Edit 27/02: after verification, the date comes from horary satellites photos, so empirical data as an input in particular for the clouds, but the hypothesis of the model regarding the quantity of sunlight going through the clouds are not very well explicited, see https://joint-research-centre.ec.europa.eu/photovoltaic-geographical-information-system-pvgis/getting-started-pvgis/pvgis-data-sources-calculation-methods_en?prefLang=fr)
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