Global incident on the ground + Sky Diffuse is different for the same design and location!

[PVsystUser]

Below are the PVsyst results for two variants, with the only difference being the module wattage. The module dimensions and bifaciality are the same. I have included a few parameters below the PVsyst loss tree.

Why has the global incident on the ground been calculated over two different square meters? Also, the sky diffuse values are different. Isn’t the sky diffuse on the rear side dependent only on the solar resource, environmental parameters like albedo, and the geometry of the system? Why do I see a difference here?

All these small differences resulted in the 625W module performing better than the 630W module.

 

[Michele Oliosi]

Hi, I think that you are not focusing the correct reason for the differences in your loss diagram. Indeed, the effective additional “Global irradiance on rear side” is the same in both cases, i.e., you can expect that the area contributions to the bifacial model are marginal.

More important than that is the module area at the stage “Effective irradiation on collectors”. This is not an energy value, so I think the highlighting method in PVsyst failed to see the difference in area. But this is more significant. If there is a difference in area here, it means that the modules have different dimensions in some way, maybe very small. Please carefully compare all parameters of the modules. Also, please make sure that the dimensions of the 3D scene tables have not changed because of that.