THE ESB says the payment of rebates to about 100,000 of its customers who were overcharged on estimated bills will be completed by the end of next month.
The utility is repaying a total of €3.5 million to customers after carrying out a review of its billing practices ordered by the Commission for Energy Regulation (CER).
The review arose as a result of a complaint made in 2007 by Independent Senator Shane Ross, who complained that customers were being overcharged at a time of frequent price increases.
An ESB spokeswoman said the review had identified issues with customers who received an estimated bill on either side of a price increase.
There were three price increases during the period examined, from January 2005 to November 2007.
At the time, the company adopted the practice of charging the price obtaining on the day a customer provided a reading. However, this meant that customers who had been billed on the basis of estimated readings were charged more, particularly when a series of estimated readings occurred.
Under an agreement with CER, the ESB is allowed issue bills based on estimated readings twice a year, out of a total of six bills. However, further estimated bills may be issued where meter readers experience problems with access.
The ESB spokeswoman said the company had used software to calculate whether each of the affected customers would have paid more or less if a pro rata approach had been taken to tariffs on either side of a price increase.
This would mean, for example, in the case of a bill for January/February with a price increase on February 1st, that about half of the electricity used would be calculated at the old rate, and half at the new one.
Where the revised approach would result in the customer paying more, no action was being taken, the spokeswoman said. Where it showed a lower bill, the rebate was being made for amounts over €5.
The average payment is €13, but some customers are receiving rebates of several hundred euro.
The spokeswoman said the billing system had been changed and the issue would not arise again.
She defended the previous approach, saying it was "neither right nor wrong" and pointed out that customers benefited under this method of calculation whenever tariffs dropped.
Affected customers have been issued with explanations along with the rebate, which also affected 14,000 business customers.