Lightning may have caused the blackout that darkened a large swath of Manhattan and the Bronx for less than an hour, Consolidated Edison Chief Executive Kevin Burke said; there were severe thunderstorms around that time.
He said energy consumption did not cause the outage. Temperatures were around 90 in the New York City region.
The storms caused temporary outages throughout the metropolitan region, but fewer than 8,000 homes and businesses still were without power late that morning.
The blackout recalled some of the confusion New York City endured during blackouts last year and in 2003, and left some residents wondering whether it was a sign of trouble to come.
"It doesn't bode well for the rest of the summer, but I'm impressed they got it back on so fast," said Nancy Marcus, a manager at an optician's store on Manhattan's Upper East Side.
The outage knocked out traffic lights, snarled subway service and forced the evacuation of the Metropolitan Museum of Art.
"It was chaos," said motorist Edward Ankudavich, who spent an hour traveling 20 blocks in the Bronx.
The blackout affected approximately 385,000 people; it began at 3:42 p.m. and all power was restored by 4:30 p.m.
"We view this as a significant event," Burke said. But with a season of high electricity demand only beginning, he strove to reassure New Yorkers that the "likelihood of this happening again is very low."
Last summer, about 174,000 people were affected by a blackout in Queens. Residents sweltered without air conditioners on some of the hottest days of the year, and estimated business losses ran into the tens of millions of dollars as stores were forced to throw out perished goods.
The Public Service Commission issued a blistering report on that blackout, charging Con Ed's performance was "unacceptable and a gross disservice to its customers."
The utility has said it learned from that experience and was making infrastructure improvements and updating emergency procedures.