The cause of the blackout is under investigation, but preliminary findings pointed to a fault in an underground line, according to Luma Energy, the island’s main power distributor. Fully restoring service could take between 24-48 hours, the company said on X.Only 13% of the island’s 1.4 million customers had power as of about 10:00 local time (14:00 GMT), according to the New York Times.

An hour later, power had been restored to some areas, as well as San Juan’s municipal hospital, Luma said.

The New Year’s Eve blackout prompted renewed calls from elected officials and residents to address the unincorporated US territory’s ongoing power issues, which have persisted since Hurricane Maria in 2017.

The island cannot continue to put up with an energy system that fails its citizens so often, Jenniffer González-Colon, Puerto Rico’s current US congressional representative and the incoming governor of Puerto Rico, wrote on X.

Blackouts continue to affect Puerto Rico’s economy and quality of life, she said.On Facebook, the current governor, Pedro Pierluisi, demanded answers and solutions from the two main power companies, Luma and Genera.

Hundreds of thousands of residents at a time have been affected by power outages this year. A June outage left about 350,000 customers without power as temperatures climbed, and more than 700,000 customers lost electricity after Hurricane Ernesto in August.

As they awoke to another day without power, Puerto Ricans expressed frustration to US media.

“They’re part of my everyday life,” Enid Núñez, 49, said of the outages to the Associated Press.

Puerto Rico’s power grid was strained even before Hurricane Maria devastated the island. US government funding helped shore up the grid, facilitate recovery projects from other natural disasters, and make other important infrastructure improvements.

But the implementation has been incomplete due to a variety of factors, such as issues starting construction and the Federal Emergency Management Agency’s requirements to authorise use of some of the funds, according to a February 2024 report from the US Government Accountability Office.

“Inexcusably the power grid has still not recovered from damage in Hurricane Maria,” Mark Levine, New York City’s Manhattan borough president, wrote on X.

New York City is home to the largest Puerto Rican community in the mainland US.

“This is 3.5M American citizens,” he wrote. “We owe them so much better.”

The National Electric Union (UNE) said on Thursday that it had boosted generation to 1,450 megawatts (MW), still less than half the typical peak demand of 3,200MW.

“The [grid] is operating normally now, but because of a generation deficit we don’t have sufficient capacity to cover demand,” said Lazaro Guerra, who oversees the sector for Cuba’s Ministry of Energy.

This week’s nationwide blackout — the third in less than two months — occurred in the early hours of Wednesday after the Antonio Guiteras power plant in Matanzas, the country’s top electricity producer, shut down.

That triggered a chain reaction, overwhelming an already strained power system and leaving the capital, Havana, in the dark.

Cuba’s oil-fired power plants are decades old and struggling to operate, but this year brought additional struggles, as oil imports from Venezuela, Russia and Mexico dwindled.

About half of the Caribbean nation’s power generation facilities are offline for maintenance or broken down, and a majority of Cuba’s residents suffer hours-long, rolling blackouts on a daily basis even when the grid is functional.

On Thursday, classes and work activities gradually returned to normal after the nationwide blackout.

Power had returned to all of Havana’s “circuits”, the local electric company said, and all of its hospitals were back online.

But the Cuban authorities said they will continue their current practice of implementing daily, five-hour power outages by block or zone as they have been doing for the past few months amid the energy crisis.

The Cuban power grid collapsed multiple times in October as fuel supplies dwindled and Hurricane Oscar struck the far-eastern end of the island, prompting authorities to close schools and nonessential workplaces.

In November, Hurricane Raphael knocked out the grid again as it made landfall on the island as a Category 3 storm.

Authorities said the Antonio Guiteras power plant in Matanzas, the country’s top electricity producer, shut down about 2am (07:00 GMT) on Wednesday, prompting the grid collapse.

The Ministry of Energy and Mines said in a post on social media that it was working to restore power.

Cuba’s oil-fired power plants, obsolete and struggling to operate, reached a full crisis this year as oil imports from Venezuela, Russia and Mexico dwindled.

The system failure on Wednesday morning left the capital, Havana, almost completely in the dark, the Reuters news agency reported, quoting a witness.

Lights before sunrise could be seen only in a handful of large hotels and government buildings across the city’s skyline.

Reports of blackouts elsewhere in Cuba on social media suggested the entire island of 10 million people was without power although the government had yet to confirm the extent of the outage.

Cuba’s power grid collapsed multiple times in October as fuel supplies dwindled and Hurricane Oscar struck the far-eastern end of the island, prompting authorities to close schools and nonessential workplaces.In November, Hurricane Raphael knocked out the grid again as it made landfall on the island as a Category 3 storm.

The storm tore across Cuba with winds hitting 185km/h (115mph), damaging homes, uprooting trees and toppling telephone poles.

The Cuban authorities have blamed previous outages on difficulties in acquiring fuel for power plants, which they have attributed to the tightening, during Donald Trump’s first presidency, of a six-decade-long United States trade embargo.

But the country has also experienced a broader economic crisis marked by soaring inflation and shortages of medicine, food and water.