66 Pa. C.S. § 1509
All bills rendered by a public utility as defined in paragraph (1)(i), (ii), (vi) or (vii) of the definition of "public utility" in section 102 (relating to definitions) to its service customers, except bills for installation charges, shall allow at least 15 days for nonresidential customers and 20 days for residential customers from the date of transmittal of the bill for payment without incurring any late payment penalty charges therefor. All customers shall be permitted to receive bills monthly and shall be notified of their right thereto. All bills shall be itemized to separately show amounts for basic service, Federal excise taxes, applicable State sales and gross receipts taxes, to the extent practicable, fuel adjustment charge, if any, State tax adjustment charge or such other similar components of the total bill as the commission may order. Any electric or gas public utility billing customers on a bimonthly or quarterly basis and rendering interim statements or bills each month shall include in such interim statement or bill an amount for the fuel adjustment charge based upon one-half of the total expected bimonthly kilowatt hour or cubic foot billing or one-third of the total expected quarterly billing and using the fuel adjustment charge rate applicable in the month of the interim statement or bill. At the time of preparing the bimonthly or quarterly bill, an appropriate adjustment shall be made in the total fuel adjustment charge billing for the period. Any public utility rendering bills on a bimonthly basis or quarterly basis shall calculate the fuel adjustment charge per kilowatt hour or cubic foot for the entire period as the weighted average of the two monthly rates or the three monthly rates whichever is applicable.
66 Pa.C.S. § 1509