Hawaiian Electric Co. yesterday asked O'ahu residents and businesses to conserve electricity at least through tomorrow because several generating units are out of service, creating the possibility of blackouts.

HECO spokesman Jose Dizon last night said the utility did not have to resort to rolling blackouts, although usage during the peak 5 to 9 p.m. period came close to reaching capacity. Dizon urged customers to continue to continue to conserve energy.

Dizon said HECO normally likes to have 180 megawatts of reserve during the peak period, but last night there were just 16 megawatts of reserve power. One megawatt supplies electricity to about 500 customers, Dizon said.

"We're definitely not out of the woods," he said.

Three of the company's power-generating units — two at Waiau and one in Honolulu — were out of service for repairs or scheduled maintenance. Then yesterday morning, the No. 6 generator at the Kahe power plant was taken offline for emergency repairs, HECO spokesman Chuck Freedman said in a written statement.

In addition, Kalaeloa Partners — an independent power producer that sells electricity to the HECO grid — was operating at less than half of normal capacity, he said.

"Our generation reserve margin is very tight today. The unexpected loss of any other generating unit, or a large upward spike in electricity use, could result in the loss of power to some of our customers," Freedman said.

The peak electricity use by residents generally is from 5 to 9 p.m. on weekdays, HECO said. The company said people can help conserve power — and avoid blackouts — by turning off air conditioners, taking hot showers later in the evening and delaying the use of energy-hungry appliances such as washing machines, dryers and dishwashers until after the peak period.