(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.