A massive power blackout in California, Arizona and Mexico affected almost five million people on Thursday.
Authorities say human error was behind the outage.
It began with a high-power line being knocked out of service, and a domino effect ended up affecting a nuclear energy plant.
Mike Niggli from San Diego Gas & Electric said: “We have a lot of demand for power here in San Diego and when you lose a transmission line you start bringing in a lot more power on the other transmission lines. And then when you lose power plants you don’t necessarily have enough power to serve the rest of the load.
“So, you have a little of a cascading event where when you don’t have enough power you start to cascade the system.”
Technicians are still attempting to restore services to some of the areas affected, but warn that many residents may be without power for a day or more.
Airports cancelled flights and traffic problems have been reported in many areas.
The police reported no major problems, though, and say hospitals managed to switch to backup power.