<PAGE>

                                                                 EXHIBIT 10.7(a)

================================================================================

                       FACILITY SITE SUBLEASE AGREEMENT

                                     (L1)

                         Dated as of December 19, 2000

                                    between

                              DICKERSON OL1 LLC,
                              as Ground Sublessor

                                      and

                      SOUTHERN ENERGY MID-ATLANTIC, LLC,
                              as Ground Sublessee

                               DICKERSON STATION

          LAND LOCATED IN MONTGOMERY COUNTY, MARYLAND, IN CONNECTION
                WITH THREE COAL-FIRED ELECTRIC GENERATING UNITS

================================================================================
<PAGE>

                               Table of Contents

<TABLE>
<CAPTION>
                                                                                                            Page
<S>                                                                                                         <C>
SECTION 1.     DEFINITIONS................................................................................     2

SECTION 2.     SUBLEASE OF THE GROUND INTEREST............................................................     3
               Section 2.1.   Sublease of the Ground Interest.............................................     3
               Section 2.2.   Basic Site Sublease Term....................................................     3
               Section 2.3.   Renewal Site Sublease Term..................................................     3
               Section 2.4.   Return of the Ground Interest...............................................     3
               Section 2.5.   Early Termination...........................................................     4

SECTION 3.     SUBORDINATION TO FACILITY SITE LEASE HEREIN................................................     4

SECTION 4.     SUBRENT....................................................................................     5

SECTION 5.     LIENS......................................................................................     5

SECTION 6.     SECURITY FOR THE GROUND SUBLESSOR'S OBLIGATION UNDER THE LESSOR NOTES......................     5

SECTION 7.     NONMERGER OF INTERESTS.....................................................................     6

SECTION 8.     INSURANCE..................................................................................     6

SECTION 9.     CONDEMNATION OR DAMAGE OR DESTRUCTION......................................................     6

SECTION 10.    HYPOTHECATION/ASSIGNMENT...................................................................     7
               Section 10.1.   Lien on Leasehold Prohibited...............................................     7
               Section 10.2.   Assignment of the Ground Sublessor's Rights................................     7

SECTION 11.    TAXES......................................................................................     7

SECTION 12.    MISCELLANEOUS..............................................................................     8
               Section 12.1.  Notices.....................................................................     8
               Section 12.2.  Governing Law...............................................................     9
               Section 12.3.  Limitation of Liability.....................................................    10
               Section 12.4.  Memorandum..................................................................    10
</TABLE>

                                       i
<PAGE>

                       Facility Site Sublease Agreement
                                     (L1)

     This FACILITY SITE SUBLEASE AGREEMENT (LI), dated as of December 19, 2000
(as amended, supplemented or otherwise modified from time to time in accordance
with the provisions hereof, this "Facility Site Sublease"), between DICKERSON
                                  ----------------------
OL1 LLC, a Delaware limited liability company (the "Ground Sublessor") created
                                                    ----------------
for the benefit of SEMA OP3 LLC, a Delaware limited liability company (the

"Owner Participant"), and SOUTHERN ENERGY MID-ATLANTIC, LLC, a Delaware limited
------------------
liability company (the "Ground Sublessee").
                        ----------------

                                  WITNESSETH:

     WHEREAS, the Ground Sublessor is governed by the Limited Liability
Agreement, dated as of December 18, 2000 (the "Lessor LLC Agreement"), between
                                               --------------------
Wilmington Trust Company acting thereunder not in its individual capacity but
solely as independent manager (the "Owner Manager") and the Owner Participant;
                                    -------------

     WHEREAS, pursuant to the Land Deed and the Bill of Sale dated as of the
date hereof, the Ground Sublessee has acquired from Potomac Electric Power
Company or its successors or assigns ("Pepco"), inter alia, that certain land,
                                       -----
in Montgomery County, Maryland, and more particularly described in Exhibit A
                                                                   ---------
attached hereto and made a part hereof (the "Dickerson Land"), together with
                                             --------------
certain improvements and fixtures located thereon or thereunder (the "Dickerson
                                                                      ---------
Improvements") other than the Facility;
------------

