The plant has not been in operation since January 2012, when a small leak occurred in one of the two reactors.
In October, Edison applied to the Nuclear Regulatory Commission to restart the other reactor. That request is still pending.
The company says it is shutting the plant down because it doesn't know when or if it will be allowed to return to service, and it is too costly to wait. With the reactors idle, the company has spent more than $500 million on repairs and replacement power.
About 7.4 million Californians live within 50 miles of San Onofre, which can power 1.4 million homes. California officials have said they would be able to make it through the summer without the plant but warned that wildfires or another disruption in distribution could cause power shortages.