Updated

Power surges hit Puerto Rico overnight as frustration mounted while the U.S. territory struggled to fully restore electricity more than two days after a rare, island-wide blackout caused by a power plant fire.

Cheers were intermixed with groans as power came and went, frustrating some who had already stocked up at the supermarket and others who complained that the surges were damaging their home appliances.

Roughly 75 percent of 1.5 million homes and businesses served by the island's power company had electricity restored by late Friday, but officials warned that number would keep fluctuating. Around 200,000 of those customers were temporarily knocked off the grid when a privately run plant failed and caused the temporary collapse of two larger public power plants that it was feeding, said Gov. Alejandro Garcia Padilla.

"There may be setbacks that we can't anticipate," he said during a press conference late Friday.

Garcia said he expected that the majority of customers would have power by the time they woke up on Saturday.

"But, again, this is a very, very ambitious goal," he said, adding: "I want people to start their week without any problems."

Javier Quintana, executive director of Puerto Rico's Electric Power Authority, warned that power would come and go given the increase in demand as more customers came online. Both he and Garcia urged Puerto Ricans to be prudent in their use of electricity.

Those without power became increasingly disgruntled by late Friday afternoon, when the heat index reached 106 degrees and meteorologists announced that a record high temperature had been set. Those who did not have generators and could not find or afford a hotel room braced for a third night in stifling heat, interrupted only by occasional cold showers to cool off.

The blackout affected the entire island of 3.5 million people and prompted Garcia to activate the National Guard and declare a state of emergency. It was a hard hit for an island already struggling through a decade-long economic crisis, with economists estimating that the outage caused millions of dollars in losses.

Tara Rodriguez Besosa, who runs an organic food restaurant, said she was forced to throw away produce including 60 pounds of arugula after businesses ran out of ice to sell.

"I was about to cry," she said. "We lost a lot of produce."

She rushed to a friend's place with the surviving produce to make sauces and salvage what was left.

As power slowly returned across the island, authorities began to investigate what cause the fire at the Aguirre power plant in the southern town of Salinas on Wednesday afternoon. Preliminary reports show that a switch at that plant exploded for unknown reasons, causing a fire that downed two transmission lines, which in turn automatically prompted the entire system to shut down as a preventive measure, Quintana said.

The fire occurred as the troubled power company seeks to restructure $9 billion in debt and struggles to find revenue to update an aging system. Garcia stressed that the switch that exploded had been properly maintained.

At least one person died the first night from exposure to carbon monoxide after setting up a personal generator. Meanwhile, four police officers were struck by vehicles while directing traffic but were expected to recover.

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Danica Coto on Twitter: www.twitter.com/danicacoto