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PVsyst v6.84: I am importing measured irradiance data from a tilted global POA pyranometer (GlobInc aka GPI). I would expect Mod GlobInc to exactly align with the Mes GlobInc that I am importing; however, this is not the case.

The first measured GlobInc value is 446.7 W/m^2. Upon importing, I look at the modeled GlobInc value at the same timestamp. GlobInc (Hay model) for the same tilt and azimuth is 416.0 W/m^2. So, there is a 7% deviation b/w Mes and Mod GlobInc at this timestamp (at times the deviations are larger than 7%).

The deviations are notably larger at the edges of the day (morning/afternoon) compared to around solar noon.

I have set PVsyst to use the Hay transposition model per the following post:

viewtopic.php?t=40/

Another note: For both Hay and Perez, it reports albedo = 0.2 for GlobInc; however, there is no option to define albedo when importing custom measured irradiance. Should albedo be a user input to the custom data import tool?

That article is quite old. Perez is the default transposition model. Using measured diffuse irradiance will result in higher transposition accuracy for GlobInc.

Thanks @solarguru but the deviations I reported are larger (>10%) using Perez vs. Hay

I am interested to hear PVsyst's opinion on this

There are no issues reported for the imported measurement data however the PVsyst POA does not match the imported values