Advance Billing

I have a different issue then usual .

My client was using annual bill for customer and they ware in advance Billing procedure .Now they want to be in monthly billing. But they want consumption charge should be monthly and service charge should be calculate in advance for the year. They decide that service charge should be calculate in May for the next Year after that only consumption charge should be come into bill.

I set up the advance billing but in May it is calculating advance charge only for the next month not year ( as the Meter reading order is for the next month ) . So anybody can guide me how to calculate it for the year.

Thanks,

Khurram