Hawaiian Electric Co. is asking customers to continue conserving electricity to avoid power failures from a system that's maxed out, with five of its 19 generating units out of service or operating at reduced capacity.

"If we lose any more units, we could experience outages," HECO spokesman Jose Dizon said. "Our margins are very tight."

The utility said it has been able to provide uninterrupted service to its 291,000 O'ahu customers.

On Tuesday night, HECO remotely turned off the water heaters of about 5,000 residents participating in its EnergyScout program for a brief time, saving 3 megawatts during peak hours.

The company also put into service auxiliary diesel generators at its 'Ewa Nui and Iwilei substations Tuesday, providing an additional 9 megawatts.

HECO received some good news Tuesday night when the H-POWER facility, which burns trash to generate electricity, completed repairs. H-POWER feeds 23 megawatts to HECO's O'ahu grid.

Even so, HECO's system remains "severely stressed" and will continue to be through the weekend and perhaps into next week, Dizon said.

"We had a record electricity load growth last year with no additional generating units (to the system), so the units require more maintenance, more frequent maintenance and longer maintenance," Dizon said.

HECO is asking the public to voluntarily cut back on non-essential electricity use as much as possible, especially during evening hours, so it can increase its reserve margin during those hours.

"We want to thank everyone — both business and residential customers — for pitching in, and we want to get the word out that we need to keep it up," Dizon said. "Our reserve margin remains very tight and will stay that way until we can get more units back online. Until then, the unexpected loss of a generating unit could result in the loss of power to some of our customers."

Two generating units at the main Kahe Point power plant are shut down and two other units at the same plant are operating at about half-capacity, Dizon said. In addition, Unit No. 8 at the Waiau power plant is not operational.

Unit No. 2 at Kahe and the unit at Waiau are down for scheduled maintenance, which involves an overhaul that takes several months to complete. Each is a 90-megawatt unit. Dizon said 1 megawatt powers 500 to 600 homes during peak hours.

Unit No. 5 at Kahe Point was shut down over the weekend for emergency boiler repairs. HECO said it was not known when the repairs would be completed.

Kahe units Nos. 1 and 6, which are 90- and 142-megawatt units, are feeding 50 and 100 megawatts, respectively, into the power grid, Dizon said.

A power failure yesterday in Kane'ohe that affected 1,600 customers was caused by a construction company and was not related to the generating units, Dizon said.

Reach Rod Ohira at rohira@honoluluadvertiser.com.