At any hearing on the application of a public utility for a change in a rate, toll, charge or schedule, the burden of proof to show that the proposed change is just and reasonable, shall be upon the public utility. The Commission shall be authorized to prescribe all necessary rules and regulations for the conduct of such hearings which shall provide for full and fair participation in such hearings by any interested person subject to such guidelines as the Commission may deem appropriate. Upon the conclusion of such hearings, the Commission shall issue an order and such opinion as is necessary to set forth fully the Commission’s findings of fact and conclusions of law. Copies of the transcripts of public hearings held to establish a fair rate of return and changes in rates, tolls and charges for investor-owned public utilities involving significant public interest shall be placed in no less than one location nor more than three locations in the geographic area served by the utility. The Commission shall determine which proceedings are of sufficient interest to require the placing of such transcripts and the location or locations to be used; provided, however, that proceedings involving investor-owned utilities serving 25,000 or more customers shall be deemed to be of sufficient public interest.