A Novel Metric for Quantifying Solar Irradiance Stability: Mapping Solar Irradiance Variability to Photovoltaic Power Generation

The daily stability of solar irradiance critically influences photovoltaic (PV) power generation, yet existing metrics for assessing such stability often fail to establish a robust and consistent correlation with PV output. Conventional metrics typically rely on simple statistical summaries of solar irradiance time series, limiting their generalizability across diverse climatic regimes and temporal scales. To address this gap, we introduce a novel metric—the Solar Instability Index (SII)—formulated by applying the Wasserstein distance to quantify the deviation of intra-day solar irradiance patterns from the idealized diurnal cycle. By leveraging the optimal transport framework, SII provides a distribution-aware measure of irradiance variability that is physically interpretable. At our case station, SII exhibits strong correlations with atmospheric moisture content and available solar energy resources, suggesting its close linkage to synoptic weather events that drive solar resource variability. Through two case studies, we evaluate the effectiveness of SII alongside two existing metrics commonly used in solar resource assessment. The results demonstrate that SII effectively captures low-frequency variations in solar irradiance without relying on arbitrarily assigned thresholds or window sizes, thereby achieving a robust correlation with PV power output compared to the established metrics. Furthermore, SII maintains consistent performance under varying sky conditions, highlighting its adaptability across different operational contexts. As such, SII offers a valuable and practical tool for assessing the potential stability of daily PV power generation. It holds particular promise for applications such as site selection for PV power plants, and for solar irradiance forecasting aimed at optimizing PV output utilization and grid integration.