Solar constant

The solar constant is the average value of perpendicular solar irradiance received per unit surface at a distance of one astronomical unit (the average distance of Earth's orbit) from the sun. It includes all types of solar radiation, not just the visible light. It is measured by satellite to be roughly 1368 watts per square meter (W/m2), although it fluctuates during a year due to the earth's varying distance from the sun, and by a few parts per thousand from day to day.

 Units are Watts per square meter.Latitude-time distrubtion of incoming solar energy at top of Earth's atmopshere: Units are Watts per square meter.

If there were no reflection, one quarter of this energy flow strikes each square meter of the atmosphere of the earth. Why ? The full energy traveling towards the earth does not vertically hit the full Earth surface, but only the cross section of the Earth. We know that half of the Earth is always in the night and the sun radiation under a low angle is weaker. Since the cross section of a sphere is one fourth of the surface of a sphere the total energy on the Earth surface is equal to one-quarter of the solar constant, or about 342 W/m2.

Despite its name, the solar constant is not constant. Since satellites started measuring its value in 1978, it averages about 1368 W/m2. Big sunspot groups can decrease its value with a few W/m2, but bright faculae fields can push it higher. This is why the solar constant is highest during a sunspot maximum, and lowest during the minimum. In the course of a solar cycle, its average value changes by only about 0.9%.


  • American Meterological Society, Glossary of Meterology, Accessed 11 January 2008.
  • Janssens, Jan, The solar cycle, The Solar Activity Monitor, Accessed 7 January 2008.
  • Pidwirny, Michael (Lead Author); Kevin Vranes (Topic Editor). 2007. Solar radiation. In: Encyclopedia of Earth. Eds. Cutler J. Cleveland (Washington, D.C.: Environmental Information Coalition, National Council for Science and the Environment). Accessed 8 January 2008.

Terms of Use:

The text of this article is original work done by the author(s) and editor(s) listed on the article.  The text of this article is freely available for non-profit educational purposes.  Complete attribution must accompany any reproduction or derivative use, and such attribution must include a link to the original Energy Library source material.  Commercial and non-educational use of material from The Energy Library is prohibited without prior approval from the owners of The Energy Library.