(a) General rule.--If, in any proceeding involving rates, the commission shall determine that any rate received by a public utility was unjust or unreasonable, or was in violation of any regulation or order of the commission, or was in excess of the applicable rate contained in an existing and effective tariff of such public utility, the commission shall have the power and authority to make an order requiring the public utility to refund the amount of any excess paid by any patron, in consequence of such unlawful collection, within four years prior to the date of the filing of the complaint, together with interest at the legal rate from the date of each such excessive payment. In making a determination under this section, the commission need not find that the rate complained of was extortionate or oppressive. Any order of the commission awarding a refund shall be made for and on behalf of all patrons subject to the same rate of the public utility. The commission shall state in any refund order the exact amount to be paid, the reasonable time within which payment shall be made, and shall make findings upon pertinent questions of fact.(b) Suit for refund.--If the public utility fails to make refunds within the time for payment fixed by any final order of the commission or court, any patron entitled to any refund may sue therefor and the findings and order made by the commission shall be prima facie evidence of the facts therein stated, and that the amount awarded is justly due the plaintiff in such suit, and the defendant public utility shall not be permitted to avail itself of the defense that the service was, in fact, rendered to the plaintiff at the rate contained in its tariffs in force at the time payment was made and received, nor shall the defendant public utility be permitted to avail itself of the defense that the rate was reasonable. Any patron entitled to any refund shall be entitled to recover, in addition to the amount of refund, a penalty of 50% of the amount of such refund, together with all court costs and reasonable attorney fees. No suit may be maintained for a refund unless instituted within one year from the date of the order of the commission or court. Any number of patrons entitled to such refund may join as plaintiffs and recover their several claims in a single action, in which action the court shall render a judgment severally for each plaintiff as his interest may appear.(c) Condition for suit.--No action shall be brought in any court for a refund, unless and until the commission shall have determined that the rate in question was unjust or unreasonable, or in violation of any regulation or order of the commission, or in excess of the applicable rate contained in an existing and effective tariff, and then only to recover such refunds as may have been awarded and directed to be paid by the commission in such order. 1978, July 1, P.L. 598, No. 116, § 1, effective in 60 days.