Today, there are many different mechanisms for storing energy, and quite a few mechanisms for converting stored energy into electric energy. Electric utilities must produce electricity on demand, with no facility to warehouse their product. Unlike the telephone system, the electric utilities have no “busy signal” by which to ask customers to come back later and try again. There is no queue line in which electric customers may wait their turn. The operation of an electric utility is a delicate balancing act in which the generation must equal the load, exactly, all of the time.