What is the accuracy of the different meteo data ?

[André Mermoud]

I have done a comparison between the different sources of data imported in PVsyst, and available for Europe: we can observe a discrepancy of the order of +/- 10 % (yearly sums) between all these sources, and this is of course the first uncertainty of the PV production forecast.

This comparison is available in the PVsyst help "Geographical and Meteorological data > Import from Meteorological data sources > Meteorological data comparisons", or summarized on our site http://www.pvsyst.com.

The differences may be attributed to the different years or periods concerned (climatic variations), the accuracy of the mesurements (sensor calibration, or models for satellite data), the quality of the data recording, etc.

For example, the European climate has significantly evolved: we observe an increase of the order of 5% on an average since the beginning of this century in all measurements. This is namely apparent with the PVGIS database: the new SAF irradiation measurements (based on recent satellite data) are about 5% over the "classic" data from terrestrial measurements of the ESRA project (1981-1990).

I don't have such a study for other regions of the world.

For the USA, the TMY3 are probably reliable data, based on 1961-1990 measurements, and used as reference by everybody. A new database managed by the NREL (SolarAnywhere - SUNY model) provides recent satellite data for the whole territory. The direct import from this database will be available in the version 6, but you can already import these data as ASCII files in the present version.

Please also refer to the works of Pierre Ineichen, available on our site http://www.pvsyst.com:

"Global irradiation: average and typical year, and year to year annual variability", Ineichen, P., 2011,

Research report of the Institut of the Environnemental Sciences, University of Geneva.

and

"Five satellite products deriving beam and global irradiance validation on data from 23 ground stations (IEA)", Ineichen, P., 2011,

Research report of the Institut of the Environnemental Sciences, University of Geneva.

Now please observe: we can have some quality criteria. But probably nobody knows which source is the most reliable, and which one will represent at best the future climate of a given site !