Document:

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                                OPTION AGREEMENT

         This OPTION AGREEMENT (this "Agreement") is dated as of May   , 2000,
by and between NORTHERN STATES POWER COMPANY, a Minnesota corporation ("NSP"),
and NRG ENERGY, INC., a Delaware corporation ("NRG").

         A. NSP beneficially owns [147,604,500] shares of Class A Common Stock,
par value $0.01 per share ("Class A Common Stock"), of NRG, which is convertible
into shares of common stock, par value $0.01 per share ("Common Stock") of NRG.

         B. On the date hereof, NRG will issue shares of Common Stock in an
initial public offering (the "Initial Public Offering") registered under the
Securities Act of 1933, as amended (the "Securities Act").

         NOW, THEREFORE, for good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, NSP and NRG, for themselves, their
successors, and assigns, hereby agree as follows:

                                    ARTICLE I

                                   DEFINITIONS

         1.1 Definitions. As used in this Agreement, the following terms shall
have the following meanings, applicable both to the singular and the plural
forms of the terms described:

         "Affiliate" means, with respect to a given Person, any Person
controlling, controlled by or under common control with such Person. For
purposes of this definition, "control" (including, with correlative meanings,
the terms "controlled by" and "under common control with"), as applied to any
Person, means the possession, directly or indirectly, of the power to vote
securities having a majority of the voting power for the election of directors
(or other Persons acting in similar capacities) of such Person or otherwise to
direct or cause the direction of the management and policies of such Person,
whether through the ownership of voting securities or by contract or otherwise.

         "Agreement" has the meaning ascribed thereto in the preamble hereto, as
such agreement may be amended and supplemented from time to time in accordance
with its terms.

         "Applicable Stock" means at any time the (i) shares of Common Stock
Equivalents owned by the NSP Entities owned on the date hereof, plus (ii) shares
of Common Stock Equivalents purchased by the NSP Entities pursuant to Article II
of this Agreement or otherwise, plus (iii) shares of Common Stock Equivalents
that were issued to NSP Entities in respect of shares described in either clause
(i) or clause (ii) in any reclassification, share combination, share
subdivision, share dividend, share exchange, merger, consolidation or similar
transaction or event.

         "Common Stock Equivalents" means the Common Stock, the Class A Common
Stock, any other class of NRG's capital stock representing the right to vote
generally for the election of
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directors and, if NRG is a subsidiary corporation includable in a consolidated
federal income tax return of NSP, any other security of NRG treated as stock for
purposes of Section 1504 of the Internal Revenue Code of 1986, as amended.

         "Follow On Offering" means any firm underwritten offering by the
Company of Common Stock Equivalents registered under the Securities Act on Form
S-1, S-2 or S-3, but excluding offerings registered in connection the a merger
or reorganization, or offerings relating to employee benefit plans on Form S-8
or its successor.

         "Market Price" of any shares of Common Stock on any date means (i) in
the event NSP is exercising the Option as a result of an Option Notice delivered
by the Company pursuant to Section 2.2(a), the price to be paid by the person
acquiring shares as described in the Option Notice, (ii) in the event NSP is
exercising the Option as a result of an Option Notice delivered by the Company
pursuant to Section 2.2(b), (a) the average of the last sale price of such
shares for the fifteen trading days immediately preceding such date on The New
York Stock Exchange or, if such shares are not listed thereon, on the principal
national securities exchange or automated interdealer quotation system on which
such shares are traded or (b) if such sale prices are unavailable or such shares
are not so traded, the value of such shares on such date determined in
accordance with agreed-upon procedures reasonably satisfactory to NRG and NSP;
(iii) in the event NSP is exercising the Option as a result of an Option Notice
delivered by the Company pursuant to Section 2.2(c), the net price paid to the
Company by the underwriters in the Follow On Offering (excluding any
underwriting discount or commissions).

         "NRG Entities" means NRG and its Subsidiaries, and "NRG Entity" shall
mean any of the NRG Entities.

         "NSP Entities" means NSP and its Affiliates (other than Affiliates that
constitute NRG Entities), and "NSP Entity" shall mean any of the NSP Entities.

         "Ownership Percentage" means, at any time, the fraction, expressed as a
percentage and rounded to the next highest thousandth of a percent, whose
numerator is the aggregate number of shares of the Applicable Stock and whose
denominator is the aggregate number of outstanding shares of Common Stock
Equivalents of NRG; provided, however, that any shares of Common Stock
Equivalents issued by NRG in violation of its obligations under Article II of
this Agreement shall not be deemed outstanding for the purpose of determining
the Ownership Percentage.

         "Person" means any individual, partnership, limited liability company,
joint venture, corporation, trust, unincorporated organization, government (and
any department or agency thereof) or other entity.

         "Subsidiary" means, as to any Person, any corporation, association,
partnership, joint venture or other business entity of which more than 50% of
the voting capital stock or other voting ownership interests is owned or
controlled, directly or indirectly, by such Person or by one or more of the
Subsidiaries of such Person or by a combination thereof. "Subsidiary," when used
with respect to NSP or NRG, shall also include any other entity Affiliated with
NSP or

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NRG, as the case may be, that NSP and NRG may hereafter agree in writing shall
be treated as a "Subsidiary" for the purposes of this Agreement.

         1.2      Internal References. Unless the context indicates otherwise,
references to Articles, Sections and paragraphs shall refer to the corresponding
articles, sections and paragraphs in this Agreement and references to the
parties shall mean the parties to this Agreement.

                                   ARTICLE II

                                     OPTION

         2.1      Option. NRG hereby grants to NSP, on the terms and conditions
set forth herein, a continuing right (the "Option") to purchase from NRG, at the
times set forth herein, the number of shares of Common Stock provided in the
first sentence of Section 2.3. The Option shall be assignable, in whole or in
part and from time to time, by NSP to any NSP Entity. The exercise price for the
shares of Common Stock purchased pursuant to the Option shall be the Market
Price of the Common Stock as of the date of first delivery of an Exercise Notice
with respect to the Option by NSP (or its permitted assignee hereunder) to NRG.

