Wash. Rev. Code § 35.23.535
No taxes shall be imposed for maintenance and operating charges of city owned water, light, power, or heating works or systems.
Rates shall be fixed by ordinance for supplying water, light, power, or heat for commercial, domestic, or irrigation purposes sufficient to pay for all operating and maintenance charges. If the rates in force produce a greater amount than is necessary to meet operating and maintenance charges, the rates may be reduced or the excess income may be transferred to the city's current expense fund.
Complete separate accounts for municipal utilities must be kept under the system and on forms prescribed by the state auditor.
The term "maintenance and operating charges," as used in this section includes all necessary repairs, replacement, interest on any debts incurred in acquiring, constructing, repairing and operating plants and departments and all depreciation charges. This term shall also include an annual charge equal to four percent on the cost of the plant or system, as determined by the state auditor to be paid into the current expense fund, except that where utility bonds have been or may hereafter be issued and are unpaid no payment shall be required into the current expense fund until such bonds are paid.
RCW 35.23.535