Four-Year Field Evaluation of Photovoltaic Module Performance and Deviations from Standard Test Conditions in Gusau, Nigeria

Manufacturer-rated photovoltaic (PV) module outputs are specified under Standard Test Conditions (STC), whereas outdoor performance varies with irradiance, module temperature, and other operating conditions. This study compared the STC ratings of nominal 10 W monocrystalline silicon, polycrystalline silicon, and amorphous silicon PV modules with electrical performance recorded outdoors in Gusau, Zamfara State, Nigeria, over four years. Environmental and electrical parameters were logged at five-minute intervals during daytime operation, and annual average field values were compared with the manufacturer-rated specifications. Three principal findings emerged from the reported maximum-power data. First, the monocrystalline module's annual average maximum power (Pmax) decreased from 0.3124 W in Year 1 to 0.0660 W in Year 4, corresponding to deviations of approximately 96.88% and 99.34% below its 10 W STC rating. Second, the polycrystalline module showed consistently low annual average Pmax, ranging from 0.5718 to 0.6386 W, or approximately 93.61–94.28% below its STC rating, with no clear monotonic decline across the four years. Third, the amorphous module's annual average Pmax ranged from 0.1195 to 0.1802 W, equivalent to approximately 98.20–98.81% below its 10 W rating. These results demonstrate a substantial difference between the recorded outdoor average power values and the modules' STC nameplate rating; however, the comparison alone cannot separate environmental and measurement effects from irreversible module degradation. The findings support improved field measurement and quality-control procedures, verification of anomalous electrical readings, and assessment of PV output using operating conditions and appropriately normalized measurements. Such steps can improve the reliability of local energy-yield estimates and technology assessments for deployments in Gusau and similar climates.