Much of Puerto Rico is still without power after Hurricane Fiona battered the island on Sept. 19. The storm laid bare how vulnerable the territory's power system still is five years after Hurricane Maria plunged it into an 11-month blackout — the longest in American history — and led to the deaths of almost 3,000 people.

Despite billions of dollars in federal aid, "very little" was done after Hurricane Maria to rebuild Puerto Rico's electric grid, Sen. Chuck Schumer, D-N.Y., said on the floor of the Senate. The island's power system, long neglected as the territory's debts soared, remains "almost 50 years out of date," Schumer added.

Yet some see signs of hope. Over the past five years, around 50,000 solar and battery power systems have been installed on homes in Puerto Rico, says Chris Rauscher, senior director of public policy at Sunrun, the biggest residential solar company in the United States. And almost all that equipment appears to have continued supplying electricity while the island's central power system went dark, according to market participants and industry observers.

Solar companies say their technology will keep getting better

Climate change is making hurricanes wetter and more powerful, increasing the risks to electric reliability in places like Puerto Rico. That bolsters the case for more investment in home solar set-ups, Rauscher says.

"It's showing that renewables paired with storage ... are really the fundamental building blocks of a clean recovery that we need to really focus on on the island and elsewhere," he says.

John Berger, chief executive of Sunnova, another big solar company, agrees, calling Puerto Rico "a window into the future."

'"The technology's just physically and fundamentally better," than the traditional power system, Berger says. "And that's not going to change."

Puerto Rico is looking to shake its fossil fuel dependence

Puerto Rico's aging power grid relies almost exclusively on fossil fuels that it ships in, and electricity prices on the island are far higher than in other parts of the U.S.

The territory moved to change that in 2019, setting a target to get all its electricity from renewable sources by midcentury, up from just 3% last year.

In February, the U.S. government and Puerto Rico signed an agreement aimed at accelerating work on the island's power system.

"One of my top priorities as Governor of Puerto Rico since I took office has been to ensure that Puerto Rico's energy transformation moves forward at a steady and reliable pace," Puerto Rico Gov. Pedro Pierluisi said in a statement in February. "I will make sure that every federal fund appropriated to Puerto Rico and allocated for the reconstruction of the power grid is used efficiently and effectively."

But big challenges are still hanging over the island. Perhaps chief among them is the fate of Puerto Rico's electric power authority, which is bankrupt.

There have also been delays in putting federal disaster aid to work on the island, due in part to political fights in Congress and restrictions put in place by the Trump administration.

The Federal Emergency Management Agency committed around $28 billion to help Puerto Rico recover from the 2017 hurricanes. Only $5.3 billion, or 19%, of that $28 billion has been spent by Puerto Rico's government as of August, according to the U.S. Government Accountability Office.

Puerto Rico has to rebuild in the face of more storms

Unspent aid "is the most challenging part of the reconstruction," Puerto Rico's resident commissioner, Jenniffer González-Colón, told NPR.

"[A] lot of reconstruction is still needed," she said. "And now on top of that, we got here again ... and in the end, the hurricane season is not over yet."

Companies like Sunrun and Sunnova are betting this latest disaster will help spur faster investment in small-scale renewables. If nothing else, customers have grown "sick and tired of not having power," Rauscher says.

"The next storm after this — and it will come, I don't know if it's next week or next year or two years from now, but it will come — we'll be in even better shape than we are right now," Berger says.