EXHIBIT 5.1

                                                               December 21, 2005

NRG Energy, Inc.
211 Carnegie Center
Princeton, NJ 08540

            Re: NRG Energy, Inc.
                Registration Statement on Form S-3

Ladies and Gentlemen:

      We have acted as special counsel to NRG Energy, Inc., a Delaware
corporation (the "Company"), in connection with the Registration Statement on
Form S-3 (the "Registration Statement"), to be filed on the date hereof by the
Company with the Securities and Exchange Commission (the "Commission") under the
Securities Act of 1933, as amended (the "Act"). The Registration Statement
relates to the issuance and sale from time to time by the Company, pursuant to
Rule 415 of the General Rules and Regulations promulgated under the Act, of the
following securities of the Company: (i) senior or subordinated debt securities,
which may be secured or unsecured, in one or more series (the "Debt
Securities"), to be issued under the Indenture (the "Indenture") proposed to be
entered into between the Company and Law Debenture Trust Company of New York, as
trustee (the "Trustee"); (ii) shares of preferred stock, $0.01 par value per
share, of the Company (the "Preferred Stock"), in one or more series; (iii)
shares of common stock, $0.01 par value per share, of the Company ("Common
Stock"); and (iv) such indeterminate amount of Debt Securities and number of
shares of Common Stock or Preferred Stock as may be issued upon conversion,
exchange or exercise of any Debt Securities or Preferred Stock, including such
shares of Common Stock or Preferred Stock as may be issued pursuant to
anti-dilution adjustments, in amounts, at prices and on terms to be determined
at the time of offering. The Debt Securities, the Preferred Stock and the Common
Stock are collectively referred to herein as the "Offered Securities."

      This opinion is being delivered in accordance with the requirements of
Item 601(b)(5) of Regulation S-K under the Act.

      In connection with this opinion, we have examined originals or copies,
certified or otherwise identified to our satisfaction, of:

      (i)   the Registration Statement relating to the Offered Securities;



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      (ii)  the Amended and Restated Certificate of Incorporation of the
            Company, as amended to the date hereof and as certified by the
            Secretary of State of the State of Delaware (the "Certificate of
            Incorporation");

      (iii) the Amended and Restated Bylaws of the Company, as currently in
            effect and as certified by the Secretary of the Company (the
            "Bylaws");

      (iv)  the form of the Indenture, filed as an exhibit to the Registration
            Statement;

      (v)   [drafts of certain resolutions (the "Resolutions") to be adopted on
            by the Board of Directors of the Company (the "Board of Directors")
            relating to the registration of the Offered Securities and related
            matters;] and

      (vi)  a specimen certificate evidencing the Common Stock.

      We have also examined originals or copies, certified or otherwise
identified to our satisfaction, of such records of the Company and such
agreements, certificates of public officials, certificates of officers or other
representatives of the Company and others, and such other documents as we have
deemed necessary or appropriate as a basis for the opinions set forth below.

      In our examination, we have assumed the legal capacity of all natural
persons, the genuineness of all signatures, the authenticity of all documents
submitted to us as originals, the conformity to original documents of all
documents submitted to us as facsimile, electronic, certified or photostatic
copies, and the authenticity of the originals of such copies. As to any facts
material to the opinions expressed herein that we did not independently
establish or verify, we have relied upon statements and representations of
officers and other representatives of the Company and others and of public
officials. In making our examination of executed documents or documents to be
executed, we have assumed that the parties thereto, other than the Company, had
or will have the power, corporate or other, to enter into and perform all
obligations thereunder and have also assumed the due authorization by all
requisite action, corporate or other, and the execution and delivery by such
parties of such documents, and, as to parties other than the Company, the
validity and binding effect thereof on such parties. We have assumed that the
Indenture will be duly authorized, executed and delivered by the parties thereto
in substantially the form reviewed by us, and that any Debt Securities that may
be issued will be issued in a form that complies with the Indenture and will be
manually signed or countersigned, as the case may be, by duly authorized
officers of the Trustee. In addition, we have also assumed that the terms of the
Offered Securities will have been established so as not to, and that the
execution and delivery by the Company of, and the performance of its obligations
under, the Indenture and the Offered Securities, will not, violate, conflict
with or constitute a default under (i) any agreement or instrument to which the
Company is



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December 21, 2005
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subject, (ii) any law, rule or regulation to which the Company is subject, (iii)
any judicial or regulatory order or decree of any governmental authority or (iv)
any consent, approval, license, authorization or validation of, or filing,
recording or registration with any governmental authority. We have also assumed
that (i) the stock certificates evidencing the Offered Preferred Stock (as
defined below) to be issued will be in a form that complies with, and the terms
of such Preferred Stock will be duly established in accordance with, the
Delaware General Corporation Law (the "DGCL"), and (ii) the stock certificate
evidencing any Offered Common Stock (as defined below) issued will conform to
the specimen certificate examined by us and will be duly executed and delivered.

