(a) Billing of net energy metering customers shall be on a monthly basis; provided that the last monthly bill for each twelve-month period shall reconcile for that twelve-month period the net electricity provided by the electric utility with:
(1) The electricity generated by the eligible customer-generator and fed back to the electric grid over the monthly billing period; and
(2) Any unused credits for excess electricity from the eligible customer-generator carried over from prior months since the last twelve-month reconciliation period.
(b) Credits for excess electricity from the eligible customer-generator that remain unused after each twelve-month reconciliation period may not be carried over to the next twelve-month period.