The ESB received over 226 unresolved complaints last year, an increase of 4% on 2000, according to an annual report by its complaints watchdog published today.
The Customer Complaints Arbitrator's findings also showed payments and account credits to customers were up by a third on figures for the previous year.
"That number of complaints is absolutely miniscule when it's related to the scale of ESB activities," said ESB arbitrator Denis Henderson.
"In the customer supply area, there are nine million bills issued every year, and twelve million payments. So really the level of unresolved complaints that comes through to us is quite small."