Solar Power » Radiation

Annual Solar Irradiance, Intermittency and Annual Variations

While no model can fully replace actual measurements to fully assess the potential of a site, solar irradiance maps can provide a first insight. The following graphs show various maps with average annual energy values on fixed, due-south facing surfaces that are optimally inclined.

In addition, system designers and project developers should take into account effects of intermittency and annual variations.

World Map - Annual global versus direct irradiance

The following maps highlight the differences between direct and global irradiance where "global" includes diffuse light. Areas with a high proportion of diffuse light include Northern Europe, South-East China and the tropical belt around the equator. (Source: )

World Annual Global Irradiance

World Annual Direct Irradiance


Europe - Annual global irradiance

In Europe, the Alps and adjacent mountain ranges form the natural border between the sunny south and the more diffuse north, as seen here on a 2011 irradiation map from .

Insolation Europe


Africa and the Middle East - Global Irradiation

The deserts of Africa, the Sahara, Namib Desert and the Arabian Peninsula, are among the places with highest irradiation on earth, especially 1,000km south of the Mediterranean where the annual global irradiation is about twice that of southern Germany.


If we assume a peak intensity of 960W/m2, the ratio of the average intensity to the peak intensity in Europe is between 12% (Iceland) – 24% (Southern Spain) with the average around 18%. This is a measure of the intermittency of the solar energy source. It also means that solar systems have to be built to cope with peak intensity, but will on average only be able to convert 18% of that peak. This factor is closely related to the so-called capacity factor.

Annual Variation of solar insolation

Annual variation of solar insolationThe energy (or insolation) received on a surface throughout the year varies relatively little from year to year.

Intuitively, more energy comes from the time intervals with high irradiation. The contribution of highly intense light can vary significantly from one year to another. The standard deviation of the overall annual energy, however, is around +/-4%.

I.e. the annual energy delivered by the sun does not vary greatly year on year. The sun provides a stable "traffic".




Downloads: Annual variation of insolation in Kassel, Germany (source: BP)

Links- European Commission - Joint Research Centre
- provided by the European Commission.

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