Solar energy is a source of inexhaustible energy. There is more solar energy that reaches the surface of the Earth each hour than the amount of energy consumed by the world in a year. The challenges of using the sun's energy – energy which can be obtained either from wind power or from solar power – is that the energy needs to either be (1) stored in physical form of fuel for when it can be used in the future, or (2) transported directly as electricity, through transmission lines. Neither is dispatchable, as there is no control over when the sun will shine or when the wind will blow. There are, however, concentrated solar power plants using thermal storage that can store energy efficiently for up to 24 hours.