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The shunt resistance Rsh is the inverse of the slope of the I/V characteristic in the neighbourhood of the short-circuit point. When a measured I/V curve is available, it is easy to find it by fitting the data around V=0.

But it is not possible to extract it from the usual manufacturer's data. Therefore PVsyst has to choose a default value. This is determined by calculating the virtual MPP conductance  ( (Isc-Imp)/Vmp ), corresponding to the absolute minimum value for Rsh, and taking a given fraction if this quantity.

With Si-crystalline cells (as well as CIS), the Rsh value is high, and its exact value has rather little effect on the general module behaviour (low shunt current loss). But with amorphous or other thin film technologies, it becomes significant, and should be ideally be specified by the manufacturer in the future.

For a very accurate modelling of all modules, and especially for amorphous, the Rsh should be corrected according to the Irradiance.