Evaluation of degradation energy productivity of photovoltaic installations in long-term case study

In the last twenty years, photovoltaic installations have become a popular form of renewable energy sources, both in Europe and around the world. One of the European pioneers in this field was Germany. Photovoltaic installations are inevitably associated with a decrease in productivity during operation years based on power degradation of photovoltaic cells. This article analyzes the decrease in energy productivity of installations over 16 years using the example of two rooftop photovoltaic installations and proposes a modified methodology for calculating the decreasing effect. The tested installations are located in the south-west of Germany, in the town of Sinsheim. The first installation, I-1, has a capacity of 37.8 kWp (Suntech panels, 150 Wp), and the second one, I-2, has a capacity of 18.48 kWp (Suntech panels, 165 Wp). In the first two years, the actual productivity calculated from the theoretical productivity estimated for the installation (modified Performance Ratio value) was 95.8% for I-1 and 96.8% for I-2. However, in the years 2020-2021 (the last two years of the installation's operation), the actual productivity calculated from the theoretical productivity estimated for the installation was 93.9% for I-1 and 93.8% for I-2. The reported yearly power degradation rate in the analyzed installations was 0.11%/year for I-1 and 0.2%/year for I-2, which is lower than in the literature examples.