Measurement reproducibility of STC-corrected maximum power of photovoltaic modules under dynamic outdoor conditions: definition, calculation, and application in power degradation evaluation

Power degradation evaluation of fielded photovoltaic (PV) modules is essential for energy yield forecast and profitability optimization of PV systems. However, accurately assessing module power degradation outdoors remains challenging due to small power degradation rates and large uncertainty in power measurements. The classical method for module field performance measurements is acquiring real-time current-voltage curves under specific conditions (above 0.8 kW/m2 irradiance) and correcting them to standard test conditions. This method entails standard uncertainty larger than 1.50% (a typical standardized laboratory-level uncertainty) following the mainstream GUM approach (JCGM 100:2008). To address the problem, in this paper, an uncertainty evaluation procedure based on the RRT approach (ISO 21748:2017) was proposed. The results indicate that the proposed procedure allows to achieve accurate module power evaluation with standard uncertainty ≤1.50% under wider irradiance ranges (0.6-1.0 kW/m2). Additionally, the determination procedure of reproducibility limit for PV field measurements was set forth. The reproducibility limit serves as the threshold that module power degradation can be confidently quantified at a 95% confidence level. Furthermore, combined with the typical 25-year and 30-year power warranties, the required minimum operation time for convincingly confirming module degradation occurring was identified. This work greatly enriches measurement uncertainty research and application for PV field measurements, and provides new insights into accurate module performance and power degradation evaluation under dynamic outdoor conditions.