Two HECO generators were restored to service last night, and a third operated by Kalaeloa Partners at Campbell Industrial Park, which sells energy to HECO, also was back on line. Pearl Harbor Naval Station also supplied HECO with additional standby power.

Although the potential for power failures was reduced significantly last night, HECO officials urged customers to continue conserving energy.

By 7 p.m. last night, peak demand for power on O'ahu was at 1,278 megawatts, down from Tuesday's peak of 1,327 mega-watts.

"It is likely a large portion, though not all, of that savings is due to individuals and businesses doing the right thing, and we are grateful," said Chuck Freedman, HECO vice president of corporate relations.

Freedman praised the efforts of Pearl Harbor Naval Station, as well as so-called interruptible-load customers, who pay a reduced rate with the understanding that HECO can shut off their power in an emergency.

"Our military friends at Pearl Harbor and our interruptible-load customers have really come through for us," Freedman said. "...Other large customers have been cooperative as well,"

Manoa Valley resident David McCauley and his neighbors lost power Tuesday night for more than two hours and then again last night for about 30 minutes.

"I was mad because it happened just when the presidential debate was on," McCauley said last night. "Then the power finally came back on."

HECO asked customers yesterday to reduce their use of appliances such as clothes dryers, dishwashers and air conditioners until after 9 p.m.

Large customers, such as hotels and military installations, also were asked to cut back by turning off lights, raising the thermostat on air conditioners and shutting doors.

University of Hawai'i officials, whose six O'ahu campuses collectively make UH one of the top three energy users on O'ahu, got HECO's e-mailed appeal yesterday, and checked for unusually high energy use but found none.

"With night classes, we're basically a 6 a.m. to 9 p.m. operation," UH spokesman Jim Manke said last night. "So it's tough. Night classes need to have the lights on and the air on if they're closed buildings. We also have a built-in problem where some buildings simply need to keep the air conditioning on for projects that have to be temperature-controlled."

In an unrelated energy problem, a leaking power transformer at the Manoa campus' Hale Wainani student residence left 650 UH students without power overnight and into today.

Earlier in the day, HECO said that if it lost another generator or if demand were too great, it would have to shut down power to "blocks of customers" to maintain the stability of the power system, said HECO spokesman Jose Dizon. The procedure, "load shedding," is designed to ensure that hospitals and key public safety organizations continue to have electricity.

Some hospitals, such as The Queen's Medical Center, said yesterday they have emergency backup generators to keep running during power failures.

Rolling blackouts are more common on the Big Island. At the beginning of the year, officials there triggered a blackout, leaving 1,800 customers without power, after an equipment failure halved power production at a plant in Hamakua. It marked the first rolling blackouts on the Big Island since 2002.

O'ahu's major blackouts — in 1983, 1988 and 1991 — happened because of transmission problems, not generator problems.

If blackouts are necessary, Dizon said, HECO will warn the public through the media, and the blackouts will not last more than two hours.

Freedman said demand for power set a record high Monday night of 1,319 megawatts. Tuesday's use also set a record.

Temperatures yesterday hit the upper 80s, but it felt much hotter because of high humidity, weather forecasters said. More of the same is expected today, although things could cool off by tomorrow.

HECO said its reserves of power generation were "very tight" yesterday.

Part of the problem was that two generators at HECO's Waiau power plant were shut down for unanticipated maintenance yesterday, and Kalaeloa Partners' generators also was down.

A third Waiau generator was out of service for planned maintenance, and the city's H-Power unit — which sells power to HECO — was operating at reduced power because of planned maintenance.

HECO has 16 generators, and would like to add another one by 2009, possibly at Campbell Industrial Park, Dizon said.

The power company activated a team of engineers and managers to monitor the situation and planned to watch it overnight, Dizon said.

"All the people in the power plants are working extra hard to ensure that everything is running and running healthy," he said.