TORONTO (CP) - Thousands of Canadian autoworkers had their hours cut or were told to stay home Monday as the Big Three automakers and businesses across Ontario cut their electrical usage by up to half to help the province cope with the aftermath of last week's blackout.

While the auto plants sat idled, offices dimmed lights, slowed elevators and increased building temperatures to lessen their reliance on power hungry air conditioners, to answer a plea by the Ontario government to conserve power while the system is brought back online.

At GM Canada, the largest of the Big Three and one of Ontario's biggest private employers, 11,500 workers or roughly half the company's workforce was affected as the vehicle maker moved to cut power usage at seven of its nine plants - including stopping production at its huge Oshawa, Ont., car plants.

"In some cases, it's people who won't work during the day but their shift time has changed to run through the middle of the night, because what we're doing in some plants is shifting the production to off-peak hours," GM spokesman Stew Low said Monday.

"We're doing our best to run some production at critical facilities between 8 p.m. and through the night to 8 a.m."

The company continued normal operations at its Oshawa truck plant and a components plant in St. Catharines that supplies parts to plants in the United States.

Viacom Outdoor Canada, the province's largest outdoor advertiser, cut its power consumption by turning off the lights on billboards and other advertising one by one across the province.

"It's a flip of about 10,000 switches because each one of these units is independently wired, so the actual physical act once you get to the location is not that complicated, its just the getting around town and around the province," Viacom's Nick Arakgi said.

Ontario officials asked business and the public to conserve energy this week as the power system remains several days away from returning to full strength.

Ontario's Independent Electricity Market Operator said capacity Monday was expected to reach 19,700 megawatts with users cutting back. By 4 p.m. ET, overall demand had climbed to 18,060 MW.

Normal consumption would be 22,000 to 24,000 MW.

Officials have warned that shortages could result in rotating power outages across the province. The IMO also has the ability to shut off power to workplaces not complying with the province's request to conserve energy.

It's too early to tell how much of an impact the power blackout will have on Canada's economy, which produced an estimated $1.1 trillion in goods and services in 2002.

CIBC World Markets economist Warren Lovely had forecast the Canadian economy to grow at an annualized rate of around three per cent in the third quarter, but he revised that estimate downward Monday because of the power outage and the ongoing threat of rolling blackouts.

"Its pretty clear that you've got a negative economic shock to the month of August and as a result to the third to the third quarter," Lovely said.

However despite the blackout, Lovely expected growth in the third quarter would still be greater than the second when it was either flat or slightly negative.

Andrew Pyle, senior economist at Scotiabank, suggested the blackout wouldn't have a huge impact unless the threat of rolling blackouts continued for an extended period of time or another major blackout occurred.

To cut back on their electricity usage, retailers such as Hudson's Bay and Sears Canada raised building temperatures, turned off outdoor signs and exterior lighting, shut off certain display lighting and the wall of TVs. Their head offices also reduced power consumption.

At Sears, its Ontario distribution centres and call centres ran off generators if possible during peak daytime hours instead of pulling power from the hydro grid. Canadian Tire also used generators at its Brampton, Ont. distribution centre and kept its head office operations to a minimium.

Grocery chain Loblaws cut back on lighting in its stores and its offices and turned off its neon store signs.

At DaimlerChrysler Canada, the company shut down its assembly plant and two research labs in Windsor, affecting 6,300 workers Monday. The company's Brampton, Ont., assembly plant and trim plants in Ajax, Ont., resumed regular production.

DaimlerChrysler expected to run its Windsor assembly plant overnight during off-peak hours of power use.

And in Hamilton, steelmaker Stelco Inc. began "very minor production" on Monday as it stabilized its systems after the emergency shutdown Thursday when the lights went out.

"We had things that just simply weren't making anything and now we've got them to the point where we've got them stabilized and starting to make a little bit, but we're not anywhere near the kinds of levels of production we'd typically have," Stelco spokesman Tim Huxley said.