<P> Energy storage is accomplished by devices or physical media that store energy to perform useful operation at a later time . A device that stores energy is sometimes called an accumulator . </P> <P> All forms of energy are either potential energy (e.g. Chemical, gravitational, electrical energy, temperature differential, latent heat, etc .) or kinetic energy (e.g. momentum). Some technologies provide only short - term energy storage, and others can be very long - term such as power to gas using hydrogen or methane and the storage of heat or cold between opposing seasons in deep aquifers or bedrock . A wind - up clock stores potential energy (in this case mechanical, in the spring tension), a battery stores readily convertible chemical energy to operate a mobile phone, and a hydroelectric dam stores energy in a reservoir as gravitational potential energy . Ice storage tanks store ice (thermal energy in the form of latent heat) at night to meet peak demand for cooling . Fossil fuels such as coal and gasoline store ancient energy derived from sunlight by organisms that later died, became buried and over time were then converted into these fuels . Even food (which is made by the same process as fossil fuels) is a form of energy stored in chemical form . </P> <P> Since prehistory, when humanity discovered fire to warm up and roast food, through the Middle Ages in which populations built windmills to grind the wheat, until the modern era in which nations can get electricity splitting the atom . Man has sought endlessly for energy sources . </P> <P> Except nuclear, geothermal and tidal, all other energy sources are from current solar isolation or from fossil remains of plant and animal life that relied upon sunlight . Ultimately, solar energy itself is the result of the Sun's nuclear fusion . Geothermal power from hot, hardened rock above the magma of the Earth's core is the result of the decay of radioactive materials present beneath the Earth's crust, and nuclear fission relies on man - made fission of heavy radioactive elements in the Earth's crust; in both cases these elements were produced in supernova explosions before the formation of the solar system . </P>

At what stage of development has mankind used the most energy