About 25,000 Oahu households and other electric customers were left in the dark for about an hour Tuesday night after two power generating units tripped offline, a Hawaiian Electric Co. spokesman said.

The two generating units stopped working shortly after 8:30 p.m. at the Kahe power plant and the Waiau power plant in Pearl City. The loss of the units created instability in the system that could have resulted in even more widespread blackouts.

Power was restored about 75 minutes later.

"In order to stabilize the system, we had to shut off power to approximately 25,000 customers, mostly in the town area," said HECO spokesman Darren Pai. "There may be some other areas that were affected."

He said communities affected were likely Nuuanu, Pauoa, Kalihi and other parts of town. Police reported outages in Waipahu, Kalihi and other areas of the island and were directing traffic in major intersections.

The system stabilized in about half an hour after the demand on the electrical grid was reduced by cutting off power to some customers. HECO crews used backup generators to replace the downed generators and restore power.

Power was restored to most customers at 9:45 p.m., Pai said. He said there may have been pockets of areas that did not immediately get power back. Customers with power problems should call HECO’s trouble line at 548-7961.

"We want to apologize to our customers for the inconvenience," Pai said. "We know this has been a disruption in their evening."

He said crews will conduct an investigation to determine what caused the generators to go offline, and take steps to address any problems discovered.