     WHEREAS, pursuant to the Facility Deed and the Bill of Sale, the Ground
Sublessor has acquired from Pepco an undivided ownership interest in the
Facility (which interest is defined in Appendix A to the Participation Agreement
as the "Undivided Interest") and which Facility includes (1) the Dickerson Base-
        ------------------
Load Units 1, 2 and 3 comprised of the 546 megawatt (net) coal-fired electric
generating units and (2) certain other improvements and fixtures which are
located above-ground, all as more particularly described in Exhibit B attached
                                                            ---------
hereto and made a part hereof, as a tenant-in-common with the other owners of
undivided ownership interests in the Facility;

     WHEREAS, the Facility is located on that certain portion of the Dickerson
Land more particularly described in Exhibit C attached hereto and made a part
                                    ---------
hereof (the "Facility Land") and with respect to the transmission lines only
             -------------
(which transmission lines are included as part of the Facility, as described on
Exhibit B attached hereto) on a portion of the Retained Dickerson Land (that
---------
portion of the Retained Dickerson Land on which the transmission lines are
located is referred to as the "Retained Dickerson Land Transmission Line Area");
                               ----------------------------------------------
<PAGE>

     WHEREAS, all of the Dickerson Improvements located under the Facility Land
are herein referred to as the "Leased Dickerson Improvements";
                               -----------------------------

     WHEREAS, the Facility Land and the Leased Dickerson Improvements
(collectively, the "Facility Site") do not include the Facility or any part(s)
                    -------------
thereof, and no part of the Facility is being leased to the Ground Sublessee
hereunder;

     WHEREAS, pursuant to that certain Facility Site Lease and Easement
Agreement (L1) dated as of December 19, 2000 between the Ground Sublessee, as
ground lessor, and the Ground Sublessor, as ground lessee (as amended,
supplemented or otherwise modified from time to time in accordance with the
provisions thereof, the "Facility Site Lease"), (i) the Ground Sublessee has
                         -------------------
reserved unto itself, its successors and assigns, certain non-exclusive
easements, (ii) the Ground Sublessee has leased to the Ground Sublessor a 63.3%
undivided leasehold interest in the Facility Site as a tenant-in-common with,
pursuant to the Other Facility Site Leases, three (3) other owners of undivided
leasehold interests in the Facility Site  and has granted to the Ground
Sublessor certain non-exclusive easements (such undivided leasehold interest,
together with the non-exclusive easements granted to the Ground Sublessor
thereunder, and subject to the non-exclusive easements reserved by the Ground
Sublessee thereunder, and together with, and subject to, all easements and
appurtenances to the Facility Site (including those granted by Pepco pursuant to
the Easement, License and Attachment Agreement (Dickerson Station) dated on or
about the date hereof between Pepco and the Ground Sublessee, which is intended
to be recorded in the appropriate recorder's office in Montgomery County,
Maryland), the "Ground Interest");
                ---------------

     WHEREAS, pursuant to that certain Facility Lease Agreement (L1) dated as of
December 19, 2000 between the Ground Sublessor, as lessor, and the Ground
Sublessee, as lessee (as amended, supplemented or otherwise modified from time
to time in accordance with the provisions thereof, the "Facility Lease") the
                                                        --------------
Ground Sublessee is leasing the Undivided Interest from the Ground Sublessor;
and

     WHEREAS, pursuant to this Facility Site Sublease, the Ground Sublessor is
subleasing the Ground Interest to the Ground Sublessee for a term coterminous
with that of the Facility Lease.

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

SECTION 1.  DEFINITIONS

     Capitalized terms used in this Facility Site Sublease, including the
recitals, and not otherwise defined herein shall have the respective meanings
set forth in Appendix A to the Participation Agreement (L1), dated as of
December 18, 2000 (as amended, supplemented or otherwise modified from time to
time, the "Participation Agreement"), among the Ground
           -----------------------
<PAGE>

Sublessee, the Ground Sublessor, the Owner Manager, the Owner Participant, and
State Street Bank and Trust Company of Connecticut, National Association, as
Pass Through Trustee and as Lease Indenture Trustee. The general provisions of
Appendix A to the Participation Agreement shall apply to terms used in this
Facility Site Sublease.

