Up to 20,000 ESB customers are facing massive arrears bills due to an error with their meters.
The customers may have been undercharged by up to 25% because the company failed to adjust household coin meters to take account of price rises more than three years ago.
The ESB said it had yet to decide whether to ask the customers to pay the arrears.
Dermot Jewell of the Consumers’ Association said it would be unfair for the ESB to demand payment.
“This is the management’s fault, so they’re going to have to either absorb the cost or certainly extend extremely decent credit facilities to those who will suffer as a result of this,” he said.