In Miami-Dade County, where roads flooded and debris flew, around three-quarters of residents (820,000 out of 1.1 million) are without power as of Monday.
High winds snapped power lines and left about 5.8 million Florida homes and businesses without power.
WILLIAM PITTMAN: Our lights have been out, went out at about 4:30 this morning, and they're still out. That's why he hasn't opened his refrigerator yet. Do not come back to the evacuation zones. For many, that will be the first step to moving past Irma. That's 64 percent of the state's power customers, and there are several counties where 80-90 percent of customers are without power.
Florida Governor Rick Scott said on Twitter he was joining members of the U.S. Coast Guard to view damage from Irma in the Florida Keys.
Collier, Lee, Volusia, Brevard and Hillsborough counties each had more than 200,000 customers in the dark. He called the scope unprecedented. Most of the outages were in Southeast Florida. I don't think any utility in the country ever has.
Its ferocity as it bore down on hurricane-prone Florida prompted one of the largest evacuations in USA history.
Storm chasers stood outside to measure the storm's strength. Incredibly, Irma was a hurricane for 11.25 days, which is the most since Ivan in 2004.
The storm is expected to further weaken as it moves through Columbus and then northwest through Alabama and into Tennessee. FPL's president asked for patience. FEMA chief Brock Long said some places won't have electricity for weeks.
Residents who needed help getting out were urged to put a white flag, T-shirt, or other white object in front of their home to grab the attention of search and rescue teams. The crews will focus on critical infrastructure like hospitals and police stations. "We expect the west coast to be the hardest-hit area, requiring an extensive rebuild of our energy grid". And we'll work to get some of those areas up as quickly as possible as well.
Electric service could be out indefinitely for many across the South as crews prepared to fix fallen lines in the storm's wake.
Unsafe storm surge is still expected, ranging from 1-2 feet at north Miami Beach to Cape Sable, including the Florida Keys, to 4-6 feet at Clearwater Beach and 2-4 feet at Tampa Bay. It could remain a risky storm throughout that time. The city has its own power grid.
"We're going to start demobilizing assets today, resources, that we don't think we need any more", the director of Emergency Management said. He tells member station WFSU they're anxious about risky conditions and plan to ramp up restoration efforts tomorrow. We're drawing every resources we can.