In carrying the peak loads of a system, pumped storage hydro-electric plants decrease the operating costs of the steam plants working in combination with them. off peak energy from steam or Diesel plants is used to drive the pumps. pumped storage peak load plants have an advantage over the usual reservoir peak load plants. If the reservoir is drawn too low, the firm capacity of the reservoir plant must be sacrificed, but the firm capacity of a pumped storage plant can be maintained by additional off peak pumping. Peak load hydro-electric plants are especially useful in systems subject to sharp and unexpected short time peaks.