The term “solar energy” refers to a wide variety of techniques for using the energy available as sunlight. Well-known examples are active and passive thermal solar energy and photovoltaic solar energy but, strictly speaking, hydropower, wind energy, and biomass are also forms of solar energy. Today, only hydropower is used in significant quantities, covering approximately 6% of the world's energy demand. Traditional use of biomass, mainly in developing countries, accounts for more than 10% of the total energy consumption, but is sometimes left out of statistics because it falls outside the category of organized and commercial use.
The global potential for solar energy is huge, since the amount of energy that reaches the earth's surface every year exceeds the total energy consumption by roughly a factor of 10,000. There are, however, various barriers to the large-scale use of solar energy technologies. Most technologies have in common that the power density of the generator is low; in other words, one needs large areas to generate significant amounts of energy. This is especially true for biomass, with typical conversion efficiencies (solar energy to chemical energy) of 1% or less. Further, many solar energy technologies have proved technically feasible, but have yet to be proved economically feasible. Last, but not least, the large-scale use of solar energy requires substantial modification of our global energy supply system, which is based largely on fossil fuels.