WASHINGTON — As Hurricane Irma rampaged through Florida, snapping power lines and damaging vital equipment, it left as many as 15 million people in the state without electricity, the Department of Homeland Security said on Tuesday. While some homes may see power restored within days, utilities said that other customers may have to wait weeks.

“This is going to be a very, very lengthy restoration, arguably the longest and most complex in U.S. history,” Robert Gould, vice president for communications at Florida Power & Light, which supplies electricity to roughly half the state, told ABC News on Sunday.

Why does it take so long?

Repairing the grid after a major hurricane is a complex task, experts said. Utilities first have to send crews out to inspect the damage before they can figure out how best to restore service. That assessment can take days, and heavy flooding and debris from the storm can delay workers trying to reach key areas. 

Restoration is not always as simple as replacing wires and poles toppled by high winds or fallen trees. Floods can cause damage to electrical substations that link transmission lines with local distribution lines. Many buildings connected to the grid may also have sustained damage to their electrical systems. Those places need to be identified and isolated from the rest of the network before power starts flowing again, in order to prevent short-circuiting and other safety hazards.

Power Outages Across Florida

Sept. 12, 2017

“You might have an area where most houses are O.K.,” said Mark McGranaghan, vice president of distribution and energy utilization at the Electric Power Research Institute, a nonprofit group that does research for the nation’s power companies. “But crews still need to check all the buildings and disconnect the ones with damage before they can restore service.”

Florida Power & Light plans to send out 16,000 workers, including crews on loan from other utilities, said Eric Silagy, the company’s president, at a news conference on Monday. The company has also deployed drones to assess problems from the air.

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Making coffee in Miami on Tuesday. The outlet was drawing power from a generator.CreditKevin Hagen for The New York Times

Typically, a utility will focus on restoring power to critical facilities like hospitals and communication networks before moving to major population centers. Less-populated areas are usually last in line.

Mr. Silagy said that southwest Florida, where the damage is most extensive, could experience the longest waits. As Irma moved north on Monday, it also left more than a million people without power in Georgia and South Carolina.

There are steps that utilities can take to protect their power grids from storm damage. Florida Power & Light has invested more than $3 billion in such measures since 2006, including replacing wooden poles with sturdier concrete poles, burying some power lines and installing flood monitors at 223 substations to protect equipment. Without those steps, Mr. Silagy said, “we would have seen much more prevalent structural damage.”

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A fallen power line in Bonita Springs, Fla., on Monday. CreditNicholas Kamm/Agence France-Presse — Getty Images