SECTION 2.  SUBLEASE OF THE GROUND INTEREST

     Section 2.1.   Sublease of the Ground Interest.  The Ground Sublessor
hereby subleases to the Ground Sublessee, and the Ground Sublessee hereby
subleases from the Ground Sublessor, the Ground Interest, upon the terms and
conditions set forth herein.  The Ground Sublessor and the Ground Sublessee
understand and agree that (a) fee title to the Facility Site remains vested in
the Ground Sublessee and, (b) this sublease of the Ground Interest is subject to
the Permitted Encumbrances and (c) fee title to the Undivided Interest is vested
in the Ground Sublessor.  Title to the Facility, all Components, Replacement
Components and every portion thereof is severed, and shall be and remain severed
from title to the real estate constituting the Facility Land and (with respect
to the transmission lines only) the Retained Dickerson Land Transmission Line
Area on which the Facility and other equipment is located, even if physically
attached thereto.  The parties agree that the Undivided Interest, Components and
Replacement Components shall be and shall remain the property of the Ground
Sublessor (except as to any Severable Improvement, title to which, in each case,
is reserved to the Ground Sublessee) and shall not be or become fixtures or
otherwise part of the real estate constituting the Facility Land or (with
respect to the transmission lines only) the Retained Dickerson Land Transmission
Line Area.

     Section 2.2.   Basic Site Sublease Term.  The term of this Facility Site
Sublease shall commence on the Closing Date and shall terminate at 11:59 p.m.
(EST) on  June 19, 2029 (the "Basic Site Sublease Term"), subject to earlier
                              ------------------------
termination pursuant to Section 2.5 hereof and extension pursuant to Section 2.3
hereof.

     Section 2.3.   Renewal Site Sublease Term.  If the Ground Sublessee
exercises its option to renew the Facility Lease, for one or more Renewal Lease
Terms pursuant to Section 15 of the Facility Lease, this Facility Site Sublease
shall automatically and without further act by any Person be renewed for a term
or terms which shall be coterminous with the Renewal Lease Term(s) under the
Facility Lease but in no event beyond the expiration, surrender or termination
of the Site Lease Term (each a "Renewal Site Sublease Term" and, together with
                                --------------------------
the Basic Site Sublease Term, the "Site Sublease Term").
                                   ------------------

     Section 2.4.   Return of the Ground Interest.

          (a) Return.  Upon any expiration or earlier termination of this
Facility Site Sublease, the Ground Sublessee shall return the Ground Interest to
the Ground Sublessor by surrendering the Ground Interest into the possession of
the Ground Sublessor without representation or warranty other than a warranty
that the Ground Interest is free and clear of all Liens other than Permitted
Liens and, upon the request of the Ground Sublessor, shall execute,

                                       3
<PAGE>

acknowledge and deliver a release and termination of this Facility Site Sublease
to be prepared by the Ground Sublessee at its expense and in a form reasonably
satisfactory to the Ground Sublessor to be duly recorded at the Ground
Sublessee's expense in the appropriate recorder's office located in Montgomery
County, Maryland.

          (b)  Condition Upon Return.   In addition to the terms set forth in
Section 2.4(a):

          (i)  the Facility Site shall be in at least as good condition as on
the date hereof, ordinary wear and tear excepted, as if it had been maintained
during the Site Sublease Term in compliance with the provisions of this Facility
Site Sublease; and

          (ii) the Ground Sublessee shall assign, without representation or
warranty, an undivided interest equal to the Owner Lessor's Percentage in any
and all licenses and permits of any Governmental Authorities or other Persons
that are then in effect in connection with the use, operation or maintenance of
the Facility Site and the Retained Dickerson Land Transmission Line Area and
which are not already issued in the name of the Ground Sublessor, to the Ground
Sublessor or a transferee or designee of the Ground Sublessor, to the extent
such assignment is permitted by Requirements of Law and the provisions of such
license or permit.

