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By Ka Yan Ng
TORONTO, Aug 18 (Reuters) - Ontario began its work week with the first real test of its fragile power supply on Monday and pleaded for business and the public to halve energy consumption as generators are restarted and build to full capacity following North America's biggest power blackout.
The Independent Electricity Market Operator, which regulates the provincial electricity grid, told Reuters that Ontario was capable of generating 19,700 megawatts of power on Monday, still short of the 21,900 megawatts the province would need on a typical weekday.
A hot summer day could lift demand to 24,000 megawatts, close to the province's normal generating capacity of 25,500 MW.
Temperatures were forecast to be in the mid-20s Celsius (mid-70s Fahrenheit) across the country's most populous province on Monday, but later this week could prove to be more demanding as the mercury rises to around 30 C (86 F) and demand for power jumps as air conditioners are switched back on.
Ontario Premier Ernie Eves, who declared a state of emergency following Thursday's widespread blackout, praised industry and commerce for co-operating with his request to reduce energy consumption by 50 percent.
He said major sectors such as automotive manufacturers, steel, mining, forestry, and retailing have helped, but warned there was no abundance of power and conservation efforts must be upheld.
"The challenge is still there. We need to have a reserve. We can't use every single megawatt of power that we have in case of a maintenance problem somewhere in the system," said Eves. "So we do need to conserve energy and we are starting off on a good foot. There's no doubt about that."
The IMO said its requires a reserve margin of at least 1,500 megawatts.
Automakers across southern Ontario agreed to scale back production to lower demand, but did not know whether production would be stepped up on Tuesday.
In Ottawa, most of the city's 55,000 federal civil servants were told not to come into work. Only essential staff at in the provincial government were working, as was the case in the city of Toronto.
But officials warned that the threat of rolling blackouts was not yet gone and conservation was key to maintaining a safe gap between supply and demand.
"If people continue their conservation efforts and really work hard to curtail power use, we should be able to meet demand without requiring rotating power outages," an IMO spokeswoman said.
Toronto's first rush hour since the lights went out began at 6 a.m. as subways and streetcars returned to service to begin transporting more than a million people to work.
The subway, a major power user, had been shut down since Thursday afternoon when tens of millions of people across the northeastern United States and Ontario were left without electricity. It remained off over the weekend on concerns that rolling blackouts could trap passengers in tunnels.
Buses were deployed to help replace the subway during the outage, but one train can carry about about 1,200 passengers, a number that would require 12 to 14 buses.
The city said Toronto Hydro had assured it would feed the 40 substations needed to operate the subway system and would notify the public before a blackout occurred.
As a result of the blackout three people died in Ontario and a fourth death from heat exhaustion is under investigation.