Wash. Rev. Code § 80.04.220
When complaint has been made to the commission concerning the reasonableness of any rate, toll, rental or charge for any service performed by any public service company, and the same has been investigated by the commission, and the commission has determined that the public service company has charged an excessive or exorbitant amount for such service, and the commission has determined that any party complainant is entitled to an award of damages, the commission shall order that the public service company pay to the complainant the excess amount found to have been charged, whether such excess amount was charged and collected before or after the filing of said complaint, with interest from the date of the collection of said excess amount.
RCW 80.04.220