Energy from the Sun

Solar power as it is dispersed on the planet and radiated back to space. Values are in PW =10¹⁵ W

Annual average insolation at the top of Earth's atmosphere (top) and at the surface (bottom). The black dots represent the land area required to replace the total primary energy supply with electricity from solar cells.

Earth receives 174 petawats of incoming solar radiation (insolation) at the upper atmosphre at any given time. When it meets the atmosphere, 6 percent of the insolation is rflected and 16 percent is aborbed. Average atmospheric conditions (clouds, dust, pollutants) further reduce insolation traveling through the atmosphere by 20 percent due to reflection and 3 percent via absorption. These atmospheric conditions not only reduce the quantity of energy reaching the Earth's surface, but also diffuse approximately 20 percent of the incoming light and filter portions of its spectrum. After passing through the Earth's atmosphere, approximately half the insolation is in the visile 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) drives the winds and the water cycle. Upon reaching the surface, sunlight is absorbed by the oceans, land masses and plants. The energy captured in the oceans drives the thermohalyne cycle. As such, solar energy is ultimately responsible for temperature-driven ocean curents such as the thermohaline cycle and wind-driven currents such as the Gulf stream. The energy absorbed by the earth, in conjunction with that recycled by the greenhouse effect, warms the surface to an average temperature of approximately 14 °C. The small portion of solar energy captured by plants and other phototrophs is converted to chemical energy via photosinthesys. 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 available to the earth is approximately 3850 zettajoules (ZJ) per year.
• Oceans absorb approximately 285 ZJ of solar energy per year.
• Winds can theoretically supply 6 ZJ of energy per year.
• Biomass captures approximately 1.8 ZJ of solar energy per year.
• Worldwide energy consumption was 0.471 ZJ in 2004.

The upper map (right) shows how solar radiation at the top of the earth's atmosphere varies with latitude, while the lower map shows annual average ground-level insolation. For example, in North America, the average insolation at ground level over an entire year (including nights and periods of cloudy weather) lies between 125 and 375 W/m² (3 to 9 kWh/m²/day). At present, photovoltaic panels typically convert about 15 percent of incident sunlight into electricity; therefore, a solar panel in the contiguous United States, on average, delivers 19 to 56 W/m² or 0.45 - 1.35 kWh/m²/day.