         2.2      Notice.

                  (a) At least 20 business days prior to the date NRG issues
Common Stock Equivalents (other than (i) in connection with the Initial Public
Offering or any Follow On Offering, including the full exercise of all
underwriters' over-allotment options granted in connection therewith, (ii)
issuances of Common Stock Equivalents to any NSP Entity and (iii) pursuant to
any stock option issued under any executive or employee benefit or compensation
plan maintained by NRG (an "Employee Option")) that, in the absence of a full or
partial exercise of the Option, would result in any reduction in the Ownership
Percentage (other than a reduction that does not reduce the Ownership Percentage
to below eighty percent (80%)), NRG will notify NSP in writing (an "Option
Notice") of the date on which such issuance could first occur. Each Option
Notice must specify the date on which NRG intends to issue additional shares
(such issuance being referred to herein as an "Issuance Event" and the date of
such issuance as an "Issuance Event Date"), the number of shares NRG intends to
issue or may issue, and the other terms and conditions of such Issuance Event.

                  (b) Within 20 business days prior to the beginning of each
fiscal quarter, NRG will notify NSP in writing of the number of shares that
could be issued in such quarter pursuant to any Employee Option (the "Quarterly
Plan Notice"). Each Quarterly Plan Notice must specify the number of shares NRG
could be bound to issue, the exercise price(s) at which shares would be issued,
and any other material terms and conditions of such issuances. The "Issuance
Event" described in a Quarterly Plan Notice shall be deemed to include all
shares issuable in such quarter, and the first day of such quarter shall be
deemed the "Issuance Event Date" for all shares to be issued in such quarter.
NRG shall not permit the issuance of any Common Stock Equivalents pursuant to
any Employee Option in such quarter in excess of the number identified in the
Quarterly Plan Notice.

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             (c) At least 30 business days prior to the date NRG issues Common
Stock Equivalents pursuant to a Follow On Offering that, in the absence of a
full or partial exercise of the Option, would result in any reduction in the
Ownership Percentage to below eighty percent (80%), NRG will provide an Option
Notice to NSP, which notice shall include the date on which such issuance could
first occur and the anticipated price range per Common Stock Equivalent at which
NRG would consummate such Follow On Offering.

         2.3 Option Exercise and Payment. The Option may be exercised upon the
occurrence of each Issuance Event by NSP (or any NSP Entity to which all or any
part of the Option has been assigned) for a number of shares equal to or less
than the number of shares, if any, that are necessary for the NSP Entities to
maintain, in the aggregate, the Ownership Percentage prior to giving effect to
(a) the applicable Issuance Event and (b) any prior Issuance Event with respect
to which and to the extent NSP had not exercised this Option in full; provided,
however, that the exercise price with respect to shares subject to the Option as
a result of such prior Issuance Event for which notice was given pursuant to
Section 2.2(a) or 2.2(b) shall be determined as of the Issuance Date associated
with such prior Issuance Event. The Option may be exercised at any time after
receipt of an Option Notice and prior to the Issuance Event Date by the delivery
to NRG of a written notice to such effect (an "Exercise Notice") specifying (i)
the number of shares of Common Stock to be purchased by NSP, or any of the NSP
Entities (which may be expressed as a percentage of the shares identified in the
Option Notice) and (ii) a calculation of the exercise price for such shares.
Upon any such exercise of the Option, NRG will, prior to the applicable Issuance
Event Date, deliver to NSP (or any NSP Entity designated by NSP), against
payment therefor, certificates (issued in the name of NSP or its permitted
assignee hereunder or as directed by NSP) representing the shares of Common
Stock being purchased upon such exercise. Payment for such shares shall be made
by wire transfer or intrabank transfer of immediately-available funds to such
account as shall be specified by NRG, for the full purchase price for such
shares. In the event the Exercise Notice is being delivered with respect to a
Follow On Offering, the Exercise Notice shall be delivered within 10 business
days of receipt of the Option Notice, and shall constitute an irrevocable offer
to buy the shares of Common Stock included in the Exercise Notice at the Market
Price upon closing of the Follow On Offering. If NSP fails to give the Exercise
Notice within such 10 business days, NRG shall not be obligated to sell shares
to NSP at the closing of such Follow On Offering.

         2.4 Effect of Failure to Exercise. Except as provided in Section 2.3
and 2.5, any failure by NSP to exercise the Option, or any exercise for less
than all shares purchasable under the Option, in connection with any particular
Issuance Event shall not affect NSP's right to exercise the Option in connection
with any subsequent Issuance Event.

         2.5 Termination of Option. The Option shall terminate upon the
occurrence of any Issuance Event that, after considering NSP's response thereto
and to any other Issuance Events, results in the Ownership Percentage being less
than 30%, other than any Issuance Event in violation of this Agreement. The
Option, or any portion thereof assigned to any NSP Entity other than NSP, also
shall terminate in the event that the Person to whom the Option, or such portion
thereof has been transferred, ceases to be a NSP Entity for any reason
whatsoever. The Option shall also terminate at the election of NSP.

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<PAGE>   5

                                   ARTICLE III

                                  MISCELLANEOUS

         3.1 Limitation of Liability. Neither NSP nor NRG shall be liable to the
other for any special, indirect, incidental or consequential damages of the
other arising in connection with this Agreement.

         3.2 Affiliates. NSP agrees and acknowledges that it shall be
responsible for the performance by each NSP Entity of the obligations hereunder
applicable to such NSP Entity.

         3.3 Amendments. This Agreement may be amended only by a writing duly
executed by or on behalf of each of the parties hereto; provided that, any
amendments materially adverse to NRG shall be approved by a majority of the
directors of NRG who are not affiliated with NSP.

