Long term outdoor testing of the PV systems at different climatic conditions or different systems under the same ambient conditions are extremely important in producing policies for the transition to renewables. The present research is the testing of three different PV systems under the same climatic conditions of central Anatolia, after four years of operation. The systems consist of a mono-Si, an a-Si thin film and a CdTe thin film arrays. Different calculation procedures of the input solar irradiation on tilted surface are used for comparison. Performances together with degradation rates are computed using different procedures and also using the measured solar data of 20 km away from the installed systems. The accuracy is verified using 4 months of simultaneous measurements. The results showed that the aforementioned different input calculation procedures of the performances are important and determinant. The 44-months-average efficiencies of these three systems are calculated to be 11.86%, 6.40% and 5.30%, respectively. The degradation rates of these systems are obtained as 0.40%, 1.88% and 10.60%, respectively. Although the degradation rates that are calculated for the first two systems are quite similar to those appeared in the literature, CdTe system is considerably higher than those given in the literature. (C) 2016 Published by Elsevier Ltd.