66 Pa. C.S. § 505
Every public utility shall furnish to the commission, from time to time, and as the commission may require, all accounts, inventories, appraisals, valuations, maps, profiles, reports of engineers, books, papers, records, and other documents or memoranda, or copies of any and all of them, in aid of any inspection, examination, inquiry, investigation, or hearing, or in aid of any determination of the value of its property, or any portion thereof, and shall cooperate with the commission in the work of the valuation of its property, or any portion thereof, and shall furnish any and all other information to the commission, as the commission may require, in any inspection, examination, inquiry, investigation, hearing, or determination of such value of its property, or any portion thereof.
66 Pa.C.S. § 505