Energy can be stored chemically and physically. Chemical energy is stored in reactions of various elements. Past millions of years solar energy is stored in the petroleum, natural gas and coal and by burning again it turns in heat, then converted into kinetic energy and power. Electricity can be stored through electrochemical processes in batteries, physical energy as potential or kinetic energy. Kinetic energy such as gyros, potential energy in pumped energy storage or in the tension of a spring or the compression of gas. These power-to-power technologies and the power-to-gas processes are the most important large-scale storage technologies.
General overview of all kinds of storages and performance. (M. Sterner und I. Stadler )Energy cannot be completely transformed from one form into the other. The relationship between energy input to and energy gained- efficiency - is one of the outstanding performance characteristics of a storage technology. The perfect power storage would have an efficiency of 100%, which means that a previously stored kWh electricity can be taken as well again. The efficiency of internal combustion engines is located, for example, at 30-40%, from thermal power plants at 30-50%, and the air pressure storage at up to 70%. The highest efficiencies are achieved in mechanical energy storage in pumped hydro storage, they are there in the range of 80 - 90%.