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Fraunhofer ISE has been involved in monitoring of PV systems since the “1000-roofs-program” in the 1990s. In a few of these “old systems” equipment is still in place, metering PV electricity output and plane-of-array irradiation. The majority of ~300 PV power plants in our monitoring campaign today, however, is large-scale and built in the past 10 years. In this paper, we briefly review the historical development of the Performance Ratio (PR) and how average PR for newly built systems increased to almost 90%. This rather high PR of 90% only holds, however, if calculated by on-site irradiation acquired with c-Si reference cells and for climates comparable with those in Germany. Next, we use about 500 years of monitoring data on aggregate to perform an analysis of variations of the PR over time. To this end, data points at similar environmental conditions are extracted from long-term time series as to calculate so-called “rates-of-change” of the PR. Highly scattered “rates-of-change” are obtained, however, not allowing for the estimation of specific degradation rates on the system level yet. To this end, we finally revisit uncertainties of irradiance sensors and in particular revisit our irradiance sensor re-calibration data.