Wash. Rev. Code § 81.04.350
The commission may after hearing require any public service company subject to regulation by the commission as to rates and service to carry proper and adequate depreciation or retirement accounts in accordance with such rules, regulations, and forms of accounts as the commission may prescribe. The commission may from time to time ascertain and by order fix the proper and adequate rates of depreciation or retirement of the several classes of property of each public service company. Each public service company shall conform its depreciation or retirement accounts to the rates so prescribed. In fixing the rate of the annual depreciation or retirement charge, the commission may consider the rate and amount theretofore charged by the company for depreciation or retirement.
The commission may exercise like power and authority over all other reserve accounts of public service companies.
RCW 81.04.350