      Our opinions set forth below are limited to the DGCL and those laws of the
State of New York that, in our experience, are normally applicable to
transactions of the type contemplated by the Registration Statement and to the
extent that judicial or regulatory orders or decrees or consents, approvals,
licenses, authorizations, validations, filings, recordings or registrations with
governmental authorities are relevant, to those required under such laws (all of
the foregoing being referred to as "Opined on Law"). We do not express any
opinion with respect to the law of any jurisdiction other than Opined on Law or
as to the effect of any such non-Opined on Law on the opinions herein stated.
The Offered Securities may be issued from time to time on a delayed or
continuous basis, and this opinion is limited to the laws, including the rules
and regulations, as in effect on the date hereof, which laws are subject to
change with possible retroactive effect.

      Based upon and subject to the foregoing and to the other qualifications
and limitations set forth herein, we are of the opinion that:

1.    With respect to any series of Debt Securities (the "Offered Debt
      Securities"), when (i) the Registration Statement, as finally amended
      (including all necessary post-effective amendments), has become effective
      under the Act and the Indenture has been qualified under the Trust
      Indenture Act of 1939, as amended; [(ii) the Resolutions have been adopted
      by the Board of Directors,] (iii) an appropriate prospectus supplement or
      term sheet with respect to the Offered Debt Securities has been prepared,
      delivered and filed in compliance with the Act and the applicable rules
      and regulations thereunder; (iv) if the Offered Debt Securities are to be
      sold pursuant to a firm commitment underwritten offering, the underwriting
      agreement with respect to the Offered Debt Securities has been duly
      authorized, executed and delivered by the Company and the other parties
      thereto; (v) the Board of Directors, including any appropriate committee
      appointed thereby, and appropriate officers of the Company have taken all
      necessary corporate action to approve the issuance and terms of the
      Offered Debt Securities and related matters; (vi) the Indenture and a
      supplemental indenture in respect of such Debt Securities has been duly
      executed and delivered by each party thereto; (vii) the terms of the
      Offered Debt Securities and of their issuance and sale have been duly
      established in conformity with the Indenture and any supplemental



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      indenture to be entered into in connection with the issuance of such Debt
      Securities so as not to violate any applicable law, the Certificate of
      Incorporation or the Bylaws or result in a default under or breach of any
      agreement or instrument binding upon the Company and so as to comply with
      any requirement or restriction imposed by any court or governmental body
      having jurisdiction over the Company; and (viii) the Offered Debt
      Securities have been issued in a form that complies with the Indenture and
      have been duly executed and authenticated in accordance with the
      provisions of the Indenture and any supplemental indenture to be entered
      into in connection with the issuance of such Debt Securities and duly
      delivered to the purchasers thereof upon payment of the agreed-upon
      consideration therefor, the Offered Debt Securities (including any Debt
      Securities duly issued upon conversion, exchange or exercise of any Debt
      Securities or Preferred Stock), when issued and sold in accordance with
      the Indenture, any supplemental indenture to be entered into in connection
      with the issuance of such Debt Securities and the applicable underwriting
      agreement, if any, or any other duly authorized, executed and delivered
      valid and binding purchase or agency agreement, will be valid and binding
      obligations of the Company, enforceable against the Company in accordance
      with their respective terms, except to the extent that enforcement thereof
      may be limited by (a) bankruptcy, insolvency, reorganization, fraudulent
      conveyance, moratorium or other similar laws now or hereafter in effect
      relating to creditors' rights generally, (b) general principles of equity
      (regardless of whether enforceability is considered in a proceeding at law
      or in equity), (c) public policy considerations which may limit the rights
      of parties to obtain remedies, (d) the waivers of any usury defense
      contained in the Indenture or Offered Debt Securities that may be
      unenforceable, (e) requirements that a claim with respect to any Offered
      Debt Securities denominated in a currency, currency unit or composite
      currency other than United States dollars (or a judgment denominated other
      than in United States dollars in respect of such claim) be converted into
      United States dollars at a rate of exchange prevailing on a date
      determined pursuant to applicable law, and (f) governmental authority to
      limit, delay or prohibit the making of payments outside the United States
      or in foreign currencies, currency units or composite currencies.

