Most of the electrical power used is generated in rotating machines; a sufficient number of generators are necessary to provide the maximum load required, since power used at a particular time must be generated at that time. Many generators stand idle for long periods because they are needed only to meet peak demands, which usually occur for only a few hours a day. Fewer generators would be necessary if electricity could be stored during the night for use during the day. In one method, a pumped-storage system is used in which water is pumped up to a high-level reservoir during the period of low demand at night so that it can be used to drive water turbines at tunes when demand is high. Lack of suitable sites prevents the system from being widely applied. Another method of storage is to provide chemical energy which can be converted to electrical energy as required. Although the cost of such an operation on a national scale would be prohibitive, this method is very widely used. A unit for chemical to electrical energy conversion is called a cell, and there are two types of cell, primary cells and secondary cells.