Cal. PUC § 314

(a) The commission, each commissioner, and each officer and person employed by the commission may, at any time, inspect the accounts, books, papers, and documents of any public utility. The commission, each commissioner, and any officer of the commission or any employee authorized to administer oaths may examine under oath any officer, agent, or employee of a public utility in relation to its business and affairs. Any person, other than a commissioner or an officer of the commission, demanding to make any inspection shall produce, under the hand and seal of the commission, authorization to make the inspection. A written record of the testimony or statement so given under oath shall be made and filed with the commission.

(b) Subdivision (a) also applies to inspections of the accounts, books, papers, and documents of any business that is a subsidiary or affiliate of, or a corporation that holds a controlling interest in, an electrical, gas, or telephone corporation, or a water corporation that has 2,000 or more service connections, with respect to any transaction between the water, electrical, gas, or telephone corporation and the subsidiary, affiliate, or holding corporation on any matter that might adversely affect the interests of the ratepayers of the water, electrical, gas, or telephone corporation.