Cal. Pub. Util. Code § 12811

The board may provide for the collection of fees, tolls, rates, rentals, or other charges in any lawful manner and may provide for collection by action at law, and all remedies for the collection and enforcement thereof are cumulative and may be pursued alternatively or consecutively as the board determines. In addition to the amount of the fees, tolls, rates, rentals, or other charges, the board may provide for a penalty of not more than 10 percent or interest at the prevailing prime interest rate, but not to exceed 112 percent per month, or both, in the event of nonpayment within the time and in the manner prescribed by the board, and may provide for collection of the penalty and interest.