About 120,000 Hydro-Québec clients are bracing themselves for a significant increase in monthly bills.

Hydro-Québec recently said one of its computer programs underestimated what many people should have paid in their equalized monthly bills between August 2006 and November 2007. The utility has recalculated for that period and is now sending out revised bills.

Hydro-Québec spokesman Guy Litalien said Monday the utility made a mistake but it will not consider letting customers off the hook.

“It's a matter of equity to [all of] our customers. We will not charge for the administration fees, but for the electricity that has been used we have to charge it,” said Litalien.

For example, if a customer paid $120 a month for electricity before August 2006, but only paid $100 for the same usage after that point, the customer will begin to receive a hydro bill of $145 a month for 12 months: an extra $20 a month to reflect the proper rate and an additional $25 per month for the unpaid usage.

After the owed amount is paid back, the customer's bill will revert to $120 a month if usage remains the same.

Customers are invited to call Hydro-Québec's customer service department if they need to make payment arrangements.