Direct debit payments are set by evenly spreading a year's consumption into monthly payments. This enables you to budget as the figure remains the same month by month across the year. This does mean however that during periods of high usage (winter months ) your account will show an outstanding balance but will show a credit during periods of low usage. The aim is to arrive at a 0 balance at the end of the year.
Each time we produce a statement we look at the annual consumption that results from the readings used on the statement we compare this with the annual consumption used to derive the annual cost - if there has been a change - usage is less then we will lower subsequent payments. and if there has been an increase we will increase subsequent payments. You may also make an additional payment to keep subsequent payments the same.