About 6.5 million homes in Florida, two-thirds of the total, are without power after Hurricane Irma cut a deadly path through the state, officials say.
Relief operations are under way and engineers are working to restore power, but many areas remain stranded.
The islands of the Florida Keys and western parts of the US state bore the brunt of the category-four hurricane.
Irma hit Florida on Sunday, before weakening to a tropical storm as it moved north on Monday.
Media reports link at least four deaths to the storm in Florida. Last week it killed at least 37 people in Caribbean islands.
White House Homeland Security Adviser Tom Bossert said it will be some time before residents in the Florida Keys are able to return to their homes.
"I would expect that the Keys are not fit for re-entry for regular citizenry for weeks", he said.
Speaking as he went on an aerial tour of the Keys, Florida Governor Rick Scott said: "Power lines are down throughout the state. We've got roads that are impassable, so everybody's got to be patient as we work through this."
The Keys are cut off from the mainland, as the 42 bridges that link them are being assessed for damage. Reports say that 10,000 people decided to ride out the storm.
At 18:00 GMT, its centre had moved into southern Georgia, the National Hurricane Center (NHC) said.