(a) Accounts.--Every public utility shall carry on its books or records of account, proper and reasonable sums representing the annual depreciation on its property used or useful in the public service, which sums shall be based upon the average estimated life of each of the several units or classes of depreciable property. The commission, by appropriate order, after hearing, shall, except where found to be inappropriate, establish for each class of public utilities, the units of depreciable property, the loss upon the retirement of which shall be charged to the depreciation reserve.

(b) Statements.--Every public utility shall file with the commission, at such times and in such form as the commission may prescribe, statements setting forth the details supporting its computation of annual depreciation, as recorded on the books or records of accounts of the public utility. If the commission, upon review of such statements, is of the opinion that the amount of annual depreciation so recorded by any public utility is not reasonable and proper, it may, after hearing, require that provision be made for annual depreciation in such sums as may be found by it to be reasonable and proper. In making its findings, the commission shall give consideration to the experience of the public utility, and the predecessors of the public utility in accumulating depreciation reserves, the retirements actually made, and such other factors as may be deemed relevant.

(c) Use of estimates.--The commission shall not be bound in rate proceedings to accept, as just and reasonable for rate-making purposes, estimates of annual depreciation established under the provisions of this section, but in such rate proceedings it shall give consideration to statements submitted under this section, in addition to such other factors as may be relevant.

Cross References. Section 1703 is referred to in section 1706 of this title.