## 3 Results and Discussion
### Performance Parameters  
The 10-year average values of energy yield (Ey) depending on the 10-year average values of total POA radiation reached on the total PV array area by month are given in Figure 3.  
The 10-year average values of total POA radiation on the total PV array by month range from 667.158 kWh (December) to 3204.684 kWh (July), while 10-year average values of energy yield by month range from 82.196 kWh (December) to 276.746 kWh (August), as can be seen in Figure 3.  
One of the main parameters of PV system performance in real conditions is the efficiency of the given system (\(\eta_{\text{syst}}\)), which is determined as the ratio of the PV system output (AC output from the inverter and transmitted to the city grid) to the total POA radiation on the PV array [10; 39]. The 10-year average values of PV system energy efficiency (\(\eta_{\text{syst}}\)) by month are given in Figure 4.  
The 10-year average values of PV system energy efficiency (\(\eta_{\text{syst}}\)) by month range from 8.61% (July) to 12.61% (January), while the PV system efficiency at STC is 19%. Figure 3: The 10-year average values of energy yield (Ey) depending on the 10-year average values of total POA radiation reached on the total PV array area by month. The reference yield (Y\({}_{\rm r}\)) represents the ratio between the total POA radiation (in kWh/m\({}^{2}\)) and the reference solar radiation of 1 kW/m\({}^{2}\) (reference solar radiation is radiation at STC) and is a function of the geographical location and orientation of the PV array. On the other hand, the specific yield factor (Y\({}_{\rm f}\)) represents the ratio between the PV system output (in kWh) and the total installed power of the PV system (in kW\({}_{\rm p}\)). The specific yield factor (or final yield) is usually utilized to normalize the produced system energy relative to the PV system size. It could be said that Y\({}_{\rm f}\) is a good parameter for comparing the energy produced by different sized PV systems [10]. The ratio between the specific yield factor (Y\({}_{\rm f}\)) and the reference yield (Y\({}_{\rm r}\)) for 10-year measurements is given in Figure 5.  
Figure 5 shows that there is a linear dependence between the specific yield factor and the reference yield. Plotting the regression line for each month during the 10 years (Figure 5) allows for the identification of the slope and, therefore, the average monthly performance ratio. In this case, the regression coefficient (slope in the regression model) amounts to 0.617536 and responds to the average change of the expected value of the dependent variable Y\({}_{\rm f}\) for the unit change of the independent variable Y\({}_{\rm r}\). Thus, the ratio between the specific yield factor (Y\({}_{\rm f}\)) and the reference yield (Y\({}_{\rm r}\)) represents Performance  
Figure 4: The 10-year average values of PV system energy efficiency (\(\eta_{\rm sysl}\)) by month. Figure 5: The ratio between the specific yield factor (Y\({}_{\rm f}\)) and the reference yield (Y\({}_{\rm r}\)) for 10-year measurements. Ratio (PR). Hence, the PR defines the rate of effective produced energy with the energy the PV system would generate if it continually worked on its efficiency at standard test conditions. So, the PR comprises all PV optical and electric losses in a system without being directly dependent on input parameters such as meteodata (temperature, solar radiation, wind speed, cloudiness, snow, precipitation, presence of aerosols, etc. ), module efficiency, and orientation of the PV array [10; 29; 30; 31; 32; 33; 34; 35; 36; 37]. Graphics of the change of yearly average values of the Performance Ratio (PR) for a 10-year period is given in Figure 6.  
The yearly average values of PR for the 10 years, from 2013 to 2022, range from 0.849 to 0.93. On the other hand, the 10-year average values of PR by month range from 1.01 (December) to 0.713 (July).