California Code, Public Utilities Code - PUC § 584

Every public utility shall furnishsuch reports to the commission at such time and in such form as the commission may require in which the utility shall specifically answer all questions propounded by the commission.  The commission may require any public utility to file monthly reports of earnings and expenses, and to file periodical or special reports, or both, concerning any matter about which the commission is authorized by any law to inquire or to keep itself informed, or which it is required to enforce.  All reports shall be under oath when required by the commission.