Cal. Rev. & Tax. Code § 8108

If any claim for refund of tax is paid more than 20 calendar days from the date upon which the claim was received by the Controller, interest shall be computed, allowed, and paid upon that refund of tax at the Pooled Money Investment Account’s Annual Yield Rate in effect on the date prior to the date that the interest on the refund of tax begins to accrue. Interest on the refund of tax shall begin to accrue on the 21st calendar day after the date that a properly completed claim for refund is received by the Controller. The interest shall accrue through the date the Controller refunds the tax. No interest shall be granted on credits taken on tax returns.