Homeowners and businesses in the Toronto area are being asked to reduce their energy consumption as the city bakes in the midst of a heat wave.

The Independent Electricity System Operator is asking residents to conserve energy during peak times over the next two days to prevent brownouts.

The mercury was expected to rise into the mid-30s with humidex readings reaching mid-40s on Tuesday, the hottest day of the year, CTV's Tom Brown said.

The city has issued its first extreme heat alert of the year, while a smog advisory remains in effect for Tuesday.

An extreme heat alert is issued when the high temperatures are such that the chance of additional deaths -- above those that are typically experienced during the same time of year -- is more than 90 per cent.

The record for June 26 was 36.1C, set in 1949, Brown said.

The city has opened up cooling centres around the city. For a complete list, click here.

"A cooling centre is a place where people can come in to get out of the heat," said Elaine Smyer, manager of emergency services."

"We have water, fruit juice and Red Cross volunteers to help provide information on how to cope with this oppressive heat.''

Residents should avoid consuming alcoholic drinks, caffeine and sugar, officials say.

Consumers are being asked to limit the use of dishwashers, washers and dryers between 12 p.m. and 8 p.m. and to turn off non-essential lights.

Air conditioners should be set to 26 degrees or higher and curtains and blinds should be closed to keep out the sun.

"What we need people to do is cut back," said Blair Peberdy, of Toronto Hydro. "It'll relieve pressure on the distribution system and avoid localized power outages."

The IESO says province-wide electrical demand as of 10 a.m. was 22,109 megawatts and expected Tuesday's demand to peak at 25,748 megawatts by 5 p.m.

Toronto Hydro recommends:

• Using fans instead of air conditioners;
• Using a clothesline to dry clothing;
• Setting pool heater thermostats back one degree;
• Washing laundry in cold water; and
• Unplugging secondary fridges.

Toronto Hydro is asking shops not to have their doors open if they are using their air conditioners.

Residents are also being urged not to leave young children and pets inside vehicles, and people with respiratory ailments should remain indoors and refrain from physical activity.

There is a high risk of thundershowers on Wednesday that could provide some relief from the blistering heat.

Thursday calls for a sunny and comfortable high of 25C, the normal for this time of year, while Friday was expected to hit 24C with plenty of sunshine.

With a report from CTV's Desmond Brown