         3.4 Severability. If any provision of this Agreement or the application
of any such provision to any party or circumstances shall be determined by any
court of competent jurisdiction to be invalid, illegal or unenforceable to any
extent, the remainder of this Agreement or such provision of the application of
such provision to such party or circumstances, other than those to which it is
so determined to be invalid, illegal or unenforceable, shall remain in full
force and effect to the fullest extent permitted by law and shall not be
affected thereby, unless such a construction would be unreasonable.

         3.5 Notices. All notices and other communications required or permitted
hereunder shall be in writing, shall be deemed duly given upon actual receipt,
and shall be delivered (a) in person, (b) by registered or certified mail,
postage prepaid, return receipt requested or (c) by facsimile or other generally
accepted means of electronic transmission (provided that a copy of any notice
delivered pursuant to this clause (c) shall also be sent pursuant to clause
(b)), addressed as follows:

         (a)      if to NRG, to:

                           NRG Energy, Inc.
                           1221 Nicollet Mall, Suite 700
                           Minneapolis, MN  55403
                           Attn:  General Counsel

         (b)      If to NSP, to:

                           Northern States Power Co.
                           414 Nicollet Mall
                           Minneapolis, MN  55401
                           Attn:  General Counsel

or to such other addresses or telecopy numbers as may be specified by like
notice to the other parties.

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<PAGE>   6

         3.6  Further Assurances. NSP and NRG shall execute, acknowledge and
deliver, or cause to be executed, acknowledged and delivered, such instruments
and take such other action as may be necessary or advisable to carry out their
obligations under this Agreement and under any exhibit, document or other
instrument delivered pursuant hereto.

         3.7  Counterparts. This Agreement may be executed in any number of
counterparts, each of which shall be deemed an original instrument, but all of
which together shall constitute but one and the same agreement.

         3.8  Governing Law. This Agreement and the transactions contemplated
hereby shall be construed in accordance with, and governed by, the laws of the
State of Delaware.

         3.9  Entire Agreement.  This Agreement constitutes the entire
understanding of the parties hereto with respect to the subject matter hereof.

         3.10 Successors. This Agreement shall be binding upon, and shall inure
to the benefit of, the parties hereto and their respective successors and
assigns. Nothing contained in this Agreement, express or implied, is intended to
confer upon any other person or entity any benefits, rights or remedies.

         3.11 Specific Performance. The parties hereto acknowledge and agree
that irreparable damage would occur in the event that any of the provisions of
this Agreement were not performed in accordance with their specific terms or
were otherwise breached. Accordingly, it is agreed that they shall be entitled
to an injunction or injunctions to prevent breaches of the provisions of this
Agreement and to enforce specifically the terms and provisions hereof in any
court of competent jurisdiction in the United States or any state thereof, in
addition to any other remedy to which they may be entitled at law or equity.

         IN WITNESS WHEREOF, the parties hereto have executed this Agreement the
day and year first above written.

                                           NORTHERN STATES POWER COMPANY

                                           By:
                                               ---------------------------------
                                           Name:
                                           Title

                                           NRG ENERGY, INC.
                                           By:
                                               ---------------------------------
                                           Name:
                                           Title

                                       6<PAGE>   1
                                                                   EXHIBIT 10.40

                          REGISTRATION RIGHTS AGREEMENT

     This REGISTRATION RIGHTS AGREEMENT (this "Agreement") is made and
entered into as of May   , 2000 by and among NRG Energy, Inc., a Delaware
corporation (the "Company"), and Northern States Power Company, a Minnesota
corporation (the "Investor").

                                 R E C I T A L S

     A.   The Company intends to register certain shares of its common stock,
par value $     per share (the "Common Stock") on a Registration Statement on
Form S-1 with the Securities and Exchange Commission, to apply to list such
shares on the New York Stock Exchange, and to sell such shares to the public in
an initial public offering (the "Offering").

     B.   Upon the filing of the Company's Amended and Restated Certificate of
Incorporation (the "Certificate") with the Secretary of State of the State of
Delaware, the Investor will exchange all of the shares of Common Stock held by
it for shares of the Company's Class A Common Stock (the "Class A Common
Stock"). The Class A Common Stock is convertible into shares of the Company's
Common Stock as set forth in the Certificate.

     C.   To facilitate the Offering, the Investor has agreed to certain
restrictions on the transfer or sale of equity securities of the Company. In
exchange for such restrictions, the Board of Directors of the Company has
determined that it is fair and in the best interests of the Company that the
Company grant rights to the Investor to cause the Company to register shares of
Common Stock held by it, or issuable to it upon conversion of Class A Common
Stock, as set forth in this Agreement.

     NOW, THEREFORE, in consideration of the foregoing recitals, the mutual
promises hereinafter set forth, and other good and valuable consideration, the
receipt and sufficiency of which are hereby acknowledged, the parties hereto
agree as follows:

     1.   Definitions. For purposes of this Agreement:

          (a)  Registration. The terms "register," "registered," and
"registration" refer to a registration effected by preparing and filing a
registration statement in compliance with the Securities Act of 1933, as
amended, (the "Securities Act"), and the declaration or ordering of
effectiveness of such registration statement

          (b)  Registrable Securities. The term "Registrable Securities" means:
(1) any Common Stock of the Company issued or to be issued to the Investor
pursuant to conversion of any shares of Class A Common Stock held by Investor,
(2) any shares of Common Stock of the Company issued as (or issuable upon the
conversion or exercise of any warrant, right or other security which is issued
as) a dividend or other distribution with respect to, or in exchange for or in
replacement of, any shares of Class A Common Stock described in clause (1) of
this subsection (b) and (3) any other Common Stock of the Company owned or
hereafter acquired by the Investor. Notwithstanding the foregoing, "Registrable
Securities" shall exclude (i) any Registrable Securities sold by the Investor in
a transaction in which rights under this Agreement

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are not assigned in accordance with this Agreement or any Registrable Securities
sold in a public offering, whether sold pursuant to Rule 144 promulgated under
the Securities Act, in a registered offering, or otherwise.

