Record-breaking heat approaching the century mark baked an area from Missouri to New York to Georgia, with about 2 million electrical customers without power -- down from 4 million at the peak. About 1 million power customers were down in Virginia, Maryland and Washington alone, CNN reported.
More storms were forecast for Monday and Accuweather.com said Wednesday's scheduled July Fourth celebrations could be interrupted by more severe weather.
About 92,000 Commonwealth Edison customers in the Chicago area remained without power by mid-afternoon as temperatures climbed into the 90s.
Straight-line winds as high as 90 mph raked the area Sunday and more than 1 1/4 inches of rain fell, with areas west of Chicago the hardest hit, the Chicago Tribune reported. ComEd said it may be the end of the week before power is restored in some areas.
Federal agencies in the Washington area told non-emergency personnel to stay home and some schools in Baltimore canceled classes, CNN said. Classes also were canceled in Washington.
"We've been sleeping in the basement," Mark Cohen of Mays Landing, N.J., who lost power in the storms, told CNN. "Yesterday was 95 and really humid. We just finished our basement, luckily. We put an air mattress down there."
The latest wave of storms hit before cleanup was complete from Friday's deluge.
The death toll from the widespread, violent thunderstorms that ripped through at least seven states Friday night rose to at least 17 in Virginia, Maryland and the District of Columbia, The Washington Post reported.
Most of those killed died from falling trees.
Four states and the nation's capital declared states of emergency, and cooling shelters were set up to help residents cope.
National Guard militia members patrolled intersections with broken traffic signals.
More than 500,000 Washington-area households and businesses -- about 1-in-4 electric customers -- were still without power early Monday, figures from utility Web sites indicated.
"Unlike a polite hurricane that gives you three days of warning, this storm gave us all of the impact of a hurricane without any of the warning of a hurricane," Maryland Gov. Martin O'Malley told CNN's "State of the Union" Sunday.
The National Weather Service described Friday's storm system as a derecho, a widespread, long-lasting, violent storm that can produce hurricane-force winds. It is associated with a fast-moving band of severe thunderstorms in the form of a squall line.
Derechos occur mostly in summer, especially June and July in the Northern Hemisphere.