Striking workers forced Ontario's biggest electricity utility to dramatically cut its output on Friday, prompting an official plea for reduced power usage as temperatures reached into the 30s.

Picket lines set up by Hydro One workers forced Ontario Power Generation to shut down six of eight generators at its Nanticoke station by early Friday evening.

By late afternoon, when four generators were down, the power grid had lost 2,000 megawatts of power – enough to supply half of Toronto's homes and buildings.

(CP file photo)

The Independent Electricity System Operator, which oversees the provincial electricity system, thought the reduction was serious enough to issue a warning of possible power outages.

It said the province expected to lose almost 4,000 megawatts of generation from the Nanticoke station – about its usual daily output.

"Certainly when you take this amount of generation out of the mix, then you're going to have an impact on the power system in Ontario," said Terry Young, an IESO spokesman.

The province was forced to replace the lost power by buying it from the United States, which could cost hundreds of thousands of dollars.

It said the risk of power outages was highest in the Toronto area, where temperatures soared to 34 C and felt like 43 because of the humidity. The city said it expected the heat to continue on Saturday.

The union behind the strike, the Society of Energy Professionals, said the workers are trying to force Hydro One to give them a fair deal.

"We wanted to have an impact," said a spokesman for the union, Kevin McCormick. "We don't want to cause a blackout by this and we won't."

The workers have been on strike since the beginning of June over Hydro One's plan to increase their hours without pay and other issues.