Thousands of customers in the Washington area remained without power late last night -- testing the patience of utility customers as crews raced to repair power lines downed by a string of powerful thunderstorms that moved through the region Wednesday evening.

Utilities officials said crews made significant progress yesterday, aided by cooler weather after several days of sweltering heat.

About 6,600 customers remained without power in Montgomery County, 9,600 in Anne Arundel County, 5,600 in Prince George's County and about 600 in the District. In Nothern Virginia, where the outages were fewer, power had been restored to all but 191 customers by 10 p.m.

Outages from violent storms this week provided the first major test for Pepco and other Maryland utilities since the state Public Service Commission took them to task for their response in 2003 to Hurricane Isabel, which left more than a million Maryland residents without power -- some for more than a week.

Bob Dobkin, a spokesman for Pepco, said the utility dispatched 145 crews to deal with 60,000 outages and more than 400 reports of downed lines in an effort to restore power to all customers by 8 p.m. today.

"The power goes out whenever there's a good-size storm," said Bruce Shulman of Wheaton, who is among the homeowners who said they are frustrated by the frequency of the outages.

Phyllis Fingerhut, 69, and her husband had a house full of guests from out of town when the power went off about 1:30 a.m. Saturday. The power was still off the next day as the temperature outside soared. The Fingerhuts, who are Orthodox Jews, could not drive on the Sabbath to somewhere cooler or call Pepco to complain. "It was horribly uncomfortable," Fingerhut said.

The family's power came on about 2:30 p.m. Saturday but went off again about 6:30 p.m. Wednesday as another storm swept in.

As many as 150,000 homes in the region lost power Wednesday. Northern Virginia had fewer problems than other areas, despite lightning strikes in Loudoun County that left one man hospitalized in critical condition yesterday. Dominion Virginia Power said about 20,000 customers lost power, mainly in the Leesburg area.

After Isabel, the worst storm to hit the region in 50 years, Pepco customers accused the utility of repairing storm damage too slowly and failing to provide prompt, reliable estimates of when lights would come on.

The Maryland Public Service Commission, which regulates energy providers, ordered Pepco and other utilities to improve their response to storms. Commissioners said the companies should improve communications with customers, manage tree growth and look for opportunities to bury power lines.

The utility has taken a number of steps in response, Dobkin said, including improving its computerized system that takes calls from customers and analyzes the information. The system is designed to handle 100,000 calls an hour. Even so, some customers are frustrated.

Shulman called the number at 1:45 a.m. Saturday, just after a swift and violent storm knocked out power to his Wheaton neighborhood. The computerized voice told him the power was estimated to be back up at 7 a.m. It didn't come back until 12:30 p.m.

Dobkin said the utility is continuing to make improvements and is working with local governments and homeowners to discuss the possibility of burying power lines.

"That's not a decision Pepco can make by itself," he said. "It has to be done with the community."

Utilities were under stress before the storms because of the tremendous energy demand during the heat wave. BGE, Southern Maryland Electric and Dominion Virginia Power reported record-high demands Wednesday and reduced their voltage.

Staff writers Rosalind S. Helderman and Nelson Hernandez contributed to this report.

Constance Smith of Middleburg makes her way under a downed tree on Welbourne Road, the result of storms this week.