By Erin Ailworth
Hurricane Irma had knocked out power to more than 1.6 million customers in Florida just before noon Sunday, according to outage maps from the state's utilities, who were warning that some people may not regain electricity for weeks, despite billions of dollars in investments to strengthen the power grid in recent years.
The state's largest investor-owned utility, Florida Power & Light Co., a subsidiary of NextEra Energy Inc., said that more than 2 million of its customers already had experienced a loss of power as a result of the storm, though crews had been able to fix some of those outages.
While Irma's path has shifted west, Rob Gould, a vice president for FPL, which serves nearly 5 million, cautioned customers on the state's east coast to remain vigilant.
"Let me be perfectly clear: do not let your guard down, do not become complacent," he said during a Sunday press conference, which was briefly interrupted due to a loss of power. "This is a very powerful storm and every bit of Florida is going to feel its wrath."
Meanwhile, Tampa Electric, a unit of Emera Inc., said Saturday that because of Irma's shift westward the storm could cause up to 500,000 of its customers to lose power, or up to 70% of its system.
Florida utilities have warned that due to Irma's strength and intensity, the damages it causes to electric systems could cause outages to be prolonged, some repairs could take weeks.
Duke Energy Corp., the state's second-largest utility, has estimated it could see more than 1 million outages because of Irma. The company serves about 1.8 million customers in Florida.
"We expect significant power outages and restoration in some areas could take a week or longer," Luis Ordaz, storm director for Duke Energy Florida, said Sunday.
The gigantic storm covers an area more than double the size of the state, and on Sunday had maximum sustained winds of 130 miles an hour. That is a decrease from the Category 5 designation Irma previously held. Still, utilities continued to prepare customers for extended loss of electricity.
FPL said Sunday that it had gone ahead with a partial shutdown of one of its nuclear power plants, but that the other continued to be fully operational and remained safe.
The company and its power-providing peers in Florida have spent billions of dollars over the last several years upgrading their technology and infrastructure to better handle storms.
FPL has spent $3 billion since its system was slammed by seven hurricanes in 2004 and 2005. Improvements include replacing wooden poles and transmission structures with concrete ones, installing real-time water monitors to alert the utility to flooding at 223 substations susceptible to storm surge, using smart meters to identify outages more quickly and automated switching technology to reroute around trouble spots.
Duke has spent more than $2.4 billion on similar upgrades since 2004, including using "self-healing" systems to reroute power.
The improvements, utility representatives said, are designed not to prevent outages wholesale, but rather limit damage so that it is easier to restore lost power more quickly. But given Irma's size and strength, the companies were preparing for the worst.
"We estimate millions of customers will lose power as a result of Irma," Mr. Gould, the FPL spokesman, said Sunday, adding that crews likely need to set up hundreds of new poles and string thousands of miles of new line. "A storm of this magnitude an intensity will require us in many cases to completely rebuild out electric system from the ground up, particularly on the west coast. We anticipate that restoration effort will be measured in weeks, not days."
All the utility companies were mustering thousands of employees -- including workers from utilities from more than two dozen states -- to be ready to repair outages in Irma's wake.
Florida Light & Power said it had nearly 17,000 restoration workers positioned across Florida. Duke had mobilized roughly 8,000, including 1,500 from its own Midwest operations, while Tampa Electric had called in nearly 6,000 workers.
Bonnie Erdek, a spokeswoman for Florida Public Utilities Co., a unit of Chesapeake Utilities Corp., said her company has been having storm calls twice a day to prepare for Irma. It serves about 120,000 customers, and hadn't experienced any outages as of Sunday morning.
"In South Florida, we are just standing by," Ms. Erdek said Friday. "We're bringing in linemen, many of whom just left Texas."
At Gulf Power, a Southern Co. subsidiary that serves just over 455,000 customers in Northwest Florida, spokesman Rick DelaHaya said the company was also keeping an eye on the storm in case it shifts closer to the Gulf.
Write to Erin Ailworth at Erin.Ailworth@wsj.com