Edmonton, Calgary, Red Deer and Lethbridge have been hit by rolling blackouts because of heavy demand on the power grid caused by the first heat wave of the summer.

Hot and humid weather Monday pushed demand on the province's electrical system to record highs, according to a news release from the Alberta Electric System Operator (AESO).

Doug Simpson, director of market operations for AESO, said six power generators went off-line Monday, and those shutdowns combined with heavy demand led to rolling blackouts across the province.

"We see multiple outages from time to time, that's not in itself unusual," Simpson said.

"To see the high temperature and the high load combined with six, the number six, is a little bit unusual. We made a request to utilities to reduce their demand and that's very unusual. We haven't done that since 2006."

AESO will now review what happened. However, the cause of what Simpson called "an unfortunate coincidence" won't be released to the public.

"As you know we have a competitive market and it wouldn't be appropriate for me to be explaining the problems that they have with their generators here," he said.

EPCOR issued a notice just before 6 p.m. MT that most power was been restored in Edmonton. The blackouts ended a couple of hours earlier in Calgary, according to Mayor Naheed Nenshi.

Enmax told Nenshi that heat pushed up demand for power. There was "great concern that would bring the entire power grid down," Nenshi told CBC Radio in Calgary.

AESO made a public appeal for everyone in the province to reduce their electricity usage until 6 p.m. Monday.

Albertans were asked to shut off unneeded lights and electrical appliances, minimize the use of air conditioners and not run their dishwashers, washers and dryers.

Earlier on Monday, AESO ordered electrical utilities to shed their customer load, which led to the temporary outages.

"We have to respond to the directive from them and that's what we're doing," said EPCOR spokesman Tim Le Riche in Edmonton.

AESO declared the power emergency at 2:10 p.m. MT after the power plants went off-line. Since then, operations at three generators have been restored.

Alberta is currently in the throes of its first major heat wave of the year with temperatures in the low 30s. Environment Canada issued a humidex warning on Monday for much of central and southeastern Alberta.