In Gurabo, just outside Caguas, Reynaldo De Hostos spends most of his time outside seeking relief from insufferable indoor heat. “Ever since we lost electricity, we can’t sleep,” he said, standing inside his pitch-dark home. Every hour or so at night, he said, he and his family use the air-conditioning in the car for a brief respite.

Even local utility employees are frustrated. “We need trucks, we need poles, we need crews, we need lines, we need more people,” said Javier Hernández Saurí, a lineman who led a team of repair workers making their way up Mr. Rodríguez’s street. “It’s going very slowly.”

He added, “We’re rewiring mostly with materials that can be reused.”

The Army Corps of Engineers, charged by the Federal Emergency Management Agency with restoring Puerto Rico’s power, estimated that it needed at least 2,000 additional workers. So far, the Corps has brought only about 200 workers, and most of them were dedicated not to restoring power but to installing generators at crucial locations.

After major storms, power companies typically rely on mutual aid agreements to get electricity restored. Outside companies send thousands of workers, and electric companies pay for the service with funds from FEMA.

Such agreements are “absolutely critical,” said Devon Streit, the deputy assistant secretary for infrastructure security at the Department of Energy, which is helping to coordinate the restoration of power. But in this instance, some companies were concerned about getting paid, she said, because the Puerto Rico Electric Power Authority, known as Prepa, had filed for bankruptcy protection in July after defaulting on $9 billion in debt.

Ricardo Ramos, chief executive of the power authority, said outside companies had been hesitant to come until they knew where the storm would make landfall. After that, he needed several days to assess the damage, and communications were down.