Statistical Trends in Nominal vs. Measured Performance of PV Modules and the Impact of Metastability

The Calibration and Testing Laboratory for PV Modules at Fraunhofer ISE has been conducting characterization measurements on modules for many years, placing emphasis on a stable calibration level and measurement precision. In this publication, we statistically analyze measurement data over the past 12 years and specifically examine the development of the performance conformity of the modules with the nameplate values given by the manufacturers. We present the underlying data filters and assess the results in the context of recent technological developments in cell and module technology. Notably, from 2016 to 2023, we observe a continuous trend towards a negative deviation, meaning that the measured performance under standard test conditions (STC) is lower than the promised nominal value by -1.28 % (median). In 2024, we observe stabilization or slight improvement. To contextualize this observation, we also examine the development of initial degradation due to light induced degradation (LID) along the evolution of different solar cell generations. This shows a significant improvement for newer technologies, leading to a trend reversal, where LID affects modules positively, which could be partly attributed to dark-storage induced metastability phenomena.