page_title: Solar irradiance
section_title: Irradiance on Earth's surface
hierarchical_section_title: Solar irradiance / Irradiance on Earth's surface
caption_attribution_description: English: Typical pyrheliometer, for measurement of direct solar radiation
context_page_description: Solar irradiance is the power per unit area, received from the Sun in the form of electromagnetic radiation as reported in the wavelength range of the measuring instrument.
  Solar irradiance is often integrated over a given time period in order to report the radiant energy emitted into the surrounding environment, during that time period. This integrated solar irradiance is called solar irradiation, solar exposure, solar insolation, or insolation.
  Irradiance may be measured in space or at the Earth's surface after atmospheric absorption and scattering.
  Irradiance in space is a function of distance from the Sun, the solar cycle, and cross-cycle changes.
  Irradiance on the Earth's surface additionally depends on the tilt of the measuring surface, the height of the sun above the horizon, and atmospheric conditions.
  Solar irradiance affects plant metabolism and animal behavior.
  The study and measurement of solar irradiance have several important applications, including the prediction of energy generation from solar power plants, the heating and cooling loads of buildings, and in climate modeling and weather forecasting.
context_section_description: Average annual solar radiation arriving at the top of the Earth's atmosphere is roughly 1361 W/m².  The Sun's rays are attenuated as they pass through the atmosphere, leaving maximum normal surface irradiance at approximately 1000 W /m² at sea level on a clear day. When 1361 W/m² is arriving above the atmosphere (when the sun is at the zenith in a cloudless sky), direct sun is about 1050 W/m², and global radiation on a horizontal surface at ground level is about 1120 W/m².
  The latter figure includes radiation scattered or reemitted by atmosphere and surroundings. The actual figure varies with the Sun's angle and atmospheric circumstances. Ignoring clouds, the daily average insolation for the Earth is approximately 6 kWh/m² = 21.6 MJ/m².
  The output of, for example, a photovoltaic panel, partly depends on the angle of the sun relative to the panel. One Sun is a unit of power flux, not a standard value for actual insolation. Sometimes this unit is referred to as a Sol, not to be confused with a sol, meaning one solar day.
caption: A pyrheliometer, mounted on a solar tracker, is used to measure Direct Normal Irradiance (or beam irradiance)
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