Based on satellite data a global data set covering more than 21 years was derived at the Institute of Atmospheric Physics describing the amount and the variability of the solar irradiance impinging the earth's surface. DLR-ISIS is calculated with the radiative transfer code libRadtran that was developed at the Institute. It is based on the cloud climatology ISCCP (International Satellite Cloud Climatology Project) provided by NASA and global data sets of aerosol, water vapour and ozone distribution. On the one hand the data set is used for climatological purposes. On the other hand DLR-ISIS data are needed for solar energy applications. The profitability of large solar power plants is examined through stable long-term averages as well as specifications of the interannual variability of the solar radiation and regional long-term changes. Therefore results from the DLR-ISIS data set were already used during the planning of the first large solar thermal power plant Andasol.