After swift-moving thunderstorms knocked out power to 120,000 District and Maryland residents in July, Pepco customers and critics renewed questions about the utility's ability to deliver reliable service.

Even relatively minor storms seemed to knock Pepco for a loop, some consumers complained, let alone the severe winds and heavy rain, spawned by Hurricane Isabel, that swept through the region in September 2003. It took the company 10 days to fully restore electricity to more than a half-million customers after that storm.

But a review of outage data shows that Pepco's overall reliability compares favorably with other major utilities in the mid-Atlantic region.

The company, which serves 720,000 customers in the District and Montgomery and Prince George's counties, returns power to customers more quickly than all but one of 11 utilities surveyed by The Washington Post. Pepco customers also experience fewer sustained outages -- those lasting five minutes or longer -- than customers of all but four of the other companies.

In a separate measure, a look at three-year averages going back to 1998 shows steady improvement by Pepco. From 1998 to 2000, customers experienced, on average, 1.41 sustained outages a year, lasting a total of 120 minutes. Over the next three years, the frequency fell to 1.34 outages for 105 minutes. In 2004, the numbers were lower still: 1.22 outages lasting 96.6 minutes.

The analysis did not compare the utilities' responses to major storms because the industry does not have uniform standards for gauging such performance.

But the data do dispel the notion that Pepco performs poorly even in times of moderate rain, thunder and winds, and that service has deteriorated because of deregulation, according to industry analysts who reviewed The Post's findings.

"This kind of confirms that they've been making significant progress, especially since Hurricane Isabel," said Grant L. Davies, president of a Chevy Chase consulting group whose clients include several major utilities, including Pepco.

The analysis examined outages from 2000 through 2004 for the six major utilities serving the District and Maryland and power suppliers in Virginia, Delaware, Pennsylvania and New Jersey. The data were gathered from state and District regulators and the companies.

Industry experts caution against comparing utilities because of variations in weather, geography and data collection. But the power industry frequently undertakes just such comparisons, usually in blind studies that are seldom made public.

To compensate for the variables, The Post chose companies that collect similar data in the same geographic region, where the landscape and weather are likely to be comparable.

On the advice of several analysts, The Post excluded outages caused by major storms to give a more accurate picture of a utility's day-to-day performance. Maryland regulators define major storms as those affecting at least 10 percent of a utility's customers for 24 hours or more.

There have been four such storms in the Pepco service area since 2000: three in 2003, including Isabel, and one in July.

To measure reliability, The Post used two widely accepted industry standards:

* System Average Interruption Frequency Index, which represents the average number of outages a customer experiences each year.

* System Average Interruption Duration Index, which represents the average number of outage minutes per customer.

In a look at five-year averages from 2000 through 2004, Pepco customers averaged 1.34 power outages lasting a total of 106 minutes per year.

Baltimore Gas and Electric Co., which serves 1.2 million customers in seven counties including Howard and Anne Arundel, averaged 1.49 outages per customer for a total of 233 minutes per year.

PECO, with 1.6 million customers in the Philadelphia region, had the lowest frequency rate at 1.07 and ranked just behind Pepco with an average duration of 114 minutes.

Dominion Virginia Power, which serves 2.2 million customers in Virginia, reported its data differently, based on criteria set by the Virginia State Corporation Commission. Unlike Maryland, it defines a major storm as one that cuts power to 10 percent of an operating area, rather than of an entire system.

For a more meaningful comparison with Maryland utilities, The Post separately reviewed data from Dominion's entire Virginia service area and its Northern Virginia region, which includes Alexandria, Springfield, Fairfax, Leesburg and Woodbridge.

Systemwide, Dominion had a frequency rate of 1.46 and a duration of 123. In Northern Virginia, the company had a higher frequency, 1.52, and a lower duration, 118 minutes.

James D. Bouford, an industry analyst and senior member of the Institute of Electrical and Electronics Engineers, said none of the data in The Post's study showed unacceptable performance.

