About 13,000 utility customers in the Philadelphia region remained without electrical service on Friday, three days after Tropical Storm Isaias slammed the region, knocking out service to 700,000 customers on both sides of the Delaware River.

Peco reported about 7,000 customers were still without power at 5 p.m. Friday, most in Bucks and Chester Counties. Public Service Electric & Gas reported it had about 1,500 South Jersey customers still dark, a fraction of its statewide total of 70,000. And Atlantic City Electric said it has about 4,500 customers awaiting restoration.

“We understand how frustrating it is to be among the customers that are still without power,” PSE&G said in a statement Friday. “Our crews are working 16-hour shifts and around the clock to restore every customer as soon as safely possible.”

PSE&G called Isaias the fifth most damaging storm in its history, and outpunched in recent years only by Hurricane Sandy in 2012.

Despite the assistance of out-of-state repair crews, utilities say some customers may not get their power restored until the weekend, as increasingly, each of the more than 3,300 outages affects fewer customers.

The restoration effort is following a standard pattern, where repair crews first fix outages affecting essential services, such as hospitals and public safety facilities, and then damage affecting the largest numbers of customers. That drove down outage numbers in the first two days.

Each of the remaining outages affects a smaller number of customers — many involve lines serving only a single customer. About nine customers on average were impacted by each of Peco’s remaining 750 outages Friday afternoon. Atlantic City Electric and PSE&G’s South Jersey outages serve an average of about two customers each.

PSE&G on Friday said it added 100 more out-of-state repair workers, taking its total restoration work force to 3,400. The additional help included tree crews and line workers from 15 states and Canada.

The utility is also operating comfort stations in Willingboro and Lumberton, where customers without power can pick up bottled water and ice. So far, PSE&G said, it has given out 175,000 bottles of water and 87,400 pounds of ice.

Tropical Storm Isaias knocked out power to three million electric customers, mostly in Northeastern states, before losing steam in Canada. New York and Connecticut officials chided electric utilities in their states for a “wholly inadequate” response as outages persisted, according to UtilityDive, a journal that covers the industry.

More than 200,000 customers in New Jersey remained without power on Friday — mostly in the northern half of the state, and mostly customers of Jersey Central Power & Light. The large number prompted the Sierra Club’s New Jersey chapter on Friday to call on state regulators to review utility rates and to pressure utilities to move power lines underground to protect them from high winds and trees.