## 4 Discussion  
The performance of solar modules depends on real external conditions, which is clearly visible in the results presented in this work, based on the measurements collected under temperate climate which is characterized by significant changes in both daily and yearly insolation and temperature. The role of solar radiation intensity on charge carriers generation and parameters of PV cells is direct and obvious. Nonetheless, it is worth remembering that most of the radiation absorbed by the cells is not converted into electricity, but it increases their temperature, thereby reducing the efficiency.  
The influence of the temperature on the performance of solar modules, revealed in the presented experimental results, is different for various types of modules, since they are built with different semiconductors materials. In general, the semiconductor bandgap should correlate well with the solar radiation spectrum to achieve its maximum absorption. The optimum value of the bandgap, within the range of 1.0-1.6 eV results in good efficiency, which is observed even for single solar cells [35]. Temperature increase leads to narrowing of the semiconductor bandgap and also the intensification of recombination processes, in which current carriers disappear.  
The bandgaps of the semiