Hydro rates are going up this summer, the Ontario Energy Board announced Thursday, a move that will add about $3 a month to the average household bill.

Time-of-use electricity rates have to go up because Ontarians used less power than expected during the milder winter, the OEB said.

The total monthly bill will rise 2.5 per cent for households that consume 750 kilowatt hours per month, the agency said.

Time-of-use prices will got up a half cent to 18 cents per kilowatt hour for the peak period, which runs from 11 a.m. to 5 p.m. on weekdays.

The off-peak rate, which is in effect between 7 p.m. and 7 a.m. and all day on weekends and holidays, is going up to 8.7 cents a kwh.

The changes go into effect on May 1.

"Ontarians consumed less electricity than expected over the recent milder winter," said a statement from the OEB.

"As a result of lower usage, Regulated Price Plan (RPP) prices did not recover the full cost of serving RPP customers. One of the main reasons prices are increasing in May is to recover this shortfall."

Time-of-use prices vary based on when electricity is used during the day. They encourage consumers to use power when electricity market prices are lower.

The OEB reviews electricity prices twice each year based on updated cost forecasts, and prices are designed to recover the actual cost of electricity over the forecast period.