          (c) The obligations of the Ground Sublessee under this Section 2.4
shall survive the termination of this Facility Site Sublease.

     Section 2.5.   Early Termination.  Notwithstanding anything to the contrary
in this Facility Site Sublease, the Site Sublease Term shall automatically
terminate upon the expiration, cancellation or early termination of the Facility
Lease Term without any action of the Ground Sublessor, the Ground Sublessee, or
any other Person.

SECTION 3.  SUBORDINATION TO FACILITY SITE LEASE HEREIN

          (a) This Facility Site Sublease is subject to all the terms and
conditions of the Facility Site Lease.  Except as otherwise provided herein, the
Ground Sublessee shall undertake and perform, for the benefit of the Ground
Sublessor, all duties and obligations of the Ground Sublessor under the Facility
Site Lease and shall not take any action, or fail to take any action, which
action or failure would constitute or result in an event of default under the
Facility Site Lease.

          (b) Except as set forth in this Facility Site Sublease to the
contrary, the terms and provisions of the Facility Site Lease as in effect from
time to time are incorporated by reference herein in their entirety mutatis
mutandis.  Without limiting the generality of the foregoing, (i) the Ground
Sublessee hereunder shall have rights and obligations under this Facility Site
Sublease identical to the rights and obligations of the Ground Lessee (as
defined in the Facility Site Lease) under the Facility Site Lease, as in effect
from time to time, and (ii) the

                                       4
<PAGE>

Ground Sublessor hereunder shall have rights and obligations under this Facility
Site Sublease identical to the rights and obligations of the Ground Lessor (as
defined in the Facility Site Lease) under the Facility Site Lease, as in effect
from time to time. All references in the Operative Documents to terms and
provisions of this Facility Site Sublease shall be references to the correlative
terms and provisions of the Facility Site Lease as incorporated by reference
herein.

          (c) Notwithstanding anything to the contrary provided in this Facility
Site Sublease, the Ground Sublessee shall have none of the rights and
obligations of the Ground Sublessor under the Facility Site Lease, and the
Ground Sublessor shall have none of the rights and obligations of the Ground
Sublessee under the Facility Site Lease, with respect to the following
provisions of the Facility Site Lease:  Sections 2.1, 2.2, 2.3, 2.4, 2.5(b),
2.6, 2.7,  3.3, 4.2, 4.3, 4.4, 4.5, 7.1, 7.2, 9.3, 10.1 through and including
10.11, 12, 15, 17.2, 17.13, 18 and 19.

SECTION 4.  SUBRENT

          (a) As subrent for the Ground Interest during the Basic Site Sublease
Term, the Ground Sublessee shall pay to the Ground Sublessor subrent equal to
the amount of rent payable under the Facility Site Lease for the corresponding
period.  The subrent for any partial period at the end of the Basic Site
Sublease Term shall be prorated on the basis of the actual number of days in
such period.

          (b) As subrent for the Ground Interest during each Renewal Site
Sublease Term, the Ground Sublessee shall pay to the Ground Sublessor subrent
equal to all rent due and payable under the Facility Site Lease for the
corresponding period.  Such subrent shall be prorated for any partial period at
the end of the Renewal Site Sublease Term on the basis of the actual number of
days during such period.

          (c) The subrent payable during the Basic Site Sublease Term or during
each Renewal Site Sublease Term shall be paid as and when due under the Facility
Site Lease.

SECTION 5.  LIENS

     The Ground Sublessee hereby covenants that it will not, directly or
indirectly, create, incur, assume or suffer to exist any Liens or other
encumbrances on or with respect to the Ground Interest, the Facility Site, or
the Ground Sublessor's or the Owner Participant's interest therein, as the case
may be, except Permitted Encumbrances and Permitted Liens, and the Ground
Sublessee shall promptly notify the Ground Sublessor of the imposition of any
such Lien, except Permitted Encumbrances and Permitted Liens, of which the
Ground Sublessee is aware and shall promptly take such action as may be
necessary to fully discharge or release any such Lien.

