Hydro-Quebec said 500,000 households or businesses were affected at the peak of the blackout but said service was gradually being restored.
Earlier in the day, Hydro-Quebec's president had explained the cause of the problem.
Thierry Vandal said smoke from mighty forest fires in northern Quebec had prompted a major transmission line to seize up near James Bay, causing a cascading effect.
"Smoke from the forest fires coming into the lines ionizes the air," Vandal said during a news conference.
"That (initial failure) had a cascading effect on a number of other lines. The network is automatically programmed to react to these situations."
The impact Wednesday was felt in Montreal's subway system, which was temporarily shut down. There was no subway interruption Thursday as of 6 p.m., but shopping centres around Montreal did lose power.
Quebec provincial teams were battling a number of blazes, with help from Parks Canada.
More than 280,000 hectares of forested area have been scorched.
One-third of the residents from the Quebec community of Eastmain — 275 of them — were evacuated to the town of Val d'Or last week.
Flames leapt up from both sides of Eastmain's access road, and non-essential travel along the James Bay Highway was discouraged.
In recent days, thick clouds of smoke loomed over the town. However, a statement from the region's Cree health board said the community was not under threat.
Attempts to reach the community's official spokespeople were not initially successful. Numerous other residents and evacuees reached by telephone, and asked for information about the fires, deferred to the designated spokespeople.
The fires are being blamed on the driest summer in 40 years around James Bay, the hub of the province's hydro production.
Three major forest fires are being monitored — including the biggest one, which has raged at speeds of up to 30 kilometres per hour.
Satellite images show smoke over a sizeable chunk of the northwestern part of the province. Some of the smoke could even be detected this week hundreds of kilometres away, in Montreal.