This system is economical as it flattens out the variations in the load on the power grid, permitting thermal power stations such as coal-fired plants and nuclear power plants that provide base-load electricity to continue operating at capacity, while reducing the need to build special power plants which run only at peak demand times using expensive natural gas. Modern turbines in the pumped water plants can be brought up to full power in a matter of seconds to respond to demand.
A new concept is wind-pumped water storage where vagaries in wind power can be leveled by using the wind power to fill a reservoir and generating grid power from the reservoir turbines.
In 2000 the United States had 19500 MWe capacity of pumped storage. This produced a net -5500 MWe of power because they consume more power filling their reservoirs than they generate by emptying them. Still the technique is considered a worthwhile addition to the electrical grid as the most cost effective means to date for storage of mass amounts of electrical power.
In 1999 the EU had 32 GW capacity of pumped storage out of a total of 188 GW of hydropower and representing 5.5% of total electrical capacity in the EU.
Worldwide List of Pumped Storage plants