As of Friday, only about 17 percent of the island has access to electricity, according to the Federal Emergency Management Agency, and rebuilding could take months. The New York Times wrote that the disaster turned Puerto Rico into a " generator island. "

Like much of the U.S. territory, Barrio Obrero has been running for weeks on an unreliable diesel-powered generator. Emergency services across Puerto Rico were devastated by the storm.

Birt, a Las Vegas firefighter who is assisting with the installation, said fire officials in San Juan told him that re-establishing the communications infrastructure was the top priority.

"Putting this in is the difference between life and death in this community," Capt. Richard Birt told CNBC by phone.

The fire station on Friday installed a 4-kilowatt solar system that will provide it with full power, including its communications system. The station has been without reliable power since Hurricane Maria struck the island on Sept. 20.

The muggy weather might be uncomfortable, but it is great news for firefighters at the Barrio Obrero fire station and the residents they serve, where the hot weather and shining sun could soon start saving lives.

"We will be flipping the switch any minute now," said Andy Newbold, a public policy director at Sunrun, which installed the system. Sunrun, a San Francisco-based residential solar provider, worked with nonprofit groups including Empowered by Light and Givepower to coordinate the installation.

The Barrio Obrero fire station is the first to be powered by the sun in Puerto Rico, according to Newbold, who also spoke to CNBC from San Juan. He said the setup, including solar panels and battery storage, will provide more resilient power than the diesel generator the fire station has been relying on. The solar panels on the roof, he said, can withstand 150 mph wind.

"It appears the federal emergency response is focused on diesel to repower the island," Marco Krapels, co-founder of Empowered by Light, said in a statement. "We wanted to demonstrate with real projects — with immediate impact, that renewable energy technology is available now and it's a much more resilient alternative to diesel."

When the grid is repaired, the solar system at Barrio Obrero will be able to connect to it or continue to function alone, according to Chris Rauscher, another Sunrun public policy director. It may even be able to deliver power to neighbors, he said, speaking from Barrio Obrero.

While the grid could take months to build, Rauscher said the project was able to give power to first responders in just a few days. By working directly with the fire station, they were able to deliver solar power while bypassing the larger government bureaucracy.

"It's very clear that it's going to take months and months and maybe even years to bring power," Rauscher said. But in the case of a solar-powered microgrid, people can get power in a day or two.

"We made the difference between life and death in this poor community in a day and a half," said Birt. "And today we are going to pack up our gear and in a day and a half we will have another station [powered by solar.]"

"This model should be used everywhere around the world," Birt said. A longtime solar advocate himself, Birt has lived off the grid for more than a decade.

The actor Mark Ruffalo, an Empowered by Light advisor who supported the effort, also weighed in: "Energy from the sun and wind are abundant, and can be used to power Puerto Rico," he said in a statement. "I think renewable energy solutions should be integrated into any emergency relief effort."

Other solar projects are also underway on the island. The Solar Energy Industries Association said it has received pledges of more than $1.2 million in solar products and monetary contributions since the storm hit. Separately, companies such as Tesla and Sonnen have pledged various battery and microgrid products. Utah-based New Star Solar sent a Boeing 747 jetliner with $300,000 worth of panels and solar generators that left Salt Lake City for Puerto Rico last weekend.

The project has been a labor of love and 18-hour days, with a total cost somewhere in the "hundreds of thousands" of dollars, Birt said. Sunrun and Givepower collectively donated about 15,000 pounds of equipment, Newbold said. In Friday's Caribbean heat, everyone involved pitched in to bring heavy equipment up four flights of stairs to get supplies to the roof of the fire station.

Birt said the project is a personal one for him. He pointed to last week's mass shooting in Las Vegas, where a gunman killed 58 people after spraying fire on a music festival. He said the resources that Las Vegas had access to made a difference in how the city was able to respond to the tragedy.

"What I want to do is reciprocate that to the fire departments in Puerto Rico," he said. "This is the same thing for their citizens."