Wash. Rev. Code § 81.16.030
In any proceeding, whether upon the commission's own motion or upon complaint, involving the rates or practices of any public service company, the commission may exclude from the accounts of the public service company any payment or compensation to an affiliated interest for any services rendered or property or service furnished, as described in this section, under existing contracts or arrangements with the affiliated interest unless the public service company establishes the reasonableness of the payment or compensation. In the proceeding the commission shall disallow the payment or compensation, in whole or in part, in the absence of satisfactory proof that it is reasonable in amount. In such a proceeding, any payment or compensation may be disapproved or disallowed by the commission, in whole or in part, if satisfactory proof is not submitted to the commission of the cost to the affiliated interest of rendering the service or furnishing the property or service described in this section.
RCW 81.16.030