Solar power is the part of the sun's energy which can be converted into useful power such as electricity, heating, or mechanical work.

Solar power has become of increasing interest as other finite power sources such as fusel fuels and hydro-electric power become increasingly scarce and expensive (in both fiscal and environmental terms). As the earth orbits the sun it receives 1,410 W / m2 as measured upon a surface kept normal (right angle) to the sun. Of this approx. 19% of the energy is absorbed by the atomsphere, while clouds reflect 35% of the total energy upon average.

Solar power is obtained by either direct or indirect conversion of solar energy. Direct means include photoelectric cells to produce electricity or roof top solar pannels to heat the buildings hot water supply; indirect means include heating water to drive turbines which produce the desired electricity or similar multistage systems to achieve the desired power type.

Direct Solar Power

Solar design is the architectural effort to decrease a home or buildings energy needs to maintain a desirable environment inside. Ideally most buildings should be well insulated, draw solar warmth inside during winter, and keep the heat outside during summer. Though such designs sometimes incorporate active elements such as heat pumps, these tend to be of marginal benefit compared to good building design.

Solar hot water systems are quite common in some countries where a small flat panel collector is mounted on the roof and able to most of the housholds hot water needs. Cheaper flat panel collectors are also often used to heat swimming pools extending their swimming seasons.

Solar cooking is helping in many developing countries, both reducing the demands for local firewood and maintaining a cleaner environmnet for the cooks. The first known record of a western solar oven is attributed to Horace de Saussure, a Swiss naturalist experimenting as early as 1767, a solar box cooker traps the sun's power in an insulated box; for cooking, pasteurization and fruit canning.

Solar cells are devices or banks of devices that use the photoelectric effect of semiconductors to generate electricity directlty from the suns photons striking them. As their manufacture cost has remain high during the twentieth century their use has been limited to powering very low power devices such as calculators with LCD displays or to power generation for isolated locations which could afford the technology. The most important use to date has been to power orbiting satellites.

Indirect Solar Power

Solar power plants generally use reflectors to concentrate sunlight into a heat absorber.

- Parabolic reflectors are most often used with a stirling engine or similar at its focus. As the single parabolic reflector achieves a greater focusing accuracy than any larger bank of mirrors can achieve, the focus is used to achieve a higher temperature which in turn allows a very efficent conversion of heat into mechanical power to drive a electrical generator.

- Heliostat mirror power plants focus the suns rays upon a collector tower, the vast amount of energy is generally transported from the tower and stored by use of a high temperture fluid. Liquid sodium is often used as the transport and storage fluid, the energy is then extracted as needed by means such as heating water for use in stream turbines.

- Trough concentrators have been used successfully in the US State of California to generate 350MW of power the past two decades. The parabolic troughs can concentrate the sun up to 30 or 60 times upon tubes where synthetic oil is heated to 390°C which is then pumped into a generating station and used to power a steam turbine.

Deployment of solar power depends largely upon local conditions and requirements, for example while certain European or US States could benefit from a public hot water utility, such systems would be both impractal and counter-productive in countries like Australlia or states like New Mexico. As all industrialised nations share a need for electricity, it is clear that solar power will increasingly be tunned to supplying a cheap, reliable electricity supply.

Many other types of power generation are indirectly solar-powered. Plants use photosynthesis to convert solar energy to chemical energy, which can later be burned as fuel to generate electricity; oil and coal originated as plants. Hydroelectric dams and wind turbines also take advantage of the power of the sun to drive weather.

In some areas of the U.S., solar electric systems are already competitive with utility systems. The basic cost advantage is that the home-owner does not pay income tax on electric power that is not purchased. As of 2002, there's a list of technical conditions: There must be many sunny days. The systems must sell power to the grid, avoiding battery costs. The solar systems must be inexpensively mass-purchased, which usually means they must be installed at the time of construction. Finally, the region must have high power prices. For example, Southern California has about 260 sunny days a year, making it an excellent venue. It yields about 9%/yr returns of investment when systems are installed at $9/watt (cheap, but feasible), and utility prices are at $0.095 per kilowatt-hour (the current base rate).

For a stand-alone system some means must be employed to store the collected energy during hours of darkness or cloud cover - either as electricity in batteries, or in some other form such as hydrogen (produced by electrolysis of water), flywheels in vacuum, or superconductors. Storage always has an extra stage of energy conversion, with consequent energy losses, greatly increasing capital costs.

Several experimental photovoltaic (PV) power plants of 300 - 500 kW capacity are connected to electricity grids in Europe and USA. Japan has 150 MWe installed. A large solar PV plant is planned for Crete. Research continues into ways to make the actual solar collecting cells less expensive and more efficient. Other major research is investigating economic ways to store the energy which is collected from the sun's rays during the day.

Solar not nuclear is a catch-cry of both antinuclear environmental groups and many technological optimists, particularly as advances in direct solar heating continue to be made.

See also autonomous building, renewable energy.