Every public utility shall furnish annually to the commission, at such time and in such form as the commission may require, a report in which the utility shall answer specifically all questions propounded by the commission upon or concerning which the commission may desire information to carry into effect the provisions of this title. The commission shall have the authority to require any public utility to file periodical or special reports concerning any matter about which the commission is authorized by this title to inquire or to keep itself informed, or which it is required to enforce. The reports shall be under oath when required by the commission.