A decisive factor in the technical and economic evaluation of PV systems and power plants is the reliable determination of degradation rates (performance decrease over time), which are usually in the order of a few per mill per year. Due to the considerable variation of outdoor operating conditions, sophisticated methods of data selection and analysis have to be developed and verified for this task. At ZSW, various methods were examined and then validated on the basis of long-term data series gathered by monitoring PV modules and PV generators set up on the Widderstall test field. Methods based on the regression of a large quantity of individual performance ratios taken with a high temporal resolution, which were determined at irradiance levels close to the reference value of 1,000 W/m², proved to be meaningful.

These regression methods can determine the degradation rates of PV elements with very satisfactory uncertainties (< 0.05%) and are statistically superior to other methods that aggregate operating measurement data, for example, by calculating the median over certain time periods.