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Hi dear Forum users I'm using PVSyst since a while now as a Student, but only now got into the topic of GHI Data generation (and also (synthetic) TMY). I always thought that GHI was calculated on a flat horizon model and has to be considered with the Horizon in PVSyst (subtracted). But when I saw the GHI map from Meteonorm of Switzerland, it seems that this is already taken into account, since north oriented side of a Mountain has much smaller values in GHI than south oriented. Now that I see it, it seems to make sense, but I wanted to make sure I don't miss a point. My thoughts: — When I get the monthly values of Meteonorm, the irradiation seems to already be the effective irradiance to hit this spot on the map, so the horizon has already been considered. — When I generate a synthetic TMY file, it doesn't take the horizon into account and spreads the hours throughout the day, even at times, where there wouldn't be a direct beam. — And when I subtract the horizon with the built-in function, I subtract it again from the data. Do you agree? I'm interested in your opinions. With real measured hourly data it wouldn't make a difference, because the measuring device would not measure a high GHI in the shadow anyway and therefore nothing could be subtracted in the simulation. It still wouldn't be necessary, but wouldn't do any harm. So the problem lies in the synthetic TMY generation combined with the horizon function. I even summed up the whole Hourly TMY to make sure it fits the yearly values. Only the partial near/shading would then be appropriate. Best, Rafael