## 3 Results
### Meteorological and Experimental Data
The dirt is accumulated on the pyranometer glass due to the weather conditions and must be cleaned once per year. Influence of measurement error of solar radiation seen as an inner factor of the electric model for the calculation of the output power, will be presented below. However, as an essential part of the thermal models, optical loss (absorptivity, transmissivity and reflectivity), that significantly contributes to the increase or decrease of the proportion of solar radiation, strongly depends on the angle of incidence of the Sun's rays. In our case, the measurements were performed on a dual-axis PV tracking system that tracks the Sun's path (inclination and orientation) once per 4 min. Thus, a change in the inclination and orientation was predicted to calculate absorptivity, transmissivity and reflectivity by \(\pm 3^{\circ}\) incident angle of the Sun's rays. Aforementioned, reflectivity, transmissivity and absorptivity are described by [PERSON]'s law as a function of the incident angle of the Sun's rays. Meteorological and experimental data were also used for output power calculation and validation by using a dynamic electric model (double diode model) and already validated static electric model (empirical equation [10,29,33]). The validation results of both models are shown in Figures 7 and 8. Figure 6: Comparison of static and dynamic thermal models for RMSE and MAE under different weather conditions. The largest output power deviations occur on days with a high to a medium proportion of direct solar radiation (clear and cloudy days). On average, the output power error for 15 days is 3.70 W (dynamic-electric model) and 4.06 W (static-electric model). Figure 9 shows the RMSE and MAE values obtained for different weather conditions: clear, cloudy and overcast. Figure 8: Validation of the dynamic electric model (Matlab/Simulink-double diode model) with experimental data (for 15 days in March 2021). Figure 7: Validation of the static electric model (Matlab/Simulink-empirical equation) with experimental data (for 15 days in March 2021). For the output power calculation, the static model (empirical equation) represents a simple correlation between the output power \(P\), solar radiation \(G\) and operating temperature of the PV module \(T\), while the double diode model considers a more accurate calculation, including several PV cell characteristics. However, the deviation between those two models is extremely small. The RMSE and MAE values differ by only 0.37 W and 0.19 W on average between the static and dynamic electric model. Solar radiation \(G\) has the greatest influence on the output power of the PV module, followed by the operating temperature of the PV module \(T\) and afterward, the air mass factor \(AM\) (which was neglected due to the small influence). To explain the deviation of the output power, the actual measured temperature values of the PV module were used as input variables. It turned out that the output power was only 1%-2% more accurate, which confirms the accuracy of the thermal models. Based on the deviation between the calculated temperature distribution and the output power of the PV module, the assumption of incorrect measurements of solar radiation was confirmed. The output power of PV modules is also significantly affected by degradation, which reduces the output power of the PV module by 1% annually [46]. Degradation of PV modules involves many types of effects such as cell cracking, yellowing, browning and more [47; 48]. The degradation rate takes effect when the output power of the PV module is reduced by at least 10%. However, high-efficiency PV modules with a 50% degradation rate can be more efficient than low-efficiency PV modules with a lower degradation rate [43]. Regardless of the error of measuring devices, the proposed dynamic thermo-electric model consisting of a dynamic thermal model presented in [10; 21; 22; 41; 42; 43; 44] and a dynamic electric model (double diode model) represent a good agreement between measured and simulated results.