Irma hit southwestern Florida on Sunday morning as a Category 4 storm, the second most severe on the five-step Saffir-Simpson scale.
"We restored power to about 40 percent of the customers impacted by Irma in just one day", FPL spokesman Rob Gould told a news conference on Tuesday.
Florida Power & Light (FPL) said it had shut one reactor at its Turkey Point nuclear plant on Saturday but will leave the other operating, after the forecast track for Hurricane Irma shifted toward the west and away from the South Florida plant.
At this point in time, "it is not expected that the St. Lucie nuclear power plant will be shut down as result of Irma, though we will closely monitor the changing weather conditions", the utility said.
"Restoring power to our Gulf Power customers will be our priority".
FPL said Turkey Point and St Lucie were created to withstand storms stronger than any ever recorded in the region and are elevated to protect against flooding and storm surges.
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In Georgia, more than 1 million customers - which includes households and businesses - were in the dark, according to Georgia Power and Georgia EMC.
Nearly 20,000 Gulf Power customers are in the crosshairs of Tropical Storm Irma as it moves north into the Florida Panhandle. At least seven storm-related deaths been reported in Florida, Georgia and SC, according to local officials. However, FPL, said on Tuesday that would restore power to eastern Florida by this weekend, and to western Florida by Sept 22.
"This storm has the potential to eclipse Hurricane Andrew", Gould said.
The total number of customers still out, representing about 9 million people in Florida, Georgia and the Carolinas, dipped from a peak of more than 7.8 million customers, or over 16 million people, on Monday.
Silagy said that once the storm safely passes, FPL crews will immediately go to work to restore any power that is lost.
Federal Emergency Management Agency administrator Brock Long says preliminary estimates suggested that 25 percent of the homes in the Keys were destroyed and 65 percent sustained major damage. The nuclear plants are located along Florida's Atlantic Coast, about 20 feet above sea level.