California Code, Public Utilities Code - PUC § 453.5

Whenever the commission orders rate refunds to be distributed, the commission shall require public utilities to pay refunds to all current utility customers, and, when practicable, to prior customers, on an equitable pro rata basis without regard as to whether or not the customer is classifiable as a residential or commercial tenant, landlord, homeowner, business, industrial, educational, governmental, nonprofit, agricultural, or any other type of entity.

For the purposes of this section, “equitable pro rata basis” shall mean in proportion to the amount originally paid for the utility service involved, or in proportion to the amount of such utility service actually received.

Nothing in this section shall prevent the commission from authorizing refunds to residential and other small customers to be based on current usage.