HUNDREDS of Oxfordshire residential customers of distribution giant Powergen may be paying too much for their electricity.o The problem affects anyone whose electricity bill was based on an estimate for the period that included the company's 18.4 per cent price increase in March, or its 9.7 per cent increase in August.

Many such people, who used more electricity in the cold snap in February and March than in the heatwave of July, are now finding bills landing on their doormats which have been averaged out for the entire period.

In an extreme case, a customer who ran up a bill at the old price and then left his house empty for the summer - using nil units - would still find himself paying two thirds of that bill at the higher, post-increase rate.

This happens because under the Powergen billing system customers' electricity units are simply added up and then divided by the total number of days involved, regardless of when the electricity was consumed.

A spokesman for watchdog Energywatch, Karl Brookes, said: "It's certainly not the first time this has been drawn to our attention.

"We have in the past taken swift action by referring this to the regulator, Ofgem, which has investigated this matter and satisfied us that it is more a case of mistakes rather than any fast turn of hand.

"Nevertheless, if consumers feel otherwise, we would urge them to get in touch with Energywatch and tell us what they know, so we can fight their corner."

Powergen spokesman Nick Sandham said: "In extreme cases customers should always call us to discuss their account as we will treat such cases sympathetically.

"In all cases, actual meter readings enable us to keep accounts up to date and based on actual energy usage."

All major electricity supppliers have increased prices in recent months. All point out that despite the current fall in wholesale prices - which they have to pay to their own suppliers - wholesale prices have risen by 87 per cent since January 2005 and companies have had to buy ahead (when prices were rising).

All advise customers to take an actual reading, rather than relying on estimates, whenever prices are increased, in order to avoid disputes.

nPower is due to increase prices by 9.9 per cent on October 1, having previously increased them by 13.3 per cent in April.