          (c)  Registrable Securities then outstanding. The number of shares of
"Registrable Securities then outstanding" shall mean the number of shares of
Common Stock of the Company that are Registrable Securities and (l) are then
issued and outstanding or (2) are then issuable pursuant to the conversion of
Class A Common Stock.

          (d)  Holder. For purposes of this Agreement, the term "Holder" means
any person owning of record Registrable Securities, or any permitted assignee of
record of such Registrable Securities to whom rights under this Agreement have
been duly assigned in accordance with this Agreement.

          (e)  Form S-3. The term "Form S-3" means such form under the
Securities Act as is in effect on the date hereof or any successor registration
form under the Securities Act subsequently adopted by the SEC which permits
inclusion or incorporation of substantial information by reference to other
documents filed by the Company with the SEC.

          (f)  SEC. The term "SEC" or "Commission" means the U.S. Securities and
Exchange Commission.

     2.   Demand Registration.

          (a)  Request by Holders. If the Company shall at any time after 180
days of the date hereof receive a written request from the Investor that the
Company file a registration statement under the Securities Act covering the
registration of Registrable Securities pursuant to this Section 2, then the
Company shall, within ten (10) business days of the receipt of such written
request, give written notice of such request ("Request Notice") to all Holders,
and use its best efforts to effect, as soon as practicable, the registration
under the Securities Act of all Registrable Securities that Holders request to
be registered and included in such registration by written notice given such
Holders to the Company within twenty (20) days after receipt of the Request
Notice, subject only to the limitations of this Section 2; provided that the
anticipated aggregate offering price of such Registrable Securities, net of
underwriting discounts and commissions, shall be at least $20,000,000; and
provided further that the Company shall not be obligated to effect any such
registration if the Company has, within the six (6) month period preceding the
date of such request, already effected a registration under the Securities Act
pursuant to this Section 2 or Section 4, or in which the Holders had an
opportunity to participate pursuant to the provisions of Section 3, other than a
registration from which the Registrable Securities of Holders have been excluded
(with respect to all or any portion of the Registrable Securities the Holders
requested be included in such registration) pursuant to the provisions of
Section 3(a).

          (b)  Underwriting. If the Holders initiating the registration request
under this Section 2 ("Initiating Holders") intend to distribute the Registrable
Securities covered by their request by means of an underwriting, then they shall
so advise the Company as a part of their request made pursuant to this Section 2
and the Company shall include such information in the

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written notice referred to in subsection 2(a). In such event, the right of any
Holder to include his Registrable Securities in such registration shall be
conditioned upon such Holder's participation in such underwriting and the
inclusion of such Holder's Registrable Securities in the underwriting (unless
otherwise mutually agreed by a majority in interest of the initiating Holders
and such Holder) to the extent provided herein. All Holders proposing to
distribute their securities through such underwriting shall enter into an
underwriting agreement in customary form with the managing underwriter or
underwriters selected for such underwriting by the Holders of a majority of the
Registrable Securities being registered and reasonably acceptable to the Company
(including a market stand-off agreement of up to 180 days if required by such
underwriters). Notwithstanding any other provision of this Section 2, if the
underwriter(s) advise(s) the Company in writing that marketing factors require a
limitation of the number of securities to be underwritten then the Company shall
so advise all Holders of Registrable Securities which would otherwise be
registered and underwritten pursuant hereto, and the number of Registrable
Securities that may be included in the underwriting shall be reduced as required
by the underwriter(s) and allocated among the Holders of Registrable Securities
on a pro rata basis according to the number of Registrable Securities then
outstanding held by each Holder requesting registration (including the
initiating Holders); provided, however, that the number of shares of Registrable
Securities to be included in such underwriting and registration shall not be
reduced unless all other securities of the Company are first entirely excluded
from the underwriting and registration. Any Registrable Securities excluded and
withdrawn from such underwriting shall be withdrawn from the registration.

          (c)  Maximum Number of Demand Registrations. The Company shall be
obligated to effect only five (5) such registrations pursuant to this Section 2.

          (d)  Deferral. Notwithstanding the foregoing, if the Company shall
furnish to Holders requesting the filing of a registration statement
pursuant to this Section 2, a certificate signed by the Chief Executive Officer
of the Company stating that in the good faith judgment of an independent
committee of the Board, it would be materially detrimental to the Company and
its stockholders for such registration statement to be filed, then the Company
shall have the right to defer such filing for a period of not more than one
hundred eighty (180) days after receipt of the request of the initiating
Holders; provided, however, that the Company may not utilize this right more
than once in any twelve (12) month period.

          (e)  Expenses. All expenses incurred in connection with any
registration pursuant to this Section 2, including without limitation all
federal and "blue sky" registration, filing and qualification fees, printer's
and accounting fees, and fees and disbursements of counsel for the Company (but
excluding underwriters' discounts and commissions relating to shares sold by the
Holders and legal fees of counsel for the Holders), shall be borne by the
Company. Each Holder participating in a registration pursuant to this Section 2
shall bear such Holder's proportionate share (based on the total number of
shares sold in such registration other than for the account of the Company) of
all discounts, commissions or other amounts payable to underwriters or brokers,
and the Holders' legal fees, in connection with such offering by the Holders.
Notwithstanding the foregoing, the Company shall not be required to pay for any
expenses of any registration proceeding begun pursuant to this Section 2 if the
registration request is subsequently withdrawn at the request of the Holders of
a majority of the Registrable

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Securities to be registered, unless the Holders of a majority of the Registrable
Securities then outstanding agree that such registration constitutes the use by
the Holders of one (1) demand registration pursuant to this Section 2 (in which
case such registration shall also constitute the use by all Holders of
Registrable Securities of one (l) such demand registration); provided, further,
however, that if at the time of such withdrawal, the Holders have learned of a
material adverse change in the condition, business, or prospects of the Company
not known to the Holders at the time of their request for such registration and
have withdrawn their request for registration with reasonable promptness after
learning of such material adverse change, then the Holders shall not be required
to pay any of such expenses and such registration shall not constitute the use
of a demand registration pursuant to this Section 2.

