Energy providers have been accused of over-charging millions of customers whose bills are estimated, a consumer watchdog has said. Consumer rights organisation Consumer First said that the one in three households who pay estimated bills could be charged the price their energy is worth at the end of the billing period rather than what it costs at the time they use it. Because of recent energy price hikes, rocketing more than 30 per cent in some areas, this is often significantly more than at the time it was used. Robert Hammond, of Consumer Focus, said: "Consumers could be paying higher prices for gas or electricity used in the past, which for companies makes a very nice windfall." Consumer organisation Which agreed that energy companies should be doing more to simplify billing and to keep their customers informed. John Holmes, principal economist for Which, said: "There's no obligation to tell you before an energy price rise happens, just that within 65 working days it's gone up. "It's like buying something from a shop and them coming back to you weeks later and saying 'the price has gone up, you owe us more money'." However, the Energy Retail Association (ERA), who represents the major electricity and gas suppliers in the UK, denied that companies are profiteering from energy price rises. A spokesman said: "Your bill tends to be the main point of contact with your energy company, so energy suppliers are very aware of the crucial importance of sending out correct bills that are easy to understand." The ERA said it is currently trialling an electronic smart meter reader system that could end the need for customers to check their own meters.