercisable for or convertible into Issuer Common Stock, applicable legal restrictions, prevailing market conditions, liquidity requirements of such Reporting Person and/or other investment considerations.

Other than as described above, each of the Reporting Persons reports that neither it, nor to its knowledge any of the other persons named in Item 2 of this Schedule 13D, currently has any plan or proposal that relates to, or may result in, any of the matters listed in Items 4(a)-(j) of Schedule 13D, although the Reporting Persons may, at any time and from time to time, review or reconsider their position and/or change their purpose and/or formulate plans or proposals with respect thereto.

Item 5.

Interest in Securities of the Issuer

(a) and (b)  The information contained on the cover pages to this Schedule 13D is incorporated herein by reference.

KKR Energy Associates, as the sole general partner of Millennium Energy, has the power to direct the voting and disposition of any shares of Issuer Common Stock owned by Millennium Energy.  KKR GP LLC, as a general partner of KKR Partners, has power to direct the voting and disposition of any shares of Issuer Common Stock owned by KKR Partners.  As a result, KKR Energy Associates and KKR GP LLC may be deemed to beneficially own any shares of Issuer Common Stock owned by Millennium Energy and KKR Partners, respectively.

KKR GP LLC, as the sole general partner of KKR Energy Associates, has the power to direct the voting and disposition of any shares of Issuer Common Stock deemed to be beneficially owned by KKR Energy Associates.  As a result, KKR GP LLC may be deemed to beneficially own any shares of Issuer Common Stock deemed to be beneficially owned by KKR Energy Associates.

As a member of KKR GP LLC, each of Messrs. Kravis, Roberts, Fisher, Garaïalde, Golkin, Greene, Huth, Lipschultz, Michelson, Navab and Raether may be deemed to beneficially own any shares of Issuer Common Stock that KKR GP LLC may beneficially own or be deemed to beneficially own.

Neither the filing of this Schedule 13D nor any of its contents shall be deemed to constitute an admission that any Reporting Person or any other person described in Item 2 hereof is the beneficial owner of the Issuer Common Stock referred to herein for purposes of Section 13(d) of the Exchange Act or for any other purpose, and such beneficial ownership is expressly disclaimed.

(c)  Except as set forth in this Item 5, to the best knowledge of each of the Reporting Persons, none of the Reporting Persons and no other person described in Item 2 hereof has beneficial ownership of, or has engaged in any transaction during the past 60 days in, any shares of Issuer Common Stock.

(d)  Other than the Reporting Persons, no other person has the right to receive or the power to direct the receipt of dividends from, or the proceeds from sale of, the shares of Issuer Common Stock referred to in this Item 5.

(e)  Not applicable.

Item 6.

Contracts, Arrangements, Understandings or Relationships with Respect to Securities of the Issuer

Except as set forth in Item 4 of this Schedule 13D, to the best knowledge of the Reporting Persons, there are no other contracts, arrangements, understandings or relationships (legal or otherwise) among the persons named in Item 2 and between such persons and any other person with respect to any securities of the Issuer, including but not limited to, transfer or voting of any of the securities of the Issuer, finder’s fees, joint ventures, loan or option arrangements, puts or calls, guarantees of profits, division of profits or loss, or the giving or withholding of proxies, or a pledge or contingency the occurrence of which would give another person voting power over the securities of the Issuer.

Item 7.

Material to Be Filed as Exhibits

1.  Joint Filing Agreement, dated February 10, 2006, among KKR Millennium Fund (Energy) L.P., KKR Associates Millennium (Energy) L.P., KKR Millennium GP (Energy) LLC, and KKR Partners III, L.P. (Series I) relating to the filing of a joint statement on Schedule 13D (filed herewith).

2.  Acquisition Agreement, dated as of September 30, 2005, among Texas Genco LLC, NRG Energy, Inc. and the direct and indirect owners of Texas Genco LLC party thereto (incorporated by reference to Exhibit 2.1 to NRG Energy, Inc.’s Form 8-K filed October 3, 2005).

3.  Investor Rights Agreement, dated as of February 2, 2006, by and among NRG Energy, Inc. and certain stockholders of NRG Energy, Inc. set forth on Annex A thereto (incorporated by reference to Exhibit 10.01 to NRG Energy, Inc.’s Form 8-K filed February 7, 2006).

KKR MILLENNIUM FUND (ENERGY) L.P.

By:

KKR Associates Millennium (Energy) L.P., its General Partner

By:

KKR Millennium GP (Energy) LLC, its General Partner

By:

       /s/ Marc S. Lipschultz

Name:

Marc S. Lipschultz

Title:

Member

KKR PARTNERS III, L.P. (SERIES I)

By:

KKR Millennium GP (Energy) LLC, a General Partner

By:

       /s/ Marc S. Lipschultz

Name:

Marc S. Lipschultz

Title:

Member

KKR ASSOCIATES MILLENNIUM (ENERGY) L.P.

By:

KKR Millennium GP (Energy) LLC, its General Partner

By:

       /s/ Marc S. Lipschultz

Name:

Marc S. Lipschultz

Title:

Member

KKR MILLENNIUM GP (ENERGY) LLC

By:

       /s/ Marc S. Lipschultz

Name:

Marc S. Lipschultz

Title:

Member

Dated: February 10, 2006

Exhibit Number

Description of Exhibits

1.

Joint Filing Agreement, dated February 10, 2006, among KKR Millennium Fund (Energy) L.P., KKR Associates Millennium (Energy) L.P., KKR Millennium GP (Energy) LLC, and KKR Partners III, L.P. (Series I) relating to the filing of a joint statement on Schedule 13D (filed herewith).

2.

Acquisition Agreement, dated as of September 30, 2005, among Texas Genco LLC, NRG Energy, Inc. and the direct and indirect owners of Texas Genco LLC party thereto (incorporated by reference to Exhibit 2.1 to NRG Energy, Inc.’s Form 8-K filed October 3, 2005).

3.

Investor Rights Agreement, dated as of February 2, 2006, by and among NRG Energy, Inc. and certain stockholders of NRG Energy, Inc. set forth on Annex A thereto (incorporated by reference to Exhibit 10.01 to NRG Energy, Inc.’s Form 8-K filed February 7, 2006).