More than 3 million South Florida residents were left without electricity yesterday after a fire and equipment failure at a substation in Miami caused a nuclear plant south of the city to shut down, a Florida utility company said.

Residents of counties including Miami-Dade, Broward, and Palm Beach experienced a blackout around 1 p.m. yesterday, and Florida Power & Light had restored power to all but a few areas late yesterday.

“It was nerve-wracking for a bit,” a New York online advertising executive who moved to Miami in July, Danny Alvarez, said. Mr. Alvarez was in an elevator when the power went out.

“I was in New York for 9/11,” he said. “You think back to what is going on and if there is any real issue here. Then we hear reports of eight power shutdowns, or whether there were explosions. … For a couple of hours, they didn’t know what the reason was.”

The outage occurred as temperatures in South Florida were in the mid-80s and demand for air conditioning was high.

“It crippled our lunch business and created havoc on the streets when everybody decided to go home and pick up their kids at school,” the owner of a Miami coffee shop, Giancarla Bodoni, said. A schoolteacher in Venice, Robin Howe, said he did not realize the entire city had been affected until he left work.

“My initial thought was that one of my students turned the lights off,” Mr. Howe said. “The kids were more excited than freaked out. I was more concerned that there might have been some sort of emergency.”