This is a complex problem indeed.
With the PVGIS old monthly values for example (before 2018), or any ground-measured data, the horizon loss of irradiance is already included in the meteo data.
This is very difficult to manage because:
- the horizon line is very sensitive to the exact location: the horizon at the exact sensor position may be very different of the horizon seen by the PV system at some hundreds of meters.
- if applied to monthly values, the synthetic generation doesn't "know" the horizon, and will generate values with beam component below the horizon. Which will be incorrect during the simulation of course.
Therefore in any case (except with real on-site measurements), the meteo data should ideally be horizon-free and the horizon effect should be computed within PVsyst, using the effective horizon line of the site.
This is the case, namely in the Meteonorm V7.1 or NASA-SSE data provided within PVsyst.
But other sources (like for example PVGIS, monthly values) deliver indeed meteo data already corrected for the horizon of the site.
There is no simple way for evaluating or avoiding the horizon loss in the original meteo data.
With monthly values, we could generate a full year of hourly values, perform a synthetic generation, evaluate the loss (on beam hourly values) due to the pre-specified horizon of the site, and then add these losses to the monthly original values for a re-generation of hourly file. This file will have better "free horizon" values on which we can apply the real horizon of the PV system during the simulation.
With hourly measured values, we can use the data "as such" for the simulation, but without defining an horizon line. If we really want to evaluate the loss due to horizon, we should reconstruct the supposed beam component below the horizon line. We have now models for doing this with some confidence (probabilistic continuity of the weather).
We have tested these methods, and they can give acceptable results. But sorry, I don't know any public program doing this, and I did not yet implement this special case in PVsyst.
Especially for PVGIS, this is not possible directly within PVsyst as the horizon line profile is given only as a plot in PDF, which cannot be read automatically in a numeric way by PVsyst. But you can follow the procedure proposed above manually.