HONOLULU -- Hawaiian Electric Co. on Friday said lightning was a factor in Oahu's islandwide power outage on Dec. 26.

Electricity began shutting down around the island at about 6:30 p.m. on Dec. 26, leaving most of HECO's 295,000 residential customers in the dark. Power was not fully restored until Sunday night.

Unusual ground strikes, probably five lightning strikes, caused the power outage, HECO representatives said.

One of the lightning strikes in particular was so powerful between Kahe and Waiau power plants that it knocked off seven major generators, HECO said.

After the lightning strike, HECO said another unexpected thing happened. Thousands and thousands of electrical devices all around the island began tripping themselves off, causing a big drop in demand for power.

"That had a huge impact on what happened afterwards on our system. If there was a knock out blow that was a knock out blow," Alm said.

Engineers speculated that more and more home and business devices are designed to shut off at the slightest sign of power fluctuation. When they protected themselves on that Friday night, power plants were suddenly over producing power, which led to the next unexpected and unexplained event. Seven major generators turned themselves off in 10 minutes.

"When those seven units failed after that it didn't matter what else we tried to do," Alm said.

The units appeared to be protecting themselves from instability. However, exactly what triggered their shutdown remains the big mystery, officials said.

"There is no way we are going to even guess what happened," Alm said.

As for getting the power back on, HECO said that the electricity restoration went as quickly as it could given that it had to make sure that the lines struck by lightning were intact before they could be powered up.

Hawaiian Electric Co. is accepting claims for damages from the outage. To file online, go to www.heco.com to download a claims form, or you can call HECO's claims office at 543-4624.

After the 2006 earthquake outage, 1,440 Oahu customers filed for damages. HECO has not yet paid off any of those claims. Claims are evaluated on a case-by-case basis, company officials said.