<PAGE>

As filed with the Securities and Exchange Commission on August 19, 1999

                          Registration No. 333-78285


                      SECURITIES AND EXCHANGE COMMISSION
                            WASHINGTON, D.C.  20549
                                   FORM S-8
                            REGISTRATION STATEMENT
                                     UNDER
                          THE SECURITIES ACT OF 1933

                                     NSTAR
          --------------------------------------------------------------
             (Exact name of Registrant as specified in its charter)

          MASSACHUSETTS                                   04-3466300
          -------------                                   ----------
(State or other jurisdiction of                       (I.R.S. Employer
incorporation or organization)                        Identification No.)


              800 BOYLSTON STREET, BOSTON, MASSACHUSETTS    02199
             (Address of principal executive offices)   (Zip Code)
       Registrant's telephone number, including area code (617) 424-2000

         -----------------------------------------------------------------
  Employee Savings Plan of Commonwealth Energy System and Subsidiary Companies
         -----------------------------------------------------------------
                           (Full title of the Plan)

                             DOUGLAS S. HORAN, ESQ.
                                     NSTAR
                              800 BOYLSTON STREET
                          BOSTON, MASSACHUSETTS 02199
                                (617) 424-2000
            -------------------------------------------------------
           (Name, address and telephone number of agent for service)


                  Please send copies of all communications to:

                              DAVID A. FINE, ESQ.
                                 ROPES & GRAY
                            ONE INTERNATIONAL PLACE
                          BOSTON, MASSACHUSETTS 02110

                        CALCULATION OF REGISTRATION FEE

<TABLE>
<CAPTION>
 Title of Securities     Amount To Be       Proposed Maximum     Proposed Maximum       Amount of
 To Be Registered       Registered/(1)/     Offering Price Per  Aggregate Offering    registration Fee
                                              Share/(2)/           Price/(2)/
-----------------------------------------------------------------------------------------------------
<S>                     <C>               <C>                  <C>                  <C>
Common Shares,                   525,000        $41.91             $22,002,750          $6,117.00
 $1.00 par value, of
 NSTAR
-----------------------------------------------------------------------------------------------------
</TABLE>

(1)  To be purchased from time to time for participants in the Plan.

(2)  Estimated solely for the purpose of determining the registration fee
pursuant to Rule 457(h) on the basis of the average of (i) the average of the
high and low prices of BEC Energy Common Shares reported on the New York Stock
Exchange on August 13, 1999 and (ii) the average of the high and low prices of
Commonwealth Energy System Common Shares reported on the New York Stock Exchange
on August 13, 1999.

In addition, pursuant to Rule 416(c) under the Securities Act of 1933, this
registration statement also covers an indeterminate amount of interests to be
offered or sold pursuant to the employee benefit plan described herein.
<PAGE>

                                    PART II

                            INFORMATION REQUIRED IN
                          THE REGISTRATION STATEMENT

Item 3.  Incorporation of documents by reference.

  The following documents, which have heretofore been filed by the Registrant
with the Securities and Exchange Commission pursuant to the Exchange Act
(Central Index Key No. 0001085871), are incorporated by reference herein and
shall be deemed to be a part hereof:

    (i)   The Joint Proxy Statement/Prospectus covering 65,000,000 Common Shares
          of the Registrant, as filed with the Commission as part of the Form S-
          4 on May 12, 1999.

    (ii)  Description of Common Shares included in the Registration Statement on
          Form S-4 filed under the Securities Act, including any amendment or
          report filed for the purpose of updating such description.

    (iii) All documents subsequently filed by NSTAR or the Plan pursuant to
          Sections 13(a), 13(c), 14 and 15(d) of the Exchange Act prior to the
          filing of a post-effective amendment which indicates that all
          securities offered have been sold or which deregisters all securities
          then remaining unsold shall be deemed to be incorporated herein by
          reference and shall be deemed a part hereof from the date of filing of
          such documents.

  The consolidated financial statements of BEC Energy included in BEC Energy's
Annual Report on Form 10-K/A for the year ended December 31, 1998, which is
incorporated by reference to the Joint Proxy Statement/Prospectus, have been
audited by PricewaterhouseCoopers LLP, independent accountants, as stated in its
report which is also included in the Annual Report. These consolidated financial
statements are incorporated by reference in this Registration Statement in
reliance upon the report given and upon the authority of PricewaterhouseCoopers
as experts in accounting and auditing.