2.    With respect to the shares of any series of Preferred Stock (the "Offered
      Preferred Stock"), when (i) the Registration Statement, as finally amended
      (including all necessary post-effective amendments), has become effective
      under the Act; [(ii) the Resolutions have been adopted by the Board of
      Directors,] (iii) an appropriate prospectus supplement or term sheet with
      respect to the shares of the Offered Preferred Stock has been prepared,
      delivered and filed in compliance with the Act and the applicable rules
      and regulations thereunder; (iv) if the Offered Preferred Stock is to be
      sold pursuant to a firm commitment underwritten offering, the underwriting
      agreement with respect to the shares of the Offered Preferred Stock has
      been duly authorized, executed and delivered by the Company and the other



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December 21, 2005
Page 5

      parties thereto; (v) the Board of Directors, including any appropriate
      committee appointed thereby, and appropriate officers of the Company have
      taken all necessary corporate action to approve the issuance and terms of
      the shares of the Offered Preferred Stock, the consideration to be
      received therefor and related matters, including the adoption of a
      Certificate of Designations to the Certificate of Incorporation for the
      Offered Preferred Stock (the "Certificate of Designation") in accordance
      with the applicable provisions of the DGCL; (vi) the filing of the
      Certificate of Designation with the Secretary of State of the State of
      Delaware has duly occurred; (vii) the terms of the Offered Preferred Stock
      and of their issuance and sale have been duly established in conformity
      with the Certificate of Incorporation, including the Certificate of
      Designation relating to the Offered Preferred Stock, and the Bylaws of the
      Company so as not to violate any applicable law, the Certificate of
      Incorporation or the Bylaws or result in a default under or breach of any
      agreement or instrument binding upon the Company and so as to comply with
      any requirement or restriction imposed by any court or governmental body
      having jurisdiction over the Company; and (viii) certificates in the form
      required under the DGCL representing the shares of the Offered Preferred
      Stock are duly executed, countersigned, registered and delivered upon
      payment of the agreed-upon consideration therefor, the shares of the
      Offered Preferred Stock (including any Preferred Stock duly issued upon
      conversion, exchange or exercise of any Debt Securities or Preferred
      Stock), when issued or sold in accordance with the applicable underwriting
      agreement or any other duly authorized, executed and delivered valid and
      binding purchase or agency agreement, will be duly authorized, validly
      issued, fully paid and nonassessable, provided that the consideration
      therefor is not less than $0.01 per share of Preferred Stock.

3.    With respect to any offering of Common Stock by the Company (the "Offered
      Common Stock"), when (i) the Registration Statement, as finally amended
      (including all necessary post-effective amendments), has become effective
      under the Act; [(ii) the Resolutions have been adopted by the Board of
      Directors,] (iii) an appropriate prospectus supplement or term sheet with
      respect to the Offered Common Stock has been prepared, delivered and filed
      in compliance with the Act and the applicable rules and regulations
      thereunder; (iv) if the Offered Common Stock is to be sold pursuant to a
      firm commitment underwritten offering, the underwriting agreement with
      respect to the Offered Common Stock has been duly authorized, executed and
      delivered by the Company and the other parties thereto; (v) the Board of
      Directors, including any appropriate committee appointed thereby, and
      appropriate officers of the Company have taken all necessary corporate
      action to approve the issuance of the Offered Common Stock, the
      consideration to be received therefor and related matters; (vi) the terms
      of the issuance and sale of the Offered Common Stock have been duly
      established in conformity with the Certificate of Incorporation and the
      Bylaws so as not to



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      violate any applicable law, the Certificate of Incorporation or the Bylaws
      or result in a default under or breach of any agreement or instrument
      binding upon the Company and so as to comply with any requirement or
      restriction imposed by any court or governmental body having jurisdiction
      over the Company; and (vii) certificates in the form required under the
      DGCL representing the shares of Offered Common Stock are duly executed,
      countersigned, registered and delivered upon payment of the agreed upon
      consideration therefor, the shares of Offered Common Stock (including any
      Common Stock duly issued upon conversion, exchange or exercise of any Debt
      Securities or Preferred Stock), when issued and sold in accordance with
      the applicable underwriting agreement with respect to the Offered Common
      Stock or any other duly authorized, executed and delivered valid and
      binding purchase or agency agreement, will be duly authorized, validly
      issued, fully paid and nonassessable, provided that the consideration
      therefor is not less than $0.01 per share of Common Stock.

      We hereby consent to the filing of this opinion with the Commission as an
exhibit to the Registration Statement. We also hereby consent to the use of our
name under the heading "Legal Matters" in the prospectus which forms a part of
the Registration Statement. In giving this consent, we do not thereby admit that
we are within the category of persons whose consent is required under Section 7
of the Act or the rules and regulations of the Commission promulgated
thereunder. This opinion is expressed as of the date hereof unless otherwise
expressly stated, and we disclaim any undertaking to advise you of any
subsequent changes in the facts stated or assumed herein or of any subsequent
changes in applicable laws.

                                    Very truly yours,

                                    /s/ Skadden, Arps, Slate, Meagher & Flom LLP