According to the BBC consumer programme “Watchdog”, the gas and electric company Npower can’t calculate it customers bills properly

Npower claims that it will charge its gas customers the premium rate for only 4,572 units per year and the rest is charged at a cheaper rate.

John Stanley noticed in January that he had been charged the premium rate for more than 4,572 units in the previous year.

Npower’s explanation for this was that they don’t go by the calendar year but the tariff year, which starts whenever they raise their prices.

The company raised their prices this January, meaning the previous tariff year was only 9 months long.

It is worth checking your bills to see that you have not been overcharged. If you have, then complain about it to ensure that Npower understands that a “per year” means a 12 month year.

Npower is Britain's fourth-largest energy supplier. In Febrauary, it reported a 41 per cent increase in annual profit for last year to £544m, amid soaring customer bills.

It was also the first supplier to increase bills in 2008, raising gas by 17 per cent and electricity by 13 per cent.

According to energywatch, an independent gas and electricity watchdog, Npower attracts a lot more complaints than the average company.