SECTION 6.  SECURITY FOR THE GROUND SUBLESSOR'S OBLIGATION UNDER THE LESSOR
     NOTES

                                       5
<PAGE>

     In order to secure the Lessor Notes or obligations to the Lease Indenture
Trustee, the Ground Sublessor will assign and grant a Lien to the Lease
Indenture Trustee of all of the Ground Sublessor's right, title and interest in,
to and under this Facility Site Sublease (other than Excepted Payments). The
Ground Sublessee hereby consents to such assignment and to the creation of such
Lien and security interest (and any sale arising from or in connection with the
exercise of remedies) and acknowledges receipt of a copy of the Lease Indenture,
it being understood that such consent shall not affect any requirement or the
absence of any requirement for any consent of the Ground Sublessee under any
other circumstances, except as may be otherwise contemplated in the Facility
Site Lease.  Unless and until the Ground Sublessee shall have received written
notice from the Lease Indenture Trustee that the Lien created under the Lease
Indenture has been fully released, the Lease Indenture Trustee shall have the
right to exercise the rights of the Ground Sublessor under this Facility Site
Sublease to the extent set forth in and subject in each case to the exceptions
set forth in the Lease Indenture; provided, however, that during the Site
Sublease Term, each payment of rent by the Ground Sublessor for the lease of the
Ground Interest pursuant to Section 3.1 of the Facility Site Lease and each
payment of rent by the Ground Sublessee for the sublease of the Ground Interest
pursuant to this Facility Site Sublease shall be automatically offset one
against the other, and no amounts shall be payable by the Ground Sublessor or
the Ground Sublessee in respect thereof.

SECTION 7.  NONMERGER OF INTERESTS

     The interests of the Ground Sublessee in the Other Ground Interests under
the Other Facility Site Subleases shall not merge into any interest in the
Ground Interest conveyed by this Facility Site Sublease even though such
interests of the Ground Sublessee in the Other Ground Interests and the Ground
Interest subleased hereunder are vested in or held directly or indirectly by the
same Person, and this Facility Site Sublease shall nonetheless remain in full
force and effect in accordance with its terms notwithstanding such vesting or
holding unless and until the Person holding such interests shall execute an
instrument affecting such merger and shall duly record such instrument.  No such
instrument of merger shall be executed and recorded unless and until the Lien
created under the Lease Indenture has been discharged in accordance with the
terms thereof.

SECTION 8.  INSURANCE

     At all times during the Site Sublease Term, the Ground Sublessee shall
comply with the requirements of Section 11 of the Facility Lease regarding
insurance and the proceeds thereof shall be applied as required by the Facility
Lease.

SECTION 9.  CONDEMNATION OR DAMAGE OR DESTRUCTION

          (a) If at any time the Facility Site or any portion thereof is
condemned or transferred in lieu of condemnation, the proceeds of such
condemnation or transfer shall be

                                       6
<PAGE>

divided among Ground Sublessor and Ground Sublessee in the proportion specified
in the Facility Site Lease and the Facility Lease, as applicable.

          (b) Subject to and without limiting the parties' obligations under the
other Operative Documents, if the Facility or any substantial part thereof is
damaged or destroyed by any fire or other casualty, the Ground Sublessee shall
repair or replace the Facility or any such equipment, to the extent required
pursuant to the Facility Lease, and any proceeds received by the Ground
Sublessee in connection therewith shall be applied and distributed in accordance
with the Facility Lease.

SECTION 10.  HYPOTHECATION/ASSIGNMENT

     Section 10.1.   Lien on Leasehold Prohibited.  Except as expressly
permitted pursuant to the Participation Agreement, the Facility Lease, or any
other Operative Document, the Ground Sublessee shall not have the right to
encumber, mortgage, pledge or hypothecate its interest in this Facility Site
Sublease or the Ground Interest or to grant an assignment of the rents, issues
and profits from the Ground Interest.

