In a word: storage. The island of Kauai is pushing hard to cut its carbon emissions. It has a goal of getting 70 percent of its electricity from renewable sources by 2030. So far, it's doing pretty well—between hydropower, biomass, and solar panels, it has enough capacity to meet 97 percent of its energy needs at noon on sunny days. But as on the mainland, the problem is that demand peaks in the mornings and evenings, when solar panels aren't producing nearly as much power.
Tesla has gotten involved, supplying a 52-megawatt-hour battery and a 13-megwatt solar farm to create the largest integrated solar-battery installation in the world when it was switched on in March. That's projected to allow Kauai's utility to cut the amount of diesel it burns to produce power for the island by 1.6 million gallons a year.
But the island, which has about 36,000 people, is going to need far more storage if it's to meet its emissions goals, so the local utility is planning to rapidly expand its solar-plus-storage capacity, while modernizing its grid and rethinking how it incentivizes residential solar projects. As this long read from Grist points out, the island's experience could teach us a lot about how to craft energy policy as renewable energy becomes a bigger part of the picture throughout the U.S.—though it's worth noting that despite Kauai's success story, the fact remains that building giant batteries is still really difficult.