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](https://pvpmc.sandia.gov/app/uploads/sites/243/2022/11/PVWatts-Irradiance-Error_Marion-1.png)
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.
![Image of PVWatts-SAPM-compare_2](https://pvpmc.sandia.gov/app/uploads/sites/243/2022/11/PVWatts-SAPM-compare_2.png)
Marion, B. (2008). Comparison of Predictive Models for Photovoltaic Module Performance. 33rd IEEE Photovoltaic Specialists Conference. San Diego, California.