Demand is projected to peak on Tuesday at 27,225 MW, which prompted the provincial system operator to issue a power warning early in the day. The IESO asked all consumers and businesses to pitch in by turning off lights and reducing use of air conditioners and appliances.

"As of now, we do not need to escalate (the warning) and get into more severe actions like voltage reductions or rotating blackouts, but we'll have to see how the afternoon unfolds," IESO spokeswoman Lisa Pearson said.

The grid operator is relying on about 2,000 MW of imported power to keep the lines humming.

Some large industrial users in the province were prepared to temporarily interrupt operations to conserve power if necessary.

Falconbridge Ltd. could idle an electric furnace in Sudbury and a zinc plant at its Kidd metallurgical site in Timmins, said Lauri Gregg, director of energy management at the nickel miner.

"We're there, ready to interrupt our load if the IESO calls for it," Gregg said. "But actually the situation is pretty good at this point, all the generating units are on and the imports are coming in," he added.

As eastern North America bakes in searing heat, Toronto's medical officer of health extended an extreme-heat alert.

The temperature in Toronto hit 36 degrees Celsius (97 Fahrenheit) by afternoon, according to Environment Canada. However, high humidity levels made it feel like 46C (115F).

While the grid is under stress, "the good news is the power system is in much better shape than last year right now," said Tom Adams of Energy Probe, a veteran electricity market watcher. "They're not digging deep into their toolbox of emergency measures just yet," he said of the IESO.

Although electrical loads are comparable to last year, additional supply is the key factor helping Ontario meet demand this summer, Adams said.

There's a "big difference" in water levels, leading to better hydroelectric generating capability, while the return to service of Pickering nuclear station's 515-MW unit 1 has lent a major helping hand, Adams said.

But the supply-demand balance is tight. The IESO warned in its statement that a reduction in transmission or generation availability, or any increase in demand, could create shortfalls that could lead to "protective actions" such as a system voltage reduction or rotating power cuts.

Consumers can help avoid that by setting air conditioners to 26C (79F) or higher, and using fans as an alternative, the IESO said. It suggested drawing curtains and blinds to keep out sunlight, and avoiding the use of dishwashers, washers, dryers and swimming pool pumps until 8 p.m. EDT.

Ontario used to see its peak power demand during the winter heating season, but several years ago that shifted to summer because of rising demand for air conditioning.

"People often ask me what's the most important thing I can do, and by far it's the air conditioning," said Peter Love, an executive in charge of conservation efforts at the Ontario Power Authority.