In early May, Elon Musk, the CEO of Tesla and chairman of Solar City, announced the release of the PowerWall, a new, low-cost battery system for home energy storage. The public reaction was overwhelmingly positive, and rightly so: As we build toward a clean energy future, the development of affordable, scalable batteries for electricity could be a key tool in the toolkit. Because the sun doesn’t always shine, and the wind doesn’t always blow, the thinking goes, batteries will ensure we have a reliable supply of electricity regardless of the vagaries of weather.