FPL said it expects to restore essentially all of its customers in the eastern portion of Florida by the weekend and the harder-hit western portion of the state by September 22.
Irma, categorized as one of the most powerful Atlantic storms on record when it rampaged through the Caribbean, has killed at least 77 people, including 36 in the United States, officials said.
Electricity generator Florida Power & Light, a subsidiary of NextEra Energy Inc, generates enough power for about 1.9 million homes at the Turkey Point and St. Lucie plants.
Some 5.8 million homes and businesses in Florida and nearby states still had no power on Tuesday after the pummeling from Hurricane Irma, as utility companies scrambled to get the lights back on in one of the biggest power restoration efforts in USA history.
"The industry's Irma response is one of the largest power restoration efforts in US history", Kuhn said. That was down from a peak over 1.4 million on Monday night.
Georgia reported more than 570,000 homes and businesses without electricity, and there were 80,000 in SC.
Duke warned its 1.8 million customers in northern and central Florida that outages could exceed 1 million. Locations without power have been flagged to the state to help utilities prioritize their work, the group said in a statement.
Irma hit southwest Florida on Sunday morning as a risky Category 4 storm, the second-highest level on the five-step Saffir-Simpson scale. It gradually weakened as it headed through the state, becoming a tropical storm and then a tropical depression on Monday.
FPL said its two nuclear plants in Florida were safe.
In Georgia, utilities reported over 1.2 million customers without power Tuesday morning.