Exhibit 10.20

FIRST AMENDMENT TO
PARTICIPATION AGREEMENT
(Undivided Interest in the Conemaugh Facility)

THIS FIRST AMENDMENT TO PARTICIPATION AGREEMENT dated as of November 15, 2001 (this "Amendment"), among (i) RELIANT ENERGY MID-ATLANTIC
POWER HOLDINGS, LLC (formerly known as Sithe Pennsylvania Holdings, LLC), a Delaware limited liability company, as Facility Lessee, (ii) CONEMAUGH LESSOR GENCO LLC, a Delaware limited liability
company, as Owner Lessor, (iii) WILMINGTON TRUST COMPANY, a banking corporation organized and existing under the laws of the State of Delaware, not in its individual capacity, except as
expressly provided herein and in the Participation Agreement referred to below, but solely as manager under the LLC Agreement, (iv) WILMINGTON TRUST COMPANY, a banking corporation organized and
existing under the laws of the State of Delaware in its individual capacity, (v) PSEGR CONEMAUGH GENERATION, LLC, a Delaware limited liability company, as Owner Participant, (vi) BANKERS
TRUST COMPANY, a banking corporation organized and existing under the laws of the State of New York, not in its individual capacity, except as expressly provided herein and in the Participation
Agreement referred to below, but solely as trustee under the Lease Indenture, and (vii) BANKERS TRUST COMPANY, a banking corporation organized and existing under the laws of the State of New
York, not in its individual capacity, except as expressly provided herein and in the Participation Agreement referred to below, but solely as trustee under the Pass Through Trust Agreement.

WHEREAS,
the Facility Lessee, the Owner Lessor, the Lessor Manager, Wilmington Trust Company, the Owner Participant, the Lease Indenture Trustee and the Pass Through Trustee have entered
into that certain Participation Agreement dated as of August 24, 2000 (together with any and all amendments and modifications thereof, the "Participation
Agreement"); capitalized terms used herein and not otherwise defined have the same meaning as in the Participation Agreement; and

WHEREAS,
the parties to the Participation Agreement desire to amend the terms thereof as provided for herein;

NOW,
THEREFORE, in consideration of the premises, and for other good and valuable consideration, the receipt and sufficiency of which are hereby acknowledged, the parties hereto agree as
follows:

Section 1.Amendments to the Participation Agreement.

(a)   Section 7.6
of the Participation Agreement is hereby amended by deleting it in its entirety and inserting in lieu thereof



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(b)Appendix Ato the Participation Agreement is hereby amended by inserting the following definitions in their proper
alphabetical place:



(c)   The
Participation Agreement is hereby further amended by attaching thereto a newExhibit Mwhich shall be in the
form ofExhibit Mattached hereto.

Section 2.Conditions to Effectiveness.This Amendment shall become effective as of the date (the
"Effective Date")

(a)   an
officer of each of the parties hereto shall have executed and delivered a counterpart hereof; and

(b)   that
the Facility Lessee has executed and delivered to each of the parties hereto a certificate stating that each of the following shall have occurred:



Section 3.Ratification of Related Documents.The Participation Agreement, as amended hereby, and each of
the other Operative Documents is hereby ratified and confirmed to be in full force and effect.

Section 4.Limitations.The modifications set forth herein are limited precisely as written, and shall
not be deemed to (a) be a consent to, or waiver or modification of, any other term or condition of the Participation Agreement or any of the other Operative Documents, or (b) prejudice
any right or rights which any Transaction Party may now have or may have in the future under or in connection with the Participation Agreement or any of the other Operative Documents. In the event of
a conflict between this Amendment and any of the foregoing documents, the terms of this Amendment shall be controlling.

Section 5.Choice of Law.This Amendment and the rights and obligations of the parties hereunder shall be
construed in accordance with and be governed by the laws of the State of New York without giving effect to the conflicts of laws provisions thereof except New York General Obligations Law
Section 5-1401.

Section 6.Descriptive Headings.The descriptive headings of the several Sections of this Amendment are
inserted for convenience only and shall not be deemed to affect the meaning or construction of any of the provisions hereof.

Section 7.Entire Agreement.This Amendment, the Operative Documents and the documents referred to herein
represent the entire understanding of the parties hereto regarding the subject matter hereof and
supersede all prior and contemporaneous oral and written agreements of the parties hereto with respect to the subject matter hereof. There are no unwritten oral agreements between the parties.

Section 8.Counterparts.This Amendment may be executed in any number of counterparts and by parties
hereto on separate counterparts, each counterpart, when so executed and delivered, shall constitute an original instrument, and all such counterparts shall constitute but one and the same instrument.

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IN
WITNESS WHEREOF, the parties hereto have caused this Amendment to be duly executed and delivered by their respective officers thereunto duly authorized.

| |  | RELIANT ENERGY MID-ATLANTIC POWER HOLDINGS, LLC
as Facility Lessee                                                                                             
| |  | ---------------------------------------------------------------------------------------------------------------------------------------------------------------
| |  | By:                                                                                                                                                            
| |  | CONEMAUGH LESSOR GENCO LLC
as Owner Lessor                                                                                                                     
| |  | By:                                                                                                                                                            
| |  |                                                                                                                                                                
| |  |                                                                                                                                                                
| |  |                                                                                                                                                                
| |  | PSEGR CONEMAUGH GENERATION, LLC                                                                                                                                
| |  | By:                                                                                                                                                            
| |  | Name:                                                                                                                                                          
| |  | Title:                                                                                                                                                         
| |  | BANKERS TRUST COMPANY, not in its individual capacity, except to the extent provided herein, but as Lease Indenture Trustee under the Lease Indenture          
| |  | By:                                                                                                                                                            
| |  | Name:                                                                                                                                                          
| |  | Title:                                                                                                                                                         
| |  | BANKERS TRUST COMPANY, not in its individual capacity, except to the extent provided herein, but as Pass Through Trustee under the Pass Through Trust Agreement
| |  | By:                                                                                                                                                            
| |  | Name:                                                                                                                                                          
| |  | Title:                                                                                                                                                         
| |  | WILMINGTON TRUST COMPANY, in its individual capacity                                                                                                           
| |  | By:                                                                                                                                                            
| |  | Name:                                                                                                                                                          
| |  | Title:                                                                                                                                                         

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