Energy storage involves converting energy from forms that are difficult to store to more conveniently or economically storable forms. Some technologies provide short-term energy storage, while others can endure for much longer. Bulk energy storage is currently dominated by hydroelectric dams, both conventional as well. Energy storage is the capture of produced at one time for use at a later time to reduce imbalances between energy demand and energy production. A device that stores energy is generally called an MillsThe classic application before the was the control of waterways to drive water. Storage capacity is the amount of energy extracted from an energy storage device or system; usually measured in or and their multiples, it may be given in number of hours of electricity production at power plant ;. In the 20th century grid, electrical power was largely generated by burning fossil fuel. When less power was required, less fuel was burned., a mechanical energy storage. OutlineThe following list includes a variety of types of energy storage:• Fossil fuel storage• Mechanical The (IESDB), is a free-access database of energy storage projects. The economics of energy storage strictly depends on the reserve service requested, and several uncertainty factors affect the profitability of energy.