Most outages were in Florida Power & Light's service area in the southern and eastern parts of the state. A unit of NextEra Energy Inc and the state's biggest power company, FPL said more than 3.6 million of its customers were without service.
As the storm weakens as it heads toward Georgia, outages at both FPL and Duke Energy Corp, Florida's second-largest utility, have started to level off.
Florida outages for Duke, which serves the northern and central parts of the state, so far have peaked at around 863,000.
Irma hit Florida on Sunday morning as a dangerous Category 4 storm, the second-highest level on the five-step Saffir-Simpson scale. It gradually lost strength, weakening to a tropical storm by Monday morning with maximum sustained winds of 70 miles (110 km) per hour, the U.S. National Hurricane Center said at 8 a.m. EDT (1200 GMT).
In Georgia, utilities reported 152,000 customers without power, but outages were likely to increase as the storm moves north.
Other big power utilities in Florida are units of Emera Inc and Southern Co, which also operates the biggest electric companies in Georgia and Alabama.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)