Elon Musk's nearly 100% solar island is a test run for the U.S.

Elon Musk has hyped his acquisition of SolarCity by Tesla as an opportunity to create the world’s most advanced and audacious clean energy company. And a recently revealed project on a Samoan island might be his best case for it yet.

The two companies, which shareholders last week voted to merge, are now running almost the entire island of Ta’u, the largest in the U.S. territory of American Samoa, on solar, showing the reach of their energy system.

“Ta’u now hosts a solar power and battery storage-enabled microgrid that can supply nearly 100 percent of the island’s power needs from renewable energy,” SolarCity-Tesla wrote in a blog post. “The system is expected to offset the use of more than 109,500 gallons of diesel per year.”

The 790 people who live on Ta’u now have 5,328 solar panels generating power that’s stored in 60 commercial- and utility-grade Tesla PowerPack units. The company says that the island can go three full days without the sun and that the panels can fully recharge the PowerPacks in just seven hours of sunlight.

Musk badly wants cities in the U.S. to ultimately adopt solar grids like the SolarCity-Tesla ones on display in American Samoa. And although Donald Trump’s election worries environmental experts and activists, the rapidly declining cost of wind and solar energy indicates that renewable energy will remain a booming business.

The real concern for Musk’s plan is that SolarCity was never able to make the numbers work as a standalone business; its stock fell more than 60 percent in the year before the Tesla merger. So naturally, there’s a lot of skepticism about whether Musk can combine his infrequently profitable electric carmaker with his unprofitable solar company to make a legitimately successful business.