Solar energy is energy from the Sun in the form of heat and light. This energy resource drives the climate and weather and supports virtually all life on Earth. Heat and light from the Sun, along with secondary solar resources such as wind and wave power, hydroelectricity and biomass, account for over 99.9% of the available flow of renewable energy on Earth.

Solar energy technologies harness the Sun's heat and light for practical ends. These technologies date from the time of the early Greeks, Native Americans and Chinese, who oriented their buildings toward the Sun to provide light and warmth.  Modern approaches to passive design and sustainable architecture use computer modeling to tie together solar lighting, heating, and ventilation systems within an integrated solar design package.

Sunlight can be used directly to improve agricultural yields, harvest salt, produce potable water, and cook food. Concentrating technologies can magnify the rays of the Sun for high temperature material testing, metal smelting, and lime production. Sunlight may also be converted into electricity, hydrogen and methanol, or stored in eutectic and molten salts.

Solar power is used synonymously with solar energy or more specifically to refer to the conversion of sunlight into electricity. This can be done with photovoltaics, concentrating solar thermal devices and various experimental technologies.
 

Earth continuously receives 174 PW of incoming solar radiation (insolation) at the upper atmosphere.  When it meets the atmosphere, 6% of the insolation is reflected and 16% is absorbed. Average atmospheric conditions (clouds, dust, pollutants) further reduce insolation traveling through the atmosphere by 20% due to reflection and 3% via absorption. These atmospheric conditions not only reduce the quantity of energy reaching the earth's surface, but also diffuse approximately 20% of the incoming light and filter portions of its spectrum. After passing through the atmosphere, approximately half the insolation is in the visible electromagnetic spectrum with the other half mostly in the infrared spectrum (a small part is ultraviolet radiation).

The absorption of solar energy by atmospheric convection (sensible heat transport) and evaporation and condensation of water vapor (latent heat transport) powers the water cycle and drives the winds.  Sunlight absorbed by the oceans and land masses keeps the surface at an average temperature of 14 °C.  The small portion of solar energy captured by plants and other phototrophs is converted to chemical energy via photosynthesis. All the food we eat, wood we build with, and fossil fuels we use are products of photosynthesis.

The flows and stores of solar energy in the environment are vast in comparison to human energy needs.

• The total solar energy absorbed by Earth's atmosphere, oceans, and land masses is approximately 3850  per year.  
• Winds can potentially supply 2.25 ZJ of electricity per year.  
• Biomass captures approximately 1.8 ZJ of solar energy per year.  
• Worldwide energy consumption was 0.471 ZJ in 2004.  

The output of a solar energy system will vary according to its conversion efficiency and the amount sunlight the system receives. For example, in United States, the average insolation at ground level over an entire year (including nights and periods of cloudy weather) is 10.8 to 32.4 MJ/m²/day. At present, photovoltaic panels typically convert about 15% of incident sunlight into electricity; therefore, a solar panel in the contiguous United States, on average, delivers 1.6 to 4.85 MJ/m²/day. By contrast, typical solar water heating systems operating at 60% efficiency will deliver 4.85 to 14.5 MJ/m²/day.

Wattage power ratings of solar systems refer to power output at noon on a clear day. This causes some confusion. If you want to light a 100 Watt bulb continuously you will need a battery and 500 Watts of solar panels.