     3.   Piggyback Registrations. The Company shall notify all Holders of
Registrable Securities in writing at least thirty (30) days prior to filing any
registration statement under the Securities Act for purposes of effecting a
public offering of securities of the Company (including, but not limited to,
registration statements relating to secondary offerings of securities of the
Company, but excluding registration statements relating to any registration
under Section 2 or Section 4 of this Agreement or to any employee benefit plan
or a corporate reorganization) and will afford each such Holder an opportunity
to include in such registration statement all or any part of the Registrable
Securities then held by such Holder. Each Holder desiring to include in any such
registration statement all or any part of the Registrable Securities held by
such Holder shall within twenty (20) days after receipt of the above-described
notice from the Company, so notify the Company in writing, and in such notice
shall inform the Company of the number of Registrable Securities such Holder
wishes to include in such registration statement. If a Holder decides not to
include all of its Registrable Securities in any registration statement
thereafter filed by the Company, such Holder shall nevertheless continue to have
the right to include any Registrable Securities in any subsequent registration
statement or registration statements as may be filed by the Company with respect
to offerings of its securities, all upon the terms and conditions set forth
herein.

          (a)  Underwriting. If a registration statement under which the
Company gives notice under this Section 3 is for an underwritten offering, then
the Company shall so advise the Holders of Registrable Securities. In such
event, the right of any such Holder's Registrable Securities to be included in a
registration pursuant to this Section 3 shall be conditioned upon such Holder's
participation in such underwriting and the inclusion of such Holder's
Registrable Securities in the underwriting to the extent provided herein. All
Holders proposing to distribute their Registrable Securities through such
underwriting shall enter into an underwriting agreement in customary form with
the managing underwriter or underwriters selected for such underwriting
(including a market stand-off agreement of up to 180 days if required by such
underwriters). Notwithstanding any other provision of this Agreement, if the
managing underwriter(s) determine(s) in good faith that marketing factors
require a limitation of the number of shares to be underwritten, then the
managing underwriter(s) may exclude shares (including up to seventy-five percent
(75%) of the Registrable Securities) from the registration and the underwriting,
and the number of shares that may be included in the registration and the
underwriting shall be allocated, first to the Company, and second, to each of
the Holders requesting inclusion of their Registrable Securities in such
registration statement on a pro rata basis based on the total number of
Registrable Securities then held by each such Holder; provided, however, that
the right of the

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underwriters to exclude shares (including Registrable Securities) from the
registration and underwriting as described above shall be restricted so that (i)
the number of Registrable Securities included in any such registration is not
reduced below twenty-five percent (25%) of the aggregate number of Registrable
Securities for which inclusion has been requested; and (ii) all shares that are
not Registrable Securities and are held by any other person, including, without
limitation, any person who is an employee, officer or director of the Company
(or any subsidiary of the Company) shall first be excluded from such
registration and underwriting before any Registrable Securities are so excluded.
If any Holder disapproves of the terms of any such underwriting, such Holder may
elect to withdraw therefrom by written notice to the Company and the
underwriter(s), delivered at least ten (10) business days prior to the effective
date of the registration statement. Any Registrable Securities excluded or
withdrawn from such underwriting shall be excluded and withdrawn from the
registration. For any Holder that is a partnership, the Holder and the partners
and retired partners of such Holder, or the estates and family members of any
such partners and retired partners and any trusts for the benefit of any of the
foregoing persons, and for any Holder that is a corporation, the Holder and all
corporations that are affiliates of such Holder, shall be deemed to be a single
"Holder," and any pro rata reduction with respect to such "Holder" shall be
based upon the aggregate amount of shares carrying registration rights owned by
all entities and individuals included in such "Holder," as defined in this
sentence.

          (b)  Expenses. All expenses incurred in connection with a
registration pursuant to this Section 3 (excluding underwriters' and brokers'
discounts and commissions relating to shares sold by the Holders and legal fees
of counsel for the Holders), including, without limitation all federal and "blue
sky" registration, filing and qualification fees, printers' and accounting fees,
and fees and disbursements of counsel for the Company, shall be borne by the
Company.

          (c)  Not Demand Registration. Registration pursuant to this Section 3
shall not be deemed to be a demand registration as described in Section 2 above.
Except as otherwise provided herein, there shall be no limit on the number of
times the Holders may request registration of Registrable Securities under this
Section 3.

     4.   Form S-3 Registration. In case the Company shall at any time after the
first anniversary of the date hereof receive from any Holder or Holders of a
majority of all Registrable Securities then outstanding a written request or
requests that the Company effect a registration on Form S-3 and any related
qualification or compliance with respect to all or a part of the Registrable
Securities owned by such Holder or Holders, then the Company will:

          (a)  Notice. Promptly give written notice of the proposed
registration and the Holder's or Holders' request therefor, and any related
qualification or compliance, to all other Holders of Registrable Securities; and

          (b)  Registration. As soon as practicable, effect such registration
and all such qualifications and compliances as may be so requested and as would
permit or facilitate the sale and distribution of all or such portion of such
Holders or Holders' Registrable Securities as are specified in such request,
together with all or such portion of the Registrable Securities of any other
Holder or Holders joining in such request as are specified in a written request
given within twenty (20) days after the Company provides the notice contemplated
by Section 4(a); provided,

                                       5
<PAGE>   6
however, that the Company shall not be obligated to effect any such
registration, qualification or compliance pursuant to this Section 4:

               (1)  if Form S-3 is not available for such offering by the
Holders:

               (2)  if the Holders, together with the holders of any other
securities of the Company entitled to inclusion in such registration, propose to
sell Registrable Securities and such other securities (if any) at an aggregate
price to the public of less than $10,000,000;

               (3)  if the Company shall furnish to the Holders a certificate
signed by the Chief Executive Officer of the Company stating that in the good
faith judgment of or independent committee the Board of Directors of the
Company, it would be materially detrimental to the Company and its shareholders
for such Form S-3 Registration to be effected at such time, in which event the
Company shall have the right to defer the filing of the Form S-3 registration
statement no more than once during any twelve month period for a period of not
more than one hundred eighty (180) days after receipt of the request of the
Holder or Holders under this Section 4;

               (4)  if the Company has, within the six (6) month period
preceding the date of such request, already effected a registration under the
Securities Act other than a registration from which the Registrable Securities
of Holders have been excluded (with respect to all or any portion of the
Registrable Securities the Holders requested be included in such registration)
pursuant to the provisions of Section 3(a); or

               (5)  in any particular jurisdiction in which the Company would be
required to qualify to do business or to execute a general consent to service of
process in effecting such registration, qualification or compliance.

