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The recent Times Money article that uncovered flaws in npower's billing system has struck a chord with readers. Many have contacted us to say that they think that they are among the 2.2 million npower gas customers who Energywatch, the energy watchdog, believes could have been overcharged for their fuel.

Ronen Orr, of Borehamwood, Hertfordshire, says: “Thanks to the article in Times Money I have checked my gas usage for last year. It was my understanding that I would be charged a maximum of 4,572 units at the higher rate. In reality, for 2007 plus a few days in December 2006, I was charged 8,924 units at the higher rate.

“I have now written to npower to ask for an immediate refund of all overcharged amounts since the start of my account with the company. I have also now left npower and will do all I can to warn my colleagues and friends about what I regard as the company's unfair tactics.”

Geoffrey Wheater, of Beaconsfield, Buckinghamshire, says: “I was delighted to read the article about npower. I have been fighting the company over a bill since September, when I believe that I was overcharged by between £17.50 and £23 for the year to mid-August. Subsequently, I reckon that I was overcharged by £48 for the six months to mid-February this year.

“I complained to npower in September but have had no joy. My two most recent letters, in March and April, have gone unanswered.”

The billing problem hinges on the question of how many units of gas npower can charge its customers each year at the higher of its two rates. Customers say that npower should not exceed the annual figure of 4,572 units, which it quotes in its literature and on its website.

However, npower says that its promise not to charge more than 4,572 units at the higher rate refers to “a tariff year”, which is not always the same length as a calendar year. The company starts a new tariff year each time that it changes its tariffs, which happened twice in 2007, resulting in a tariff year of only seven months from April to November.

Over those seven months npower also suspended its seasonal weighting temporarily. This weighting is designed to charge more units at the higher rate in winter and fewer in summer. The consequence was that customers were charged more higher-rate units than normal during the summer, but the same number as they would normally be charged in the winter. Npower accepts that this resulted in some customers being charged more than 4,572 units at the higher rate over a full calendar year, but not over the seven-month tariff year.

An npower spokeswoman says: “We accept that Mr Orr and Mr Wheater have both been charged more than 4,572 units of gas at the higher rate in a calendar year. However, we think this is counterbalanced by our price reductions, which took place at the same time. Overall we think they are probably better off.”