66 Pa.C.S. § 2104

The commission may, by regulation or order, require any contract with an affiliated interest to be in writing. The commission may also, by regulation or order, require that any contract with an affiliated interest shall contain a provision whereby the affiliated interest shall agree to furnish to the public utility, at the time of billing such public utility for any service, property, security, right, or thing, under such contract, a detailed statement of the cost to the affiliated interest of such service, property, security, right, or thing.