          (c)  Expenses. The Company shall pay all expenses incurred in
connection with each registration requested pursuant to this Section 4,
(excluding underwriters' or brokers' discounts and commissions relating to
shares sold by the Holders and legal fees of counsel for the Holders), including
without limitation federal and "blue sky" registration, filing and qualification
fees, printers' and accounting fees, and fees and disbursements of counsel.

          (d)  Not Demand Registration. Form S-3 registrations shall not be
deemed to be demand registrations as described in Section 2 above. Except as
otherwise provided herein, there shall be no limit on the number of times the
Holders may request registration of Registrable Securities under this Section 4.

     5.   Obligations of the Company. Whenever required to effect the
registration of any Registrable Securities under this Agreement the Company
shall, as expeditiously as reasonably possible:

          (a)  Registration Statement. Prepare and file with the SEC a
registration statement with respect to such Registrable Securities and use its
best efforts to cause such registration statement to become effective, provided,
however, that the Company shall not be required to keep any such registration
statement effective for more than ninety (90) days.

                                      6
<PAGE>   7
          (b)  Amendments and Supplements. Prepare and file with the SEC such
amendments and supplements to such registration statement and the
prospectus used in connection with such registration statement as may be
necessary to comply with the provisions of the Securities Act with respect to
the disposition of all securities covered by such registration statement.

          (c)  Prospectuses. Furnish to the Holders such number of copies of a
prospectus, including a preliminary prospectus, in conformity with the
requirements of the Securities Act, and such other documents as they may
reasonably request in order to facilitate the disposition of the Registrable
Securities owned by them that are included in such registration.

          (d)  Blue Sky. Use its best efforts to register and qualify the
securities covered by such registration statement under such other securities or
Blue Sky laws of such jurisdictions as shall be reasonably requested by the
Holders, provided that the Company shall not be required in connection therewith
or as a condition thereto to qualify to do business or to file a general consent
to service of process in any such states or jurisdictions.

          (e)  Underwriting. In the event of any underwritten public offering,
enter into and perform its obligations under an underwriting agreement
in usual and customary form, with the managing underwriter(s) of such offering.
Each Holder participating in such underwriting shall also enter into and perform
its obligations under such an agreement.

          (f)  Notification. Notify each Holder of Registrable Securities
covered by such registration statement at any time when a prospectus relating
thereto is required to be delivered under the Securities Act of the happening of
any event as a result of which the prospectus included in such registration
statement, as then in effect, includes an untrue statement of a material fact or
omits to state a material fact required to be stated therein or necessary to
make the statements therein not misleading in the light of the circumstances
then existing.

          (g)  Opinion and Comfort Letter. Furnish, at the request of any
Holder requesting registration of Registrable Securities, on the date that such
Registrable Securities are delivered to the underwriters for sale, if such
securities are being sold through underwriters, or, if such securities are not
being sold through underwriters, on the date that the registration statement
with respect to such securities becomes effective, (i) an opinion, dated as of
such date, of the counsel representing the Company for the purposes of such
registration, in form and substance as is customarily given to underwriters in
an underwritten public offering and reasonably satisfactory to a majority in
interest of the Holders requesting registration, addressed to the underwriters,
if any, and to the Holders requesting registration of Registrable Securities and
(ii) a "comfort" letter dated as of such date, from the independent certified
public accountants of the Company, in form and substance as is customarily given
by independent certified public accountants to underwriters in an underwritten
public offering and reasonably satisfactory to a majority in interest of the
Holders requesting registration, addressed to the underwriters, if any, and to
the Holders requesting registration of Registrable Securities.

     6.   Furnish Information. It shall be a condition precedent to the
obligations of the Company to take any action pursuant to Sections 2, 3 or 4
that the selling Holders shall furnish to

                                       7
<PAGE>   8
the Company such information regarding themselves, the Registrable Securities
held by them, and the intended method of disposition of such securities as shall
be required to timely effect the Registration of their Registrable Securities.

     7.   Indemnification. In the event any Registrable Securities are included
in a registration statement under Sections 2, 3 or 4:

          (a)  By the Company. To the extent permitted by law; the Company will
indemnify and hold harmless each Holder, the partners, officers and directors of
each Holder, any underwriter (as determined in the Securities Act) for such
Holder and each person, if any, who controls such Holder or underwriter within
the meaning of the Securities Act or the Securities Exchange Act of 1934, as
amended, (the "1934 Act"), against any losses, claims, damages, or Liabilities
(joint or several) to which they may become subject under the Securities Act,
the 1934 Act or other federal or state law, insofar as such losses, claims,
damages, or liabilities (or actions in respect thereof) arise out of or are
based upon any of the following statements, omissions or violations
(collectively a "Violation"):

               (i)    any untrue statement or alleged untrue statement of a
          material fact contained in such registration statement, including any
          preliminary prospectus or final prospectus contained therein or any
          amendments or supplements thereto;

               (ii)   the omission or alleged omission to state therein a
          material fact required to be stated therein, or necessary to make the
          statements therein not misleading, or

               (iii)  any violation or alleged violation by the Company of the
          Securities Act, the 1934 Act, any federal or state securities law or
          any rule or regulation promulgated under the Securities Act, the 1934
          Act or any federal or state securities law in connection with the
          offering covered by such registration statement;

and the Company will reimburse each such Holder, partner, officer or director,
underwriter or controlling person for any legal or other expenses reasonably
incurred by them, as incurred, in connection with investigating or defending any
such loss, claim, damage, liability or action; provided, however, that the
indemnity agreement contained in this subsection 7(a) shall not apply to amounts
paid in settlement of any such loss, claim, damage, liability or action if such
settlement is effected without the consent of the Company (which consent shall
not be unreasonably withheld), nor shall the Company be liable in any such case
for any such loss, claim, damage, liability or action to the extent that it
arises out of or is based upon a Violation which occurs in reliance upon and in
conformity with written information furnished expressly for use in connection
with such registration by such Holder, partner, officer, director, underwriter
or controlling person of such Holder.

