N.J. Stat. § 48:3-2.1
A public utility shall pay or credit interest at a rate prescribed by the Board of Public Utilities on any overpayment made by a residential customer due to a billing error, unless the overpayment is fully refunded or credited to the customer's account within two billing cycles after written notification by the customer to the utility of the alleged error. For the purposes of this act, "billing error" means a charge to a residential customer in excess of that approved by the board for the type of service supplied to that customer, or in excess of the charge due for the service supplied to that customer as measured or recorded by meter or other device, except that neither the amount of an estimated bill in and of itself, nor the amount due on a budget account installment, shall constitute a billing error. The board shall set and from time to time revise the interest rate in accordance with appropriate prevailing marketplace interest rates.
N.J.S. § 48:3-2.1