As a result, some 6,700 Hydro Ottawa customers underpaid an average of $740, and are receiving bills much higher than usual, the Ottawa Citizen reported.

However, the reconciliation process also found 5,700 customers had overpaid their bills, and a total of $2.9 million was refunded, the utility said.

The equal-billing system is supposed to average seasonal peak and low usage and smooth out payments, and was intended to be audited each year for anomalies, the newspaper said.

For customers getting billed extra, Alex Cullen, a city councilor who also sits on Hydro Ottawa's board of directors, said it was particularly difficult for some people on fixed incomes.

"They have caused angst amongst Hydro ratepayers, particularly seniors," Cullen said. "They will tell you ... this was not their finest hour."