Cal. Rev. & Tax. Code § 13563

(a) For purposes of determining interest on overpayments for periods beginning before July 1, 2002, interest shall be allowed and paid upon any overpayment of tax due under this part in the same manner as provided in Sections 6621(a)(1) and 6622 of the Internal Revenue Code.
(b) For purposes of determining interest on overpayments for periods beginning on or after July 1, 2002, interest shall be allowed and paid upon any overpayment of tax due under this part at the lesser of the following:
1. (1) Five percent.
2. (2) The bond equivalent rate of 13-week United States Treasury bills, determined as follows:
  1. (A) The bond equivalent rate of 13-week United States Treasury bills established at the first auction held during the month of January shall be utilized for determining the appropriate rate for the following July 1 to December 31, inclusive.
  2. (B) The bond equivalent rate of 13-week United States Treasury bills established at the first auction held during the month of July shall be utilized for determining the appropriate rate for the following January 1 to June 30, inclusive.
(c) For purposes of subdivision (b), in computing the amount of any interest required to be paid by the state, that interest shall be computed as simple interest, not compound interest. That interest shall be allowed from the date on which payment would have become delinquent, if not paid, or the date of actual payment, whichever is later in time, to the date preceding the date of the refund warrant by not more than 30 days, the date to be determined by the Controller.