Reuters: The next time some U.S. utility customers collectively pump up their air conditioners on a sweltering summer day, that power may be coming from a battery the size of a double decker bus. U.S. utilities are increasingly investing in super-sized batteries and other forms of energy storage to reduce their dependence on slow, dirty backup plants, put off building new substations, and to store power from wind farms and other growing but volatile sources of renewable energy. "We …