Wash. Rev. Code § 35.97.060
A municipality may impose rates, charges, or rentals for heat, service, and facilities provided to customers of the system if the rates charged are uniform for the same class of customers or service. In classifying customers served or service furnished, the legislative authority may consider: The difference in cost of service to the various customers; location of the various customers within or without the municipality; the difference in cost of maintenance, operation, repair, and replacement of the various parts of the system; the different character of the service furnished various customers; the quantity and quality of the heat furnished; the time heat is used; the demand on the system; capital contributions made to the system including, but not limited to, assessments or the amount of capital facilities provided for use by the customer; and any other matters which present a reasonable difference as a ground for distinction.
RCW 35.97.060