Section 57. At the beginning of each fiscal year, the manager of municipal lighting shall furnish to the mayor, selectmen or municipal light board, if any, an estimate of the income from sales of gas and electricity to private consumers during the ensuing fiscal year, and of the expense of the plant during said year, meaning the gross expenses of operation, maintenance and repair, the interest on the bonds, notes or certificates of indebtedness issued to pay for the plant, an amount for depreciation equal to three per cent of the cost of the plant exclusive of land and any water power appurtenant thereto, or such smaller or larger amount as the department may approve, the requirements of the sinking fund or debt incurred for the plant, and the loss, if any, in the operation of the plant during the preceding year, and of the cost, as defined in section fifty-eight, of the gas and electricity to be used by the town. The town shall include in its annual appropriations and in the tax levy not less than the estimated cost of the gas and electricity to be used by the town as above defined and estimated. By cost of the plant is intended the total amount expended on the plant to the beginning of the fiscal year for the purpose of establishing, purchasing, extending or enlarging the same. By loss in operation is intended the difference between the actual income from private consumers plus the appropriations for maintenance for the preceding fiscal year and the actual expense of the plant, reckoned as above, for that year in case such expenses exceeded the amount of such income and appropriation. The income from sales and the money appropriated as aforesaid shall be used to pay the annual expense of the plant, defined as above, for the fiscal year, except that no part of the sum therein included for depreciation shall be used for any other purpose than renewals in excess of ordinary repairs, extensions, reconstruction, enlargements and additions. The surplus, if any, of said annual allowances for depreciation after making the above payments shall be kept as a separate fund and used for renewals other than ordinary repairs, extensions, reconstructions, enlargements and additions in succeeding years, and for the cost of plant, nuclear decommissioning costs, the costs of contractual commitments, and deferred costs related to such commitments which the city council, the board of selectmen, or the municipal light board, if any, determines are above market value. Said depreciation fund shall be kept and managed by the town treasurer as a separate fund, subject to appropriation by the city council or selectmen or municipal light board, if any, for the foregoing purpose. Upon his own initiative or upon the request of the city council, selectmen or municipal light board, the treasurer shall invest or deposit the same as permitted by section fifty-five A of chapter forty-four, and any income thereon shall be credited to the depreciation fund. So much of said fund as the department may from time to time approve may also be used to pay notes, bonds or certificates of indebtedness issued to pay for the cost of reconstruction or renewals in excess of ordinary repairs, when such notes, bonds or certificates of indebtedness become due. All appropriations for the plant shall be either for the annual expense defined as above, or for extensions, reconstruction, enlargements or additions; and no appropriation shall be used for any purpose other than that stated in the vote making the same. No bonds, notes or certificates of indebtedness shall be issued by a town for the annual expenses as defined in this section.