          (b)  By Selling Holders. To the extent permitted by law, each
selling Holder will indemnify and hold harmless the Company, each of its
directors, each of its officers who have signed the registration statement, each
person, if any, who controls the Company within the

                                       8
<PAGE>   9
meaning of the Securities Act, any underwriter and any other Holder selling
securities under such registration statement or any of such other Holder's
partners, directors or officers or any person who controls such Holder within
the meaning of the Securities Act or the 1934 Act, against any losses, claims,
damages or liabilities (joint or several) to which the Company or any such
director, officer, controlling person, underwriter or other such Holder, partner
or director, officer or controlling person of such other Holder may become
subject under the Securities Act, the 1934 Act or other federal or state law,
insofar as such losses, claims, damages or liabilities (or actions in respect
thereto) arise out of or are based upon any Violation, in each case to the
extent (and only to the extent) that such Violation occurs in reliance upon and
in conformity with written information furnished by such Holder expressly for
use in connection with such registration; and each such Holder will reimburse
any legal or other expenses reasonably incurred by the Company or any such
director, officer, controlling person, underwriter or other Holder, partner,
officer, director or controlling person of such other Holder in connection with
investigating or defending any such loss, claim, damage, liability or action:
provided, however, that the indemnity agreement contained in this subsection
7(b) shall not apply to amounts paid in settlement of any such loss, claim,
damage, liability or action if such settlement is effected without the consent
of the indemnifying Holder, which consent shall not be unreasonably withheld;
and provided, further, that the total amounts payable in indemnity by a Holder
under this Section 7(b) in respect of any Violation shall not exceed the net
proceeds received by such Holder in the registered offering out of which such
Violation arises.

          (c)  Notice. Promptly after receipt by an indemnified party under this
Section 7 of notice of the commencement of any action (including any
governmental action), such indemnified party will, if a claim in respect thereof
is to be made against any indemnifying party under this Section 7, deliver to
the indemnifying party a written notice of the commencement thereof and the
indemnifying party shall have the right to participate in, and, to the extent
the indemnifying party so desires, jointly with any other indemnifying party
similarly noticed, to assume the defense thereof with counsel mutually
satisfactory to the parties; provided, however, that an indemnified party shall
have the right to retain its own counsel, with the fees and expenses to be paid
by the indemnifying party, if representation of such indemnified party by the
counsel retained by the indemnifying party would be inappropriate due to actual
or potential conflict of interests between such indemnified party and any other
party represented by such counsel in such proceeding. The failure to deliver
written notice to the indemnifying party within a reasonable time of the
commencement of any such action shall relieve such indemnifying party of
liability to the indemnified party under this Section 7 to the extent the
indemnifying party is prejudiced as a result thereof, but the omission so to
deliver written notice to the indemnified party will not relieve it of any
liability that it may have to any indemnified party otherwise than under this
Section 7.

          (d)  Defect Eliminated in Final Prospectus. The foregoing indemnity
agreements of the Company and Holders are subject to the condition that, insofar
as they relate to any Violation made in a preliminary prospectus but eliminated
or remedied in the amended prospectus on file with the SEC at the time the
registration statement in question becomes effective or the amended prospectus
filed with the SEC pursuant to SEC Rule 424(b) (the "Final Prospectus"), such
indemnity agreement shall not inure to the benefit of any person if a copy of
the Final Prospectus was timely furnished to the indemnified party and was not
furnished to the

                                       9
<PAGE>   10
person asserting the loss, liability, claim or damage at or prior to the time
such action is required by the Securities Act.

          (e)  Contribution. In order to provide for just and equitable
contribution to joint liability under the Securities Act in any case in which
either (i) any Holder exercising rights under this Agreement, or any controlling
person of any such Holder, makes a claim for indemnification pursuant to this
Section 7 but it is judicially determined (by the entry of a final judgment or
decree by a court of competent jurisdiction and the expiration of time to appeal
or the denial of the last right of appeal) that such indemnification may not be
enforced in such case notwithstanding the fact that this Section 7 provides for
indemnification in such case, or (ii) contribution under the Securities Act may
be required on the part of any such selling Holder or any such controlling
person in circumstances for which indemnification is provided under this Section
7; then, and in each such case, the Company and such Holder will contribute to
the aggregate losses, claims, damages or liabilities to which they may be
subject (after contribution from others) in such proportion so that such Holder
is responsible for the portion represented by the percentage that the public
offering price of its Registrable Securities offered by and sold under the
registration statement bears to the public offering price of all securities
offered by and sold under such registration statement, and the Company and other
selling Holders are responsible for the remaining portion; provided, however,
that, in any such case: (A) no such Holder will be required to contribute any
amount in excess of the public offering price of all such Registrable Securities
offered and sold by such Holder pursuant to such registration statement; and (B)
no person or entity guilty of fraudulent misrepresentation (within the meaning
of Section 11(f) of the Securities Act) will be entitled to contribution from
any person or entity who was not guilty of such fraudulent misrepresentation.

