Wash. Rev. Code § 81.20.020
Whenever the commission in any proceeding upon its own motion or upon complaint shall deem it necessary in order to carry out the duties imposed upon it by law to investigate the books, accounts, practices and activities of, or make any valuation or appraisal of the property of any public service company, or to investigate or appraise any phase of its operations, or to render any engineering or accounting service to or in connection with any public service company, and the cost thereof to the commission exceeds in amount the ordinary regulatory fees paid by such public service company during the preceding calendar year or estimated to be paid during the current year, whichever is more, such public service company shall pay the expenses reasonably attributable and allocable to such investigation, valuation, appraisal or services. The commission shall ascertain such expenses, and, after giving notice and an opportunity to be heard, shall render a bill therefor by registered mail to the public service company, either at the conclusion of the investigation, valuation, appraisal or services, or from time to time during its progress. Within thirty days after a bill has been mailed such public service company shall pay to the commission the amount of the bill, and the commission shall transmit such payment to the state treasurer who shall credit it to the public service revolving fund. The total amount which any public service company shall be required to pay under the provisions of this section in any calendar year shall not exceed one percent of the gross operating revenues derived by such public service company from its intrastate operations during the last preceding calendar year. If such company did not operate during all of the preceding year the calculations shall be based upon estimated gross revenues for the current year.
RCW 81.20.020