The failure began at 2:51 p.m. when AES, the island's largest independent power generator, went offline. AES sells electricity to HECO from its Campbell Industrial Park plant.

A short while later the H-Power and Kahe power plants went offline, said HECO spokesman Chuck Freedman. The three plants provide about 30 percent of the island's electricity.

To protect the stability of the system, Freedman said, HECO began "load shedding," a process of systematically disconnecting customers around the island. As many as 40,000 of HECO's 280,000-plus O'ahu customers lost electricity.

Power to most of them was restored by 5 p.m.

Rush hour was a mess because of inoperable traffic lights.

But as the island reached the peak demand for power in the evening, the AES and Kahe plants were unable to produce electricity, Freedman said. HECO asked large hotels to conserve energy, but that wasn't enough.

At 6 p.m., HECO again had to shut off electricity to portions of O'ahu, including Makaha, 'Aiea, Kaka'ako, Hawai'i Kai, Kapalama, Pearl City, Waipahu, Pauoa Valley and Mililani. By 7 p.m., the Kahe plant was up and running and power to most communities was restored a half-hour later.

Only about 50 customers in Wai'anae were without power at 9 p.m.