  The consolidated financial statements of Commonwealth Energy System
("COM/Energy") included in COM/Energy's Annual Report on Form 10-K for the year
ended December 31, 1998, which is incorporated by reference in the prospectus
included in the Joint Proxy Statement/Prospectus have been audited by Arthur
Andersen LLP, independent public accountants, as indicated in their reports with
respect thereto, and are included herein in reliance upon the authority of said
firm as experts in giving said reports.

Item 4.   Description of Securities.

  Not applicable.

                                      -2-
<PAGE>

Item 5.   Interests of Named Experts and Counsel.

  Thomas G. Dignan, Jr., a partner of Ropes & Gray (counsel to the Company), is
a trustee of the Company.

Item 6.   Indemnification of Directors and Officers.

  NSTAR's Declaration of Trust (the "Declaration of Trust") provides that, to
the extent legally permissible, each of NSTAR's Trustees and officers shall be
indemnified by NSTAR's trust estate against any loss, liability or expense,
including amounts paid in satisfaction of judgments, in compromise or as fines
and penalties, and counsel fees, imposed upon or reasonably incurred by such
person in connection with the defense or disposition of any action, suit or
other proceeding, whether civil or criminal, in which such person may be
involved or with which such person may be threatened, while in office or
thereafter, by reason of such person's being or having been such a Trustee or
officer, except with respect to any matter as to which such person shall have
been adjudicated in such action, suit or proceeding not to have acted in good
faith in the reasonable belief that his or her action was in the best interests
of NSTAR; provided, however, that as to any matter disposed of by a compromise
payment by such Trustee or officer, pursuant to a consent decree or otherwise,
no indemnification either for said payment or for any other expenses shall be
provided unless such compromise shall be approved as in the best interests of
NSTAR, after notice that it involves such indemnification, (i) by a
disinterested majority of the Trustees then in office, or (ii) by a majority of
the disinterested Trustees then in office, provided that there has been obtained
an opinion in writing of independent legal counsel to the effect that such
Trustee or officer appears to have acted in good faith in the reasonable belief
that his or her action was in the best interests of NSTAR, or (iii) by the vote,
at a meeting duly called and held, of the holders of a majority of the shares
outstanding and entitled to vote thereon, exclusive of any shares owned by any
interested Trustee or officer.

  In discharging his or her duties, a Trustee or officer of NSTAR, when acting
in good faith, shall be fully protected in relying upon the books of account of
NSTAR or of another organization in which he or she serves as contemplated by
the indemnification provisions of the Declaration of Trust, reports made to
NSTAR or to such other organization by any of its officers or employees or by
counsel, accountants, appraisers or other experts or consultants selected with
reasonable care by the Trustees or similar governing body of such other
organization, or upon other records of NSTAR or of such other organization.  The
rights of indemnification provided in the Declaration of Trust shall not be
exclusive of or affect any other rights to which any Trustee or officer may be
entitled and such rights shall inure to the benefit of his or her successors,
heirs, executors, administrators and other legal representatives.  As used in
this provision, the terms "Trustee" and "officer" include persons who serve at
the request of NSTAR as directors, officers, or trustees of another organization
in which NSTAR has any direct or indirect interest as a shareholder, creditor or
otherwise.

                                      -3-
<PAGE>

  Expenses, including counsel fees, reasonably incurred by any Trustee or
officer with respect to the defense or disposition of any action, suit or
proceeding referred to in the indemnification provisions of the Declaration of
Trust may be advanced by NSTAR prior to the final disposition of such action,
suit or proceeding, upon receipt of an undertaking by or on behalf of the
recipient to repay such amount unless it is ultimately determined that he or she
is entitled to indemnification.  Nothing contained in these provisions affects
any rights to indemnification to which NSTAR personnel other than Trustees and
officers may be entitled by contract or otherwise under law.  No Trustee shall
be obligated to give any bond or other security for the performance of any of
his or her duties.

Item 7.  Exemption from Registration Claimed.

  Not applicable.

Item 8.

  (a)    Exhibits:  See "Exhibit Index"

  (b)    The Registrant undertakes that it will submit or has submitted the
Employee Savings Plan of Commonwealth Energy Companies System and Subsidiary
(the "Plan") or any amendment thereto to the Internal Revenue Service ("IRS") in
a timely manner and has made or will make all changes required by the IRS in
order to qualify the Plan under ERISA.

Item 9.  Undertakings.

