Bill Marion from NREL developed a correction method that improves the ability of PVWatts to represent the nonlinear behavior at low irradiance, which is evident in x-Si module performance (Marion, 2008).

Figure from Marion (2010) presentation: “Overview of the PV Module Model in PVWatts”

Marion found that the error could be accurately characterized by the following equation:

$k=\frac{0.2P_{mp0}-P_{200}}{P_{mp0}}$.

$k$ is determined from a measurement of $P_{mp}$ at 200 W/$m^{2}$, which add an additional input parameter.  However, IEC 61215 and 61646 require this measurement to be made.

Figure from Marion (2010)

Marion, B. (2008). Comparison of Predictive Models for Photovoltaic Module Performance. 33rd IEEE Photovoltaic Specialists Conference. San Diego, California.