Dear Andre, You are right however I'm not comparing daily values; instead I'm comparing the monthly sum. If you check the latest image in my post; at the bottom you will see monthy sum of irradiation is 101,5 kWh/m2. My corresponding production is 75MWh. In the simulation; it gives 95MWh for 104,3 kWh/m2. This I believe has to be inline with the irradiation. Difference between real measured irradiance (monthly sum) and simulated one ise 104,3kWh/m2 / 101,5 kWh/m2 = %2 However corresponding montly production is 95MWh / 75 MWh = %26. So that's my point. Input irradiance value is almost the same to the system however the output is very different. I believe PVsyst is overestimating it due to some errors in my config. I can share the meteo data file If you prefer to do the simulation by yourself. BR,