A novel metric for quantifying solar irradiance stability: Mapping solar irradiance variability to photovoltaic power generation

The daily stability of solar irradiance significantly influences photovoltaic (PV) power generation; however, existing metrics for assessing it normally fail to robustly correlate with daily PV output. To address this gap, we introduce anew metric, the solar instability index (SII), formulated by applying the Wasserstein distance to assess the deviation of intra-day solar irradiance pattern from the anticipated diurnal cycle. In our case station, SII closely correlates with atmospheric moisture and available solar energy, suggesting its strong association with synoptic weather events that lead to solar resource loss. We further scrutinize the efficacy of SII alongside two existing metrics through two case studies. The results demonstrate that SII excels in capturing low-frequency variations in solar irradiance without relying on arbitrarily assigned parameters, thereby outperforming the other two metrics in establishing a robust correlation with PV power output. As such, in scenarios involving site selection for PV power plant, SII stands as a valuable metric for assessing the potential stability of daily PV power generation.