The distribution of yearly Irradiation values for several years (at least 10-15 years) is evaluated using a Gaussian distribution.
The year-to-year variability is the width (RMS) of this gaussian function.
|-||The year-to-year variability is now provided by the Meteonorm database. |
|It is displayed in the "Geographic site" dialog, page "Monthly Meteo", after importing the data from Meteonorm 7.2. |
|-||If you get multi-years data from a meteorological data provider, you can evaluate the variability of these data using the tool "Databases > Compare Meteo data". Here you can choose the years to be accumulated, and choose the "Histo and probabilities" option to see the corresponding plot. |
|-||The variability of the meteo data has been studied in detail by Pierre Ineichen for about 30 different sites, in a document available from our web site (cf ) |
|He has collected data from different sources, and analyzed time series of 10 to 30 years for each of the 30 sites studied. |
|The main results are gathered on the table given below. |
|It appears that the variability is dependent on the annual irradiance: this may be explained by the fact that with sunny climates with many clear days, the possibility of middle or bad days is lower.|
|Therefore for use of these data in the evaluation of the P90 values, we have established an upper estimate which is used as default value. |