52 Pa. Code § 56.22 : PUBLIC UTILITIES — PUBLIC UTILITY COMMISSION — FIXED SERVICE UTILITIES — STANDARDS AND BILLING PRACTICES FOR RESIDENTIAL PUBLIC UTILITY SERVICE — BILLING AND PAYMENT STANDARDS —PAYMENTS — Accrual of late payment charges

(a) Every public utility subject to this chapter is prohibited from levying or assessing a late charge or penalty on any overdue public utility bill, as defined in § 56.21 (relating to payment), in an amount which exceeds 1.5 % interest per month on the overdue balance of the bill. These charges are to be calculated on the overdue portions of the bill only. The interest rate, when annualized, may not exceed 18% simple interest per annum.
(b) An additional charge or fixed fee designed to recover the cost of a subsequent rebilling may not be charged by a regulated public utility.
(c) Late payment charges may not be imposed on disputed estimated bills, unless the estimated bill was required because public utility personnel were willfully denied access to the affected premises to obtain an actual meter reading.
(d) A public utility may waive late payment charges on any customer accounts. The Commission may only order a waiver of late payment charges levied by a public utility as a result of a delinquent account for customers with a gross monthly household income not exceeding 150% of the Federal poverty level. See 66 Pa.C.S. § 1409 (relating to late payment charge waiver).

52 Pa. Code § 56.22