With subways and streetcars set to resume service Monday in Canada's largest city, Toronto officials joined their provincial counterparts in calling for people to use as little electricity as possible.
"(Monday) the entire province will be faced with a possible shortfall of the power we need to return to normal," Mayor Mel Lastman said.
Ontario Premier Ernie Eves broadcast live on television to urge conservation.
"We currently do not have enough generation back on line to see us through a regular weekday," Eves said, noting Ontario would have the capacity to supply 20,500 megawatts Monday compared with the 23,000 megawatts or more it normally uses on a hot summer weekday.
Eves also complained that Canadian authorities were not notified about problems occurring in the interconnected power system ahead of the blackout, as required by a protocol established after a severe blackout in 1965.
"After the famous 1965 blackout, North American protocols were established to ensure that interconnected jurisdictions are notified immediately when problems develop in one province or state on the grid," Eves told reporters at a Toronto news conference. "For some reason, that did not happen this past Thursday."
Bruce Campbell, vice president of the Independent Electricity Marketing Operator that regulates Ontario's power, said the province can import power, as it did Saturday when it received help from Manitoba, Quebec, New York and Minnesota to prevent rolling blackouts.
Eves and other officials said the best tool for preserving the still-fragile electricity delivery system was for everyone to use as little power as possible.
"If people don't cooperate, the system will break," said Jim Young, Ontario's commissioner of public safety.
Eves said the federal and provincial governments would provide only essential services Monday as a way to cut power. Major industrial and manufacturing users agreed to reduce power consumption by as much as 50 percent in the coming week, he said, urging businesses and consumers to do the same.
"If you must use major appliances, such as washing machine, do so between 8 p.m. and 8 a.m.," Eves said in providing energy conservation tips.
Wash clothes in cold or warm water, because heating the water comprises 85 percent of a washing machine's energy needs, and air dry clothes, he said.
Turn off unnecessary lights and computers, and don't use the air conditioner unless absolutely necessary, Eves said.
Toronto officials decided to resume service on the public transit system, which transports more than 1 million people a day, despite the possibility of rolling blackouts in the city Monday.
"We have assurances from Toronto Hydro that it will continue to feed the 40 substations that are integral to the city's transit system," said Rick Ducharme, general manager of the Toronto Transit Commission.
"But we still have a situation out there with electrical systems, so be patient."