     Section 10.2.   Assignment of the Ground Sublessor's Rights.   The Ground
Sublessor shall have the right to encumber, mortgage, pledge or hypothecate its
interest in this Facility Site Sublease, the Facility Lease or the Ground
Interest or to grant an assignment of the rents, issues and profits from this
Facility Site Sublease or the Ground Interest (any such encumbrance, mortgage,
pledge, hypothecation or assignment, the "Leasehold Lien") in favor of or to a
                                          --------------
Leasehold Mortgage entered into in accordance with the provisions of the
Facility Site Lease.  The Ground Sublessee specifically acknowledges that,
concurrently with the execution and delivery of this Facility Site Sublease, the
Ground Sublessor will, as security for the Lessor Secured Obligations (as
defined in the Lease Indenture), grant the Lease Indenture Trustee a Lien in and
to all of the Ground Sublessor's right, title and interest in, to and under the
Ground Interest (other than Excepted Payments).  The Ground Sublessee hereby
consents to such assignment and creation of such Lien and acknowledges receipt
of copies of the Lease Indenture, it being understood that such consent shall
not affect any requirement or the absence of any requirement for any consent
under any other circumstances. Unless and until the Ground Sublessee shall have
received written notice from the Lease Indenture Trustee that the Lien of the
Lease Indenture have been fully released, the Lease Indenture Trustee under the
Lease Indenture shall have the rights of the Ground Sublessor under this
Facility Site Sublease to the extent, and subject in each case to the exceptions
set forth in, the Lease Indenture.

SECTION 11.  TAXES

          The Ground Sublessee shall pay and indemnify the Ground Sublessor, as
provided in and subject to the terms, conditions and limitations in, Section
10.2 of the Participation Agreement with respect to Taxes imposed upon, or in
connection with, or applicable to, the Ground Interest during the Site Sublease
Term, this Facility Site Sublease, the Ground Sublessee

                                       7
<PAGE>

and those that result upon termination hereof. Such provisions of Section 10.2
of the Participation Agreement shall survive the expiration or termination of
this Facility Site Sublease.

SECTION 12.  MISCELLANEOUS

     Section 12.1.   Notices.  Unless otherwise expressly specified or permitted
by the terms hereof, all notices and other communications provided to a party
hereto shall be in writing or shall be produced by a telecommunications device
capable of creating a written record, and any such notice shall become effective
(a) upon personal delivery thereof, including, without limitation, by overnight
mail or next business day or courier service, (b) in the case of notice by
United States mail, certified or registered, postage prepaid, return receipt
requested, upon receipt thereof, or (c) in the case of notice by such a
telecommunications device, upon transmission thereof, provided such transmission
is promptly confirmed by either of the methods set forth in clause (a) or (b)
above, in each case addressed to such party and copy party at its address set
forth below or at such other address as such party or copy party may from time
to time designate by written notice to the other parties:

     If to the Ground Sublessee:

          Southern Energy Mid-Atlantic, LLC
          1155 Perimeter Center West
          Atlanta, Georgia  30338-4780
          Telephone No.: (678) 579-5000
          Facsimile No.: (678) 579-7001
          Attention: President

     with a copy to:

          Southern Energy, Inc.
          1155 Perimeter Center West
          Atlanta, Georgia  30338-4780
          Telephone No.: (678) 579-5000
          Facsimile No.:  (678) 579-6904
          Attention: President, with a copy to General Counsel

     If to the Ground Sublessor:

          Dickerson OL1 LLC
          c/o Wilmington Trust Company
          Rodney Square North
          1100 North Market Street
          Wilmington, Delaware 19890-0001
          Telephone No.: (302) 651-1000

                                       8
<PAGE>

          Facsimile No.:  (302) 651-8882
          Attention:  Corporate Trust Administration


     with a copy to the Owner Participant:

          SEMA OP3 LLC
          c/o Wilmington Trust Company
          Rodney Square North
          1100 North Market Street
          Wilmington, Delaware 19890-0001
          Telephone No.: (302) 651-1000
          Facsimile No.:  (302) 651-8882
          Attention:  Corporate Trust Administration

     and to the Lease Indenture Trustee:

          State Street Bank and Trust Company of Connecticut, National
          Association
          225 Asylum Street, Goodwin Square
          Hartford, Connecticut  06103
          Attention: Corporate Trust Department

     with a copy to:

