The utility bolstered its force of 3,000 employees and contractors with an additional 500 utility workers under mutual aid agreements Wednesday night. Peco expected to receive 1,000 more workers in the next two days. Some are being supplied by Commonwealth Edison, a Chicago utility owned by Peco's parent, Exelon Corp.
"We're mobilizing a massive field force," Engel Menendez said.
Health and safety officials encouraged the public to check on elderly or infirm neighbors and relatives, and to not use unsafe forms of heating and lighting, like gas ovens or candles.
In New Jersey, the storm damage was less extensive.
"Typically an ice storm is the worst-case scenario for an overhead-line utility like us," said Karen A. Johnson, a spokeswoman for Public Service Electric & Gas Co., New Jersey's largest utility.
The storm knocked out power for 40,000 of PSE&G's 2,200,000 customers, mostly in Mercer and Burlington Counties.
The storm was the second-worst in Peco's history in terms of power loss, exceeded only by Hurricane Sandy in 2012, which knocked out 850,000 customers.
More than 60 percent of Peco's customers in the four counties surrounding Philadelphia lost power as trees, limbs, and power lines sagged from the accumulated burden of Monday's heavy, wet snow, followed by Wednesday's freezing rain.
In Chester County, power was knocked out to 87 percent of Peco's 216,000 customers as trees that provide shady comfort during the summer months turned into frozen pikes that cut power lines into ribbons.
"The snow-covered trees look beautiful, but they're dangerous," said James P. Fama, the vice president of energy delivery for the Edison Electric Institute, an industry trade group.
Peco said it was prepared for the worst, and had been operating under emergency conditions since Monday's snowfall.
"The forecast of ice is always extremely concerning to a utility," Engel Menendez said.
The Pennsylvania Public Utility Commission said it was monitoring response to the storm. Spokeswoman Jennifer Kocher said the PUC would withhold judgment on Peco's performance until it could conduct a thorough analysis.
"Everybody thinks mistakes are made - that always happens in the middle of a storm," Kocher said. "We won't know that for sure until after the fact. We're just focused now on getting everybody restored."
Peco said it positioned about 3,000 employees and contractors Wednesday to field customer calls, remove downed limbs, and restore power.
The utility is functioning under what it called "Operation Condition 5" - the highest response level for storms, which will remain in effect until power is restored to all customers in the region,
Utilities are required to restore power first to vital public services - hospitals, water-treatment systems, police and fire facilities. Then areas are ranked in priority by the number of customers whose service would be restored.
Outlying customers and individuals at the end of circuits are last in line. Peco said some were likely to remain powerless until the weekend.
Gov. Corbett said five hospitals were operating on generators. Aqua America, the largest water utility in Philadelphia's hard-hit suburbs, said it fired up diesel generators to keep its water treatment plants and pumping system operating.
Utility crews were impeded by downed trees and the need to remove broken limbs before some repairs could take place. Chainsaw crews from the state Department of Conservation and Natural Resources were called in to help clear the way.
The storm's destruction, caused by freezing rain rather than wind, was largely concentrated in an area of Southeastern Pennsylvania where temperatures dipped just below freezing early Wednesday. Northern Pennsylvania received heavy snow, but suffered few power outages. Further east and south, the rain just rolled off trees.
"The difference of one or two degrees can be huge," Fama said.
Officials were worried that melting ice would refreeze Wednesday night, causing vehicles to skid on slippery streets and collide with utility poles, causing more outages.
The last bad ice storm, on Jan. 7, 1994, left 549,000 customers without power.
Officials warned residents to stay away from all downed lines and to consider everything "hot."
About 50 Peco employees were clustered in a windowless basement "war room" at the utility's Plymouth Meeting Service Center on Wednesday to organize the emergency response.
Most of the employees work at offices elsewhere, Engel Menendez said. A majority will work 12- to 16-hour shifts, she said. But there are perks - a table in a nearby hallway was lined with coffee, candy, and snacks, and around 11 a.m., hoagies and chips were wheeled in for lunch.
How to Get Help
Customers who lose power are urged to notify Peco by calling 800-841-4141, or through its website, Peco.com.
PSE&G customers can report an outage to 1-800-436-7734 or by logging into "MyAccount" at pseg.com.
Inquirer staff writers Maria Panaritis and Chris Palmer contributed to this article.