It is common practice in PV system simulation to use the De Soto model, which describes how to use the 1-diode equivalent circuit model for modules. De Soto’s model scales the shunt with irradiance, making it disappear towards zero W/m 2. Also, the commercial software PVsyst uses a parameterization that reduces the shunt effect when the irradiance goes down. However, the Si solar cells that make up a module typically do not have an illumination dependent shunt. We therefore investigate the origin of the intensity dependent apparent shunt in modules. We show that this apparent shunt (derived from the slope of the quasi-linear region from I SC onwards) is a misinterpretation and has little to do with a shunt conductance. Instead, the module I- V curve slope of the quasi-linear region from I SC onwards stems from I SC mismatches between the cells. Such mismatch can occur from small illumination inhomogeneity or cell production variation. Abandoning the practice of using the I- V curve slope to determine the shunt value for equivalent circuit models of modules (and the corresponding shunt scaling in the De Soto model or PVsyst), contributes to physically more meaningful I- V curve parameterizations and possibly also more accurate PV system energy yield prediction.