If the utility has willfully overcharged any customer, the utility shall refund the difference, plus interest, as stated in the South Carolina Code of Laws Section 34-31-20(A) or such future Code of Laws of South Carolina governing the legal rate of interest of cases of accounts stated and in all cases where any sum or sums of money shall be ascertained and shall draw interest according to law for the period of time that can be determined that the customer was overcharged.
S.C. Code Regs. § 103-733.4