They’ve got the juice. Can they bottle it? At midday, Kauai’s Island Utility Cooperative generates 97 percent of its energy needs from renewable sources, more than double its 2016 capacity. But at night, fossil fuels keep the lights on, which is why Tesla chose the island to launch its most ambitious power storage project yet, installing 50,000 solar panels and 272 batteries in March. If the company can prove the fridge-sized power cells can dispense electricity on a public scale, it could provide a model for making solar work round-the-clock nationwide.