"You need to understand particularly if there's structural damage it absolutely could be weeks if we have to rebuild parts of the system", said Mr. Silagy, who estimated that the FPL outages affected about 9 million people.
The Cuban authorities said they had evacuated more than a million people as a precaution, including about 4,000 in the capital.
FEMA Chief Brock Long said he believed the Keys and Collier County in southwest Florida took the brunt of the storm.
The system, which has already knocked out power to nearly 500,000 customers across the state, was expected to churn-up life-threatening storm surges of up to 15 feet along parts of southwestern Florida and on other parts of the coast, forecasters warned.
Ana Gibbs, a spokeswoman with Duke Energy Corp., the state's second-largest investor-owned utility, said the company had been given the all clear to start work in the Tampa and Orlando areas.
In the Keys, where the storm roared ashore Sunday morning with winds of 130 miles per hour (209 kph), video showed houses shoved from their foundations and boats tossed onto the pavement.
Now the entire Georgia coast is under a storm surge warning as Irma continues its destructive march north.
In Jacksonville, where bridges are closed and a flash flood emergency has been issued by the National Weather Service, law enforcement warned residents to heed evacuation orders and said rescue teams will be looking for people trapped in their homes. "We have no cell service, no electricity, and no water", Monroe County Commissioner Heather Carruthers said.
"I say we didn't dodge a bullet, we dodged a cannonball", Miami Beach mayor Philip Levine told CNBC on Monday. "Travel into the Downtown Charleston area is not advised due to unsafe flooding conditions". As of Monday morning, 6.2 million customers were without power in Florida, according to the state's Department of Emergency Management.
What else should I know about the hurricane?
But on Saturday, all that changed when Irma's course shifted.
Some 400 miles north of the Keys, people in the Tampa-St. Petersburg area, once thought to be relatively safe from harm, suddenly found itself in the storm's crosshairs.
Winter Park police spokesman Garvin McComie says officers responded Monday morning following reports of a man lying in the roadway. "There is now a potential landfall as a Category 3 in Sarasota County, or it's still possible that the eye could remain off the west coast of Florida as it moves northwest". Evacuees Of The Keys shared school closure notices, videos of destruction, and many posts from friends and relatives searching for loved ones. "All the beauty kind of got sucked out of the city". "We didn't want to be that far away from our home", she added, noting that her husband and their dog have stayed at home.
Florida Gov. Rick Scott flew over the hard-hit but isolated Keys on Monday and said he saw "devastation" that included boats washed ashore and mobile homes pummeled by the storm.