Cal. Water Code § 72072.12

Notwithstanding the provisions of Section 72072.11, a service supplier may claim a refund or take as credit against taxes collected and remitted the amount overpaid, paid more than once, or erroneously or illegally collected or received when it is established in a manner prescribed by the board that the service user from whom the tax has been collected did not owe the tax; provided, however, that neither a refund nor a credit shall be allowed unless the amount of the tax so collected has either been refunded to the service user or credited to charges subsequently payable by the service user to the person required to collect and remit.