Improvements to PVWatts

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).

Image of PVWatts-Irradiance-Error_Marion-1
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:


$$k$$ 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.

Image of PVWatts-SAPM-compare_2
Figure from Marion (2010)

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