The Impact of Cloudy Weather on the Calculation Accuracy of the Soiling Loss Ratio in Photovoltaic Systems

Accurate calculation of the soiling loss ratio (SLR) is essential for photovoltaic power prediction and cleaning optimization. While theoretical power-based methods perform reliably under stable, clear-sky conditions, their accuracy in fluctuating, cloudy weather remains uncertain. This study evaluates the impact of cloudy conditions on SLR calculation through a 20-day comparative experiment using a 10 kW PV system. The power difference between cleaned and soiled arrays defined the benchmark soiling loss ratio (SLRbm), against which theoretical power-derived soiling loss ratio (SLRtpd) was compared. Results show strong agreement between SLRtpd and SLRbm under sunny days but significant fluctuations (mean daily dispersion ratio: 17.8) and frequent anomalous negative values under cloudy conditions. These findings indicate that rapid irradiance changes and MPPT lag in cloudy weather amplify inherent model errors, highlighting the need for revised models adapted to complex meteorological conditions.