The ESB has written to 3,000 customers telling them they have been overcharged for the last six years.

The company said the standing charge, which must be paid by all electricity users, had been incorrectly applied to some bills. The standing charge is the cost of supplying electricity to a premises. It is similar to a line-rental charge in the telecoms industry.

The company said a review of the opening up of the electricity market had disclosed that 3,000 customers were paying the wrong charge. It has agreed to pay each of the customers €300. It will cost the company about €900,000.

The ESB works out the standing charge on the basis of how many premises are supplied by local infrastructure. The standing charge for rural customers is traditionally higher than for urban customers.

The company last carried out a review of the standing charges in 1998. A spokesman said increased development had brought a lot of previously rural-based customers into the urban category.

In June, thousands of ESB token-meter customers unwittingly found themselves in arrears due to an error in adjusting for electricity price rises. It emerged that some of the ESB's 20,000 such customers throughout the State had been undercharged because pre-paid tokens did not take account of price-rises of up to 29 per cent since the deregulation of the electricity market in 2001.

It is understood the error emerged when a customer attempted to buy goods on credit at an ESB shop and was denied credit by the finance company because of his ESB arrears.