Our most important primary source of energy is the sun. Its radiation regulates the energy budget of the atmosphere. The total short-wave radiation falling on a horizontal surface is termed "global radiation."

Global radiation is measured in the units W per m² or kJ per minute-cm², both of which indicate an "energy flux density" or "work density." The conversions are 1 kJ/min-cm² = 1.67x10⁵ W/m² and 41.67 W/m² = 1 kWh/m²d.

Radiation measurements taken over hours, days, or months are known as "irradiation" or "energy density" and correspondingly use the units kJ/cm² or kWh/m². The conversion is 1 kJ/cm² = 2.78 kWh/m².

Global radiation deals with the total radiation from both direct sunlight and diffuse celestial radiation. This difference is based on the fact that daylight also exists in cases where the sun is obstructed (e.g. by buildings or by cloudy skies), thus the diffuse components of the sunlight still exert an effect in the form of celestial radiation.

The importance of celestial radiation, especially for the daytime ambient lighting to be discussed in Chapter 3.2.4, results from the cloudiness frequently present at northerly latitudes. This portion of the global radiation actually increases under cloudy skies, but only when the sky is up to 80% covered.

Since diffuse celestial radiation and direct solar radiation comprise nearly identical proportions of global radiation in relation to a horizontal surface, an analysis of the energy budget using only solar radiation would lead to entirely inaccurate values.

Direct sunlight is at its maximum under clear skies. It can be approximated by data for the duration of sunshine at a location. Directly connected with the sunshine, however, is the appearance of shadow. (The sundial that relies on shadows to tell time "only keeps track of the sunny hours.") The considerable patterns of light and shadow resulting from solar geometry are dealt with in detail in Chapter 3.2.2.

In Stuttgart-Hohenheim the average annual total of global radiation amounts to 402 kJ/cm² for the 30-year period from 1961 to 1990. This corresponds to an average value of 127 W/m² or 1,116 kWh/m² of annual radiation power.

Figure 3/4 depicts the distribution of average durations of sunshine and the annual energy supply of the sun on 1 m² surfaces in the Federal Republic of Germany. The duration of sunshine here varies between 1,300 and 2,000 annual hours, while the global radiation varies between 780 and 1,240 kWh/m².

The values of the global radiation correlate very closely with the duration of sunshine, as shown in Table 3/1. This shows the average monthly global radiation totals for Stuttgart-Hohenheim. The table also shows the average actual (measured) duration of sunshine in monthly hours relative to the astronomically possible duration of sunshine. The yearly variation of actual and astronomically possible durations of sunshine is the subject of Figure 3/5.

The detailed statistics necessary for the use of solar technology are found in the comprehensive ATLAS OF EUROPEAN SOLAR RADIATION (1984), Volume 1 of which relates to horizontal surfaces and Volume 2 to inclined surfaces. (Figures 3/7 and Figures 3/8 are based on data from Volume 2 of this atlas.)

Appendix J to the VDI-GUIDELINE 3789, Sheet 2 (1994), contains instructions for the collection of data about radiation patterns in Germany, Europe, and non-European regions.