Are you sure, Mary?
I've experienced that with BG, Npower and Powergen, sometimes to the tune of 20% overpayment over the billing year.
I'm not in the business of lending money to utility companies - especially not at zero interest.
I therefore have two simple rules
1) I don't allow them to set the direct debit amount or to change it. There are a few who will do so as a matter of course. I won't do business with them.2) I calculate the next year's usage as being the same as the past year's usage, apply any price increases and work out a projected cost. I then offer them no more than 85% of that spread over the year, thus pretty much guaranteeing that I owe them money at the end of the year. Finally, I carry forward the shortfall into the next year and factor that into the cost and offer no more than 85% again.
I sometimes get squeaks from them and they have to go and get approval from the supervisor. A couple of times I've had to discuss with the supervisor. However, they are often on bonuses or targets for customer retention. I only once met with a refusal to co-operate, and that company lost the business.