Irene smashed power poles, ripped transmission wires and flooded electrical stations over thousands of square miles as it whipped north from South Carolina to Maine. Most of damage came from downed trees.
Of the almost 8 million who lost power, 4.7 million remain in the dark.
Northern cities were still surveying the damage. The storm ranks among the worst in terms of power outages. Vermont experienced its worst floods in a century. Parts of New Jersey were cut off by swollen rivers. Half of Connecticut Light & Power customers were in the dark.
"This is just unprecedented," the Connecticut utility's spokesman, David Radanovich, said. "The largest storm we've ever faced."
As 750,000 of the utility's customers lost power over the weekend, Connecticut Light requested outside help. About 200 to 300 additional crews are headed to the state.
Public Service Electric and Gas Company, New Jersey's largest utility, said several thousand homes lost power when electrical substations were flooded. Those stations will remain offline until crews can pump out the water and dry the equipment.
"That takes days," PSE&G spokeswoman Karen Johnson said. "The flooding is kind of overwhelming."
In Vermont, hundreds of people were ordered to evacuate as streams and rivers jumped their banks. Gov. Peter Shumlin called it the worst flooding in a century.
More than 37,000 Vermonters were without power Monday, and officials said it could take weeks for power to come back.
"In many places, we can't even get to the damage," said Joe Kraus, operations chief at Central Vermont Public Service.
The 7.9 million homes and businesses that lost power over the weekend are nearly double the number of outages from the last hurricane to make landfall in the United States. At least 31 people in 10 states died in the storm.
Across the East Coast, power companies said most of the damage came from trees that smashed into transmission lines and other electrical equipment.
"Just lots of trees down," said Linda Foy with Baltimore Gas & Electric. "We've got whole trees knocked into equipment; large limbs the size of small trees hanging on power lines."
Repair crews focused on the most damaged parts of the grid. Utilities say they'll first repair damaged lines that power hospitals, jails, emergency call centers and other critical services. They'll also focus on transmission lines and substations that feed the largest parts of the grid.
Much of the damage is expected to be repaired with a few days, while some hard-to-reach areas could be cut off for days or weeks.
PECO, which serves southern Pennsylvania, said that 90 percent of its power outages will be fixed by Wednesday.
In Washington D.C., Pepco Holdings Inc. said it will complete most repairs by Thursday. Baltimore Gas & Electric, Long Island Power Authority in New York and Dominion Resources in Virginia said most of their outages should restored by Friday.
Other power companies weren't so sure. Some said it may take a few days, while others were hunkering down for many more nights in the dark.
"In some places we still need to wait for the flood waters to recede," said Ron Morano with Jersey Central Power & Light in New Jersey. "I'm not sure how long that takes."