## 1 Introduction
The parameters of reference, array and final yields, system losses, PV and inverter efficiencies, and PR of a 13 kWp grid-connected PV system in Northern Ireland are analyzed on the hourly, daily, and monthly levels over three years of its operations, as can be seen in [16], and the results were compared with the simulation results obtained by TRNSYS simulation tool [17]. Experimental results and simulation results obtained by SAM and PVSyst simulation tools of PR, efficiency, and losses of a grid-connected 960 kW PV system in Italy are analyzed in [18]. Reference [19] provides the results of efficiencies, PR, and degradation rates by testing three PV systems with different PV technologies (c-Si, a-Si, and CdTe) under the same climatic conditions in Ankara (Anatolia) after 4 operation years. A comparison of the experimental and simulated performance of a grid-connected 49.92 kW PV system in the Irish climate over one year is presented in [20]. The energy efficiency analysis, specifically in terms of specific yield, of eight grid-connected and one hybrid PV system with various system configurations, tilt angles, and orientations, installed in southeastern Poland, is presented in [21]. A comprehensive review of PV system performances with a focus on comparative performance studies of different PV systems worldwide is analyzed in [22]. The outdoor performances (PR, system efficiency, and capacity utilization factor) of three grid-connected photovoltaic (PV) systems with c-Si, p-Si, and a-Si PV technologies and total installed power of 5.94 kWp in Morocco are analyzed in [23]. The analysis of PV-generated power in dependence on insolation, ambient, and module temperatures of PV systems with different PV technologies (c-Si, p-Si, and CIGS) in Poland is given in [24].An experimental efficiency analysis of three PV systems with different PV technologies (p-Si, CdTe, and CIGS) under a temperate climate in Poland is given in [25].The performance parameters of a 1.75 kW grid-connected PV system over the four years of its operation in Sydney (Australia) are determined and compared with simulation results obtained by PVSyst. This analysis is given in [26], while the performance indicators, such as performance index, solar fraction etc., are analyzed in [27]. The performance and I-U parameters of two 20-year-old PV systems installed on the west and east side of the building at NREL (USA) are statistically analyzed in [28]. Special attention is focused on the degradation rate, which was found to be 0.8%/year, which is consistent with historical averages. The degradation rate of PV systems with different powers and PV technology and under different climate conditions is also analyzed in [29; 30; 31; 32; 33; 34; 35; 36; 37]. All those studies provide helpful insight into energy prediction, the economics and profitability of PV system utilization, and PV systems' operation planning and maintenance. However, there are not many experimental studies on long-term PV system operation in Serbia and surrounding countries with similar climates. This study provides the original experimental results of PV system performances in Nis (Serbia) for 10 years of its operations. The collected results are statistically analyzed in order to obtain the PV system performances under real operating conditions for a specific region. Based on experimental measurements, the PV system performance parameters over the 10 years of its operations were determined, and a mathematical prediction model was created. Compared with other similar studies, the difference and originality of this study are reflected in terms of 10-year results obtained for a specific location as well as the application of statistical methods for the analysis of a large number of data obtained by experimental measurements of POA radiation and PV electricity. A regression analysis was performed to obtain a prediction model, and an Analysis of Variance (ANOVA) was applied for testing the model's validity.