Mechanism analysis and quantitative evaluation of series mismatch losses in photovoltaic modules

Performance degradation among photovoltaic (PV) modules during long-term outdoor operation causes heterogeneous electrical characteristics within series-connected strings, leading to series mismatch losses that are often inaccurately assessed using conventional empirical or analytical approaches. This study aims to accurately quantify such mismatch effects by developing a simulation framework based on a two-diode equivalent circuit capable of reproducing the current–voltage (I–V) characteristics of PV modules with high fidelity. The model parameters were extracted through a hybrid analytical and iterative fitting procedure and experimentally verified against measured data. Sixteen representative current-mismatch scenarios were designed to investigate the influence of the mean degradation (μ) and standard deviation (σ) of the module maximum power (P max ) degradation within the string on output performance.The simulation and experimental results demonstrate that the commonly used “barrel effect” model tends to overestimate series mismatch losses, as it neglects the dynamic regulation introduced by inverter maximum power point tracking (MPPT). Statistical analysis reveals that σ is the dominant factor governing mismatch losses, which increase markedly with larger σ values, whereas μ has a relatively minor effect. When σ ≤ 1.71 %, series mismatch losses are negligible and the total string degradation can be approximated by μ alone. For σ > 2 %, mismatch losses become significant and require circuit-level modeling or field testing for accurate evaluation. Furthermore, the study discusses the threshold's applicability by considering the superimposed effects of bifacial gain, inverter scanning logic, and dynamic climatic factors.The outdoor validation confirms both the predictive accuracy of the two-diode model and the applicability of the μ-based simplified assessment method. The findings elucidate the critical role of σ in determining string performance and provide a quantitative foundation for improving power yield estimation and optimizing operation and maintenance strategies in large-scale PV power plants. • Developed a two-diode-based model for accurate series mismatch loss evaluation. • Identified σ ≤ 1.71 % as a negligible-loss threshold for series-connected PV strings. • Demonstrated μ-dominated simplified loss estimation with 0.21 % deviation from field data.