Difference between GIPMeas in .MET file and GlobInc in 8760 output

[Andrés Fernández]

Hi everyone,

I've noticed something curious while working with a custom .MET file that includes field-measured irradiance in the plane of array (GIPMeas). After running a simulation and generating the 8760 output file, I've seen that the GlobInc values sometimes differ from the GIPMeas values input.

I understand that GlobInc represents the global irradiance on the plane of the array, but I was expecting it to match the GIPMeas data directly, since that's what I used in the .MET file.

I’m attaching a screenshot showing a comparison between GIPMeas and GlobInc for reference.

Does anyone know why these differences occur? Is there some internal processing or adjustment done during simulation that could explain this?

Thanks in advance!

[Linda Thoren]

Hello,

By importing irradiance data in the plane, PVsyst applies a reverse transposition to determine the global and diffuse horizontal irradiation required for the simulation. For the simulation, PVsyst will utilize the Global on horizontal plane as starting point. This is necessary as the optical treatment (shadings, IAM, etc) requires the beam, diffuse and albedo components. The reverse transposition and re-transposition can indeed cause some discrepancies with the original data. Verify that you in the Simulation use the Hay transposition. You define this is the project settings.

You can read more about the transposition model in the following help pages:
Transposition model - PVsyst documentation
The Hay transposition model - PVsyst documentation