          (f)  Survival. The obligations of the Company and Holders under this
Section 7 shall survive until the fifth anniversary of the completion of any
offering of Registrable Securities in a registration statement, regardless of
the expiration of any statutes of limitation or extensions of such statutes.

     8.   Termination of the Company's Obligations. The Company shall have no
obligations pursuant to Sections 2 through 4 with respect to any Registrable
Securities proposed to be sold by a Holder in a registration pursuant to Section
2, 3 or 4 more than seven (7) years after the date of this Agreement, or, if, in
the opinion of counsel to the Company, all such Registrable Securities proposed
to be sold by a Holder may then be sold under Rule 144 in one transaction
without exceeding the volume limitations thereunder.

     9.   No Registration Rights to Third Parties. Without the prior written
consent of the Holders of a majority in interest of the Registrable Securities
then outstanding, the Company covenants and agrees that it shall not grant, or
cause or permit to be created, for the benefit of any person or entity any
registration rights of any kind (whether similar to the demand, "piggyback" or
Form S-3 registration rights described in this Article 2, or otherwise) relating
to shares of the Company's Common Stock, Class A Common Stock or any other
voting securities of the Company, other than rights that are on a parity with or
subordinate in right to the Investor.

                                       10
<PAGE>   11
     10.  Assignment. The rights under this Agreement hereof may be assigned by
Investor to any entity controlling, under common control with or controlled by
the Investor; provided, however, that no party may be assigned any of the
foregoing rights unless the Company is given written notice by the Investor at
the time of such assignment stating the name and address of the assignee and
identifying the securities of the Company as to which the rights in question are
being assigned; and provided further that any such assignee shall receive such
assigned rights subject to all the terms and conditions of this Agreement,
including without limitation the provisions of this Section 10.

     11.  Amendment. Any provision of this Agreement may be amended and the
observance thereof may be waived (either generally or in a particular instance
and either retroactively or prospectively), only with the written consent of the
Company and the Holders of a majority of the Registrable Securities then
outstanding and entitled to the registration rights set forth herein. Any
amendment or waiver effected in accordance with this Section 11 shall be binding
upon the Investor, each Holder, each permitted successor or assignee of such
Investor or Holder and the Company.

     12.  Notices. Except as may be otherwise provided herein, all notices,
requests, waivers and other communications made pursuant to this Agreement shall
be in writing and shall be conclusively deemed to have been duly given (a) when
hand delivered to the other party; (b) when received when sent by facsimile at
the address and number set forth below; (c) three business days after deposit in
the U.S. mail with first class or certified mail receipt requested postage
prepaid and addressed to the other party as set forth below; or (d) the next
business day after deposit with a national overnight delivery service, postage
prepaid, addressed to the parties as set forth below with next-business-day
delivery guaranteed, provided that the sending party receives a confirmation of
delivery from the delivery service provider.

        To the Company:                       To Investor:

        NRG Energy, Inc.                      Northern States Power Company
        1221 Nicollet Mall, Suite 700         414 Nicollet Mall
        Minneapolis, MN  55403                Minneapolis, MN  55401
        Attn:    General Counsel              Attn:  General Counsel
        Fax:     (612) 373-5392               Fax:
                                                   ---------------------

     Each person making a communication hereunder by facsimile shall promptly
confirm by telephone to the person to whom such communication was addressed each
communication made by it by facsimile pursuant hereto but the absence of such
confirmation shall not affect the validity of any such communication. A party
may change or supplement the addresses given above, or designate additional
addresses, for purposes of this Section 12 by giving the other party written
notice of the new address in the manner set forth above.

     13.  Entire Agreement. This Agreement, together with all the Exhibits
hereto, constitutes and contains the entire agreement and understanding of the
parties with respect to the subject matter hereof and supersedes any and all
prior negotiations, correspondence, agreements, understandings, duties or
obligations between the parties respecting the subject matter hereof.

                                       11
<PAGE>   12
     14.  Governing Law. This Agreement shall be governed by and construed
exclusively in accordance with the internal laws of the State of Delaware,
excluding that body of law relating to conflict of laws and choice of law.

     15.  Severability. If one or more provisions of this Agreement are held to
be unenforceable under applicable law, then such provision(s) shall be excluded
from this Agreement and the balance of this Agreement shall be interpreted as if
such provision(s) were so excluded and shall be enforceable in accordance with
its terms.

     16.  Third Parties. Nothing in this Agreement, express or implied, is
intended to confer upon any person, other than the parties hereto and their
permitted successors and assigns, any rights or remedies under or by reason of
this Agreement.

     17.  Successors and Assigns. Subject to the provisions of Section 10, the
provisions of this Agreement shall inure to the benefit of, and shall be binding
upon, the successors and permitted assigns of the parties hereto.

     18.  Captions. The captions to sections of this Agreement have been
inserted for identification and reference purposes only and shall not be used to
construe or interpret this Agreement.

     19.  Counterparts. This Agreement may be executed in counterparts, each of
which shall be deemed an original, but all of which together shall constitute
one and the same instrument.

     20.  Adjustments for Stock Splits, Etc. Wherever in this Agreement there is
a reference to a specific number of shares of Class A Common Stock or Common
Stock of the Company, then, upon the occurrence of any subdivision, combination
or stock dividend of Class A Common Stock or Common Stock, as the case may be,
the specific number of shares so referenced in this Agreement shall
automatically be proportionally adjusted to reflect the affect on the
outstanding shares of such class or series of stock by such subdivision,
combination or stock dividend.

                          -- Signature Page Follows --

                                       12
<PAGE>   13

     IN WITNESS WHEREOF, the parties hereto have executed this Agreement as of
the date and year first above written.

NRG ENERGY, INC.                             NORTHERN STATES POWER COMPANY

By:                                          By:
   -------------------------------              -------------------------------

Name:                                        Name:
     -----------------------------                -----------------------------

Title:                                       Title:
      ----------------------------                 ----------------------------

                 SIGNATURE PAGE TO REGISTRATION RIGHTS AGREEMENT

                                       13

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