The moon shines behind buildings during a power outage in downtown Tijuana
The moon shines behind buildings during a power outage in downtown Tijuana September 8, 2011. A massive blackout caused by "human failure" left nearly 5 million people without power in parts of California, Arizona and Mexico on Thursday, and officials said many residents may be out of service for a day or more. The outage, apparently triggered by an employee who carried out a procedure at a substation in Arizona, snarled traffic on Southern California freeways, knocked out water supplies in parts of San Diego County and Tijuana and sent some elderly residents to emergency rooms. Reuters

Power companies in Southern California restored electricity to most customers by early Friday after a massive blackout on Thursday left nearly 5 million people in parts of California, Arizona and Mexico in the dark.

Although the Sept. 8 outage, apparently caused by human error, was just a tenth the size of the 2003 blackout that left about 50 million people without power in the eastern United States and Canada, it will surely rank as one of the biggest blackouts in recent history -- certainly one of the biggest caused by human error.

Sempra Energy's (SRE.N) San Diego Gas & Electric power company said it restored power to its 1.4 million customers at 3:25 a.m. Western time on Friday.

That was almost 12 hours after a major electric transmission system outage in western Arizona and the loss of a key connection with the 2,150-megawatt San Onofre nuclear power plant in California resulted in the most widespread power outage in the company's history, SDG&E said.

Across the border in Baja California, where 3.5 million people were affected by blackouts, power had been restored to 1.1 million customers by 6 a.m. local time, Mexico's Federal Electricity Commission said in a statement.

Service is practically back to normal, with only some specialized industrial clients awaiting reconnection, it said.

San Onofre, which is operated by Edison International's (EIX.N) Southern California Edison, shut on Thursday and remained out of service early Friday, according to the U.S. Nuclear Regulatory Commission.

Restoring power in the aftermath of the loss of the entire local grid serving San Diego and southern Orange counties was a monumental task, David Geier, SDG&E vice president of electric operations, said in a release.

The restoration process, however, has left our local power grid very fragile and we are asking our customers to conserve electricity throughout the day Friday, Geier said.

SDG&E and the California ISO, which operates the power grid for much of the state, said they would focus on maintaining and ensuring the integrity of the local power system for the next few days before determining the sequence of events that led to the outage and establishing practices and procedures to ensure that outages such as the Sept. 8 event are not repeated.

There appears to be two failures here -- one is human failure and the other is a system failure. Both of those will be addressed, said Damon Gross, a spokesman for Pinnacle West Capital's (PNW.N) Arizona utility Arizona Public Service.