The crisis, which chilled homes, forced many schools to close and shut down a huge chunk of the city's subway system during the morning rush hour, had city-owned Toronto Hydro scrambling to restore power to some customers until about 9:40 p.m., when Mayor David Miller announced it was back on.
"It is extraordinary that this kind of thing happens on almost the worst possible night," the mayor said at a news conference. "But I must say my feeling right now is relief."
The widespread outage, which lasted 24 hours in some areas, has sparked an investigation by Hydro One and raised new questions about the fragility of the province's electricity distribution system.
"There are several weak links in the system, and so Ontario continues to be at risk of these very serious power outages," Ontario New Democrat Leader Howard Hampton said in an interview. "Something as simple as a burst water pipe is all it takes to do it."
The trouble began just before 10 p.m. January 15, when a valve that triggers a sprinkler system in the Dufferin Street transformer station suddenly opened, causing alarm bells to ring. Repair crews rushed to the automated site that takes electricity coming into the city from Hydro One and distributes it to Toronto Hydro customers.
But the crews were unable to shut off the valve, which in turn activated the sprinklers meant to douse a fire or overheated electrical equipment, said Hydro One spokeswoman DaniÃ¨le Gauvin. Within minutes, the station was waist-deep in water, soaking Toronto Hydro transformers and refrigerator-sized circuit breakers.
"It's not yet clear what caused the valve to open," Ms. Gauvin said.
The mishap forced crews to shut down the entire station in order to pump out the water and then use propane-powered, hot-air blowers to dry the breakers before they could start restoring power.
A huge chunk of west-end Toronto was initially affected, an area spanning Spadina Avenue in the east to Jane Street in the west, and from Queen Street north to St. Clair Avenue. In all, an estimated 22,000 Toronto Hydro customers, or 250,000 people, were left without power and heat on a night when the temperature dipped to -19.
The morning rush hour was snarled as a long chunk of the city's Bloor-Danforth subway line was shut down, between St. George and Jane Streets, forcing thousands of commuters to walk in the frigid cold, jockey for taxis or cram onto shuttle buses.
Some pockets within the affected zone never lost power, or had it restored fairly quickly. But one set of breakers, affecting about 5,000 customers, suffered substantial water damage and required more extensive repairs that were completed later.
Industry officials said the power outage could have been avoided if systems were in place that would allow Toronto Hydro to divert electricity from other stations to customers.
"This is going to happen again and again and again," said an industry executive who asked not to be named.
Ms. Gauvin said Hydro One will conduct a "detailed investigation" into why the valve opened and then remained open. She said the utility will also examine whether it should use chemical foam instead of water-based fire prevention systems.
David O'Brien, chief executive officer of Toronto Hydro, said at a news conference that his utility relies on chemical foam at its facilities instead of water-based, fire-prevention systems.
This is not the first time a flood has caused a power outage in Toronto. In 2005, a broken water main caused an outage in the downtown core, leaving some residents without power for nearly 12 hours. And just recently, Toronto Hydro launched an unrelated investigation into a "stray voltage" problem after two dogs out for walks on the same street in the city's west end died in separate electrocutions near hydro poles.
In the latest incident, Toronto Hydro said 75 per cent of the people affected had their power back on that same day. Full subway service resumed well before the afternoon rush hour. City officials were making contingency plans in case power was not restored to everyone, and in case some of the 100 people who ended up using the "warming centres" opened up by the city needed a place to stay overnight.
The mayor, whose west-end home was among those unaffected, praised the city's emergency efforts, which included having staff knocking on the doors of the elderly and disabled to ensure their safety.