  (a)  The Registrant hereby undertakes:

       (1) to file, during any period in which offers or sales are being made,
  a post-effective amendment to this registration statement; (i) to include
  any prospectus required by Section 10(a)(3) of the Securities Act of 1933
  (the "Securities Act"); (ii) to reflect in the prospectus any facts or
  events arising after the effective date of the registration statement (or
  the most recent post-effective amendment thereof), which, individually or in
  the aggregate, represent a fundamental change in the information set forth
  in the registration statement; and (iii) to include any material information
  with respect to the plan of distributi on not previously disclosed in the
  registration statement or any material change to such information in the
  registration statement; provided, however, that paragraphs (a)(l)(i) and
  (ii) shall not apply if the information required to be included in a
  post-effective amendment by those paragraphs is contained in periodic
  reports filed by the registrant pursuant to Section 13(a) or 15(d) of the
  Securities Exchange Act of 1934 (the "Exchange Act")that are incorporated by
  reference in the registration statement;

       (2) that, for the purpose of dete rmining any liability under the
  Securities Act, each such post-effecti ve amendment shall be deemed to be a
  new registration statement relating to

                                      -4-
<PAGE>

    the securities offered herein, and
    the offering of such securities at that time shall be deemed to be the
    initial bona fide offering thereof; and

         (3) to remove from registration by means of a post-effective amendment
    any of the securities being registered which remain unsold at the
    termination of the offering.

    (b)  The Registrant hereby undertakes that, for purposes of determining any
liability under the Securities Act , each filing of the Registrant's annual
report pursuant to Section 13(a) or 15(d) of the  Exchange Act (and, where
applicable, each filing of the Plan's annual report pursuant to Section 15(d) of
the Exchange Act) that is incorporated by reference in the registration
statement shall be deemed to be a new registration statement relating to the
securities offered therein, and the offering of such securities at that time
shall be deemed to be the initial bona fide offering thereof.

    (c)  Insofar as indemnification for liabilities arising under the
Securities Act may be permitted to directors, officers and controlling persons
of the Registrant pursuant to the foregoing provisions, or otherwise, the
Registrant has been advised that in the opinion of the Securities and Exchange
Commission such indemnification is against public policy as expressed in the
Securities Act and is, therefore, unenforceable.  In the event that a claim for
indemnification against such liabilities (other than the payment by the
Registrant of expenses incurred or paid by a director, officer or controlling
person of the Registrant in the successful defense of any action, suit or
proceeding) is asserted by such director, officer or controlling person in
connection with the securities being registered, the Registrant will, unless in
the opinion of its counsel the matter has been settled by controlling precedent,
submit to a court of appropriate jurisdiction the question whether such
indemnification by it is against public policy as expressed in the Securities
Act and will be governed by the final adjudication of such issue.

                                      -5-
<PAGE>

                                 EXHIBIT INDEX

Exhibit Number  Description of Exhibit

4.1             NSTAR Declaration of Trust, dated April 20, 1999 (incorporated
                by reference to Annex D to the Proxy Statement/Prospectus in
                Part I of Registration Statement on Form S-4 of NSTAR (No. 333-
                78285).

4.2             By-laws of NSTAR, dated April 20, 1999 (incorporated by
                reference to Annex E to the Proxy Statement/Prospectus in Part I
                of Registration Statement on Form S-4 of NSTAR (No. 333-78285)).

5.1             Opinion of Ropes & Gray.

15.1            PricewaterhouseCoopers LLP's Letter Re Unaudited Interim
                Financial Information.

23.1            Consent of PricewaterhouseCoopers LLP

23.2            Consent of Arthur Andersen LLP

23.3            Consent of Ropes & Gray (included in Opinion filed as Exhibit
                5.1).

24              Power of Attorney.

                                      -6-
<PAGE>

                                  SIGNATURES

  Pursuant to the requirements of the Securities Act of 1933, the registrant has
duly caused this Registration Statement on Form S-8 to be signed on its behalf
by the undersigned, thereunto duly authorized, in the City of Boston,
Commonwealth of Massachusetts, on the 18th of August 1999.

                  NSTAR

                BY: /s/ Thomas J. May
                    __________________________________

                TITLE: Chairman of the Board,
                       Chief Executive Officer,
                       Treasurer and Trustee


  Pursuant to the requirements of the Securities Act of 1933, as amended, this
Registration Statement on Form S-8 has been signed below by the following
persons in the capacities stated below on the 18th of August 1999.


/s/ Thomas J. May          Chairman of the Board,
------------------------   Chief Executive Officer,
THOMAS J. MAY              Treasurer, and Trustee



/s/ Russell D. Wright      President, Chief Operating Officer,
------------------------   Clerk, and Trustee
RUSSELL D. WRIGHT

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