SAN DIEGO — Utility crews today brought electricity back to much of California, Arizona and Mexico, a day after a power outage left millions in the dark, paralyzed freeways and halted flights at San Diego’s airport.

But officials warned that the electrical grid was still too fragile and asked residents and businesses to go easy on — or even put off using — major appliances, such as air conditioners.

“Conservation will really help reduce the strain,” said Stephanie McCorkle at the California Independent System Operator, which manages the power grid. “Put off doing your major chores until California is fully restored.”

Electricity was restored in San Diego early today, signaling that the blackout was essentially over because most of the affected customers were in the nation’s eighth-largest city.

City schools, state universities and community colleges in the area remained shuttered. City beaches were closed because the blackout caused a 3.2-million sewage spill.

Mexico’s electrical utility said the lights are on for 1.1 million customers, or 97 percent of those who lost power.