Storing this energy when the sun isn't shining or a breeze isn't blowing has remained an expensive hurdle. Battery believers say that's changing. They've invested more than $5 billion in the past decade, racing to get technologies to market. They're betting new batteries can hold enough clean energy to run a car, home, or campus; store power from wind or solar farms; and make dirty electricity grids greener by replacing generators and reducing the need for more fossil fuel plants. This market for storage capacity will increase almost 10-fold in three years to 2,400 mw, equal to six natural gas turbines, Navigant Consulting says.