With the photovoltaics (PV) market reaching the terawatt scale, assessing the performance of PV systems accurately has become essential. Traditional approaches to fault detection and diagnosis (FDD) and performance loss rate (PLR) evaluation often operate independently, leaving a critical gap in reliability analysis. Reversible faults can bias PLR calculations, leading to misjudgements of system health and misallocated responsibilities in warranty claims. This study integrates FDD into PLR evaluations, analysing over 300 system strings in the built environment, and introduces new metrics including the fault time factor and fault time rate. Results show that smaller systems tend to exhibit more extreme PLR outliers and higher fault time factors, while newer systems show rising fault incidence. Furthermore, a strong correlation between the fault time rate and PLR suggests that increasing fault frequency over time can significantly bias the PLR if not accounted for. Overall, this work emphasises the value of combining FDD with long‐term performance analysis to decouple operational faults from degradation mechanisms, in order to optimise PV system design, maintenance, and operational efficiency. These insights contribute to global energy transition goals and provide actionable strategies for enhancing PV system reliability and minimising levelised cost of electricity (LCOE).