As the cost of solar and wind power continues to drop, sustainable energy is in the middle of a growth spurt. And one of the technologies that may fuel the next phase of sustainable power for homes, companies, and cities has nothing to do with turbines and rooftop installations. It’s all about batteries. Consider the new Peña Station Next project taking shape in Denver, a joint venture among local government, Xcel Energy, and Panasonic. This forthcoming 400-acre “sustainable small town” will run on the energy coming from a large series of solar panels connected to a 1-megawatt, grid-connected battery. With this massive battery extending the usability of solar energy beyond daylight hours, this new development seeks to become independent of outside energy sources.