          State Street Bank and Trust Company
          2  Avenue de Lafayette
          Boston, Massachusetts  02111
          Telephone No.: (617) 662-1681
          Facsimile No.: (617) 662-1465
          Attention: Corporate Trust Department

     Section 12.2.   Governing Law. This Facility Site Sublease was negotiated
in the State of New York which the Ground Sublessee and the Ground Sublessor
agree has a substantial relationship to the parties and to the underlying
transaction embodied hereby, and, in accordance with Section 5-1401 of the New
York General Obligations Law, in all respects, including matters of
construction, validity and performance, this Facility Site Sublease shall be
governed by, and construed in accordance with, the laws of the State of New York
applicable to contracts made and performed in such State and any applicable law
of the United States of America, except that the provisions for the creation and
                                 ------
enforcement of the subleasehold interest created hereby shall be governed by and
construed according to the law of the state in which the Facility Site is
located, it being understood that, to the fullest extent permitted by the law of
the state in which the Facility Site is located, the law of the State of New
York shall govern the validity and enforceability of the representations,
warranties, covenants and obligations of the Ground

                                       9
<PAGE>

Sublessee and the Ground Sublessor under this Facility Site Sublease and all
other Operative Documents (excluding the Lessor LLC Agreement and the OP LLC
Agreement which shall be governed by and construed as set forth therein) and all
of the indebtedness arising hereunder or thereunder. To the fullest extent
permitted by law, the Ground Sublessee and the Ground Sublessor hereby
unconditionally and irrevocably waive any claim to assert that the law of any
other jurisdiction governs this Facility Site, except as expressly otherwise
provided above.

     Section 12.3.   Limitation of Liability.   It is expressly understood and
agreed by the parties hereto that (a) this Facility Site Sublease is executed
and delivered by the Owner Manager, not individually or personally but solely as
Owner Manager of the Ground Sublessor under the Lessor LLC Agreement, in the
exercise of the powers and authority conferred and vested in it pursuant
thereto, (b) each of the representations, undertakings and agreements herein
made on the part of the Ground Sublessor is made and intended not as a personal
representation, undertaking and agreement (as applicable) by the Owner Manager
but is made and intended for the purpose of binding only the Ground Sublessor,
(c) nothing herein contained shall be construed as creating any liability on the
Owner Manager, individually or personally, to perform any covenant either
expressly contained or implied herein, all such liability, if any, being
expressly waived by the parties hereto or by any person claiming by, through or
under the parties hereto and (d) under no circumstances shall the Owner Manager
be personally liable for the payment of any indebtedness or expenses of the
Ground Sublessor or be liable for the breach or failure of any obligation,
representation, warranty or covenant made or undertaken by the Ground Sublessor
under this Facility Site Sublease.

     Section 12.4.   Memorandum.   A memorandum of the Facility Site Sublease
shall be duly recorded against the Facility Site in the appropriate recorder's
office located in Montgomery County, Maryland by the Ground Sublessee, at the
Ground Sublessee's expense.


                      (Signatures Appear on Following Page(s))

                                      10
<PAGE>

     IN WITNESS WHEREOF, the parties hereto have caused this Facility Site
Sublease Agreement (L1) to be duly executed and delivered under seal by their
respective officers thereunto duly authorized.

                          GROUND SUBLESSOR:

                          DICKERSON OL1 LLC,
                          a Delaware limited liability company

                          By:  Wilmington Trust Company,
                               not in its individual capacity but solely as
                               Owner Manager under the Lessor LLC Agreement


                          By:  /s/ W. Chris Sponenberg
                               --------------------------------------------
                               Name: W. Chris Sponenberg
                               Title: Assistant Vice President
                               Date:  December 19, 2000


                          GROUND SUBLESSEE:

                          SOUTHERN ENERGY MID-ATLANTIC, LLC,
                          a Delaware limited liability company


                          By:  /s/ Raymond V. Haley
                               --------------------------------------------
                               Name: Raymond V. Haley
                               Title: Vice President
                               Date:  December 19, 2000


                          FACILITY SITE SUBLEASE (L1)
                          ---------------------------