"I've seen places where it's much, much worse," he said. "If one of these [interruption durations] were 400, I'd start to get concerned."

Why one utility performs better or worse than others is difficult to quantify, experts say. Pepco's service area, for example, has among the highest concentration of trees of any urban-suburban area in the country, increasing the probability of downed power lines during moderate to heavy storms.

Population density can also affect performance. Electricity providers serving large rural areas, for example, tend to experience more outages for longer periods. With fewer customers spread over a wider region, distribution lines, or feeders, are longer. That increases the chances of problems and the time to repair them.

"The probability of an outage is proportionate to the length of a feeder," said Rich Christie, an energy systems reliability expert at the University of Washington in Seattle. "Urban areas should have better reliability."

Delmarva Power, which had among the highest frequency and duration of outages, serves largely rural Delaware and the Eastern Shore, where feeder lines run as long as 100 miles, said William M. Gausman, a vice president for Pepco Holdings Inc., parent company for Pepco and Delmarva.

By contrast, "a long distribution line for Pepco is seven miles because the [population] density is so heavy," Gausman said.

State regulators typically judge a utility on its year-to-year performance, rather than comparing it with other utilities. Dominion uses targets set by the company, and monitored by the corporation commission, to gauge reliability. The utility's duration target typically ranges from 110 to 130 minutes, said Phillip Powell, director of planning and reliability.

Maryland and the District don't set performance goals but do require annual reliability reports. Maryland also requires utilities to identify the worst-performing feeders each year and submit a plan for improving them.

"It's those plans that we pay very close attention to," said Richard Schafer, chief engineer for the Maryland Public Service Commission.

BGE officials criticized direct comparisons.

"Each utility varies. Its terrain varies, the design varies, there's a lot of things that can make this an apples-to-oranges comparison," said John Borkoski, BGE's director of systems and reliability planning.

The commission, he said, recognized that when it developed reliability reporting standards for the state's major utilities in 2000. "Comparing any reliability indices of one utility to another is inappropriate," the working group that developed the standards wrote to the commission that year.

But Davies, the industry consultant, said: "I think these are fair comparisons. There are differences in geography, which we need to recognize, and some differences in the way things are counted, but if I'm looking at these numbers, and I'm BGE, I would wonder why performance looks worse than the others."

Comparing BGE's data against its own historical performance shows an increase in the duration of outages. From 1995 to 1997, its average was 168 minutes. From 1998 to 2000, it was 218 and from 2001 to 2003, it climbed to 228.

The number was higher still in 2004, at 259 minutes.

Borkoski said the increases were the result of an improved outage tracking system, installed in mid-2003 and giving a truer picture of outage data, and not a decline in reliability. The company, however, has been unable to determine just what impact the new system has had on the numbers, he said.

Industry surveys show large upward swings in frequency and duration when utilities switch from a manual outage management system to an automated system. But the jumps are generally much less distinct when they move from one automated system to another, as BGE did. "In most cases, it will go up slightly but nowhere near to that when your switching from a manual to automated," said Bouford, the industry analyst.

The utility serves an area of 2,300 square miles, more than four times the size of Pepco's. That increases the possibility that localized weather disturbances are elevating its numbers. A December 2004 windstorm, for example, knocked out power to 119,000 customers, but because it was not 10 percent of the utility's total customer base, it did not count as a major storm and was included in the outage data compared in this study.

"They look like they're not knocking out as many major storm events," Davies said. "What that suggests to me is they could be getting hit by a whole lot of small storms . . . which would make their numbers higher."

Schafer said the Public Service Commission did not see a problem with BGE's numbers.

"We really don't, given their geography and demographics," he said. "We also feel they're right in line in terms of the budget they're using for system improvements."

Fierce thunderstorms in July knocked out power to 120,000 D.C. and Maryland residents, renewing criticism of Pepco.