Wash. Rev. Code § 81.04.220
After a complaint is made to the commission concerning the reasonableness of any rate, fare, toll, rental or charge for any service performed by any public service company subject to regulation by the commission as to rates and service, and the complaint is investigated by the commission, and the commission determines both that the public service company has charged an excessive or exorbitant amount for the service and that any party complainant is entitled to an award of damages, the commission shall order the public service company to pay the complainant the excess amount found to have been charged, whether the excess amount was charged and collected before or after the filing of the complaint, with interest from the date of the collection of the excess amount.
RCW 81.04.220