10 year statistics of global horizontal irradiance

In the figures below statistics for the six DRY irradiance stations is shown for the ten year period 2001-2010. The locations of the stations can be here: http://irradiance.dmi.dk/irradiance-data/dry-2001-2010/global-irradiance/

The plots show monthly insolation on a horizontal surface with a shortwave albedo similar to that of grass in units of MJ/(m2⋅month). The data can be divided with 3.6 to obtain results in units of kWh/(m2⋅month). In the plots 10 year minimum, maximum and mean values (μ) are included together with the values in the Design Reference Year (DRY) data set. The mean values are plotted with error bars that show +/- two standard deviations (2σ). For data with a Gaussian distribution more than 95% of the data are within +/- 2σ, or, said in another way, there is less than 2.5% probability of receiving a monthly insolation that is below the lower end of the error bars.

It should be noted here that a 10-year period in meteorology is considered to be too short to obtain robust statistics. A period of 30 years is preferred. On the other hand, by using a 10-year period of data, this data will be more similar to current insolation conditions than data from the 1980'ies, where aerosol pollution levels in Europe were higher than they are now. Visually the effect of of not have the most robust statistics can be seen in months were the error bars are unusually small or large. The statistics for those months should be taken with a grain of salt.


  • Wang, P. R., M. Scharling & K. P. Nielsen: "2001 - 2010 Design Reference Year for Denmark," beta.dmi.dk/fileadmin/Rapporter/TR/tr12-17.pdf, Technical Report 12-17, DMI, Copenhagen, Denmark, 2012.
  • Wang, P. G., M. Scharling, K. P. Nielsen, K. B. Wittchen & C. Kern-Hansen: "2001 - 2010 Danish Design Reference Year - Reference Climate Dataset for Technical Dimensioning in Building, Construction and other Sectors," www.dmi.dk/fileadmin/user_upload/Rapporter/TR/2013/TR13-19.pdf, Technical Report 13-19, DMI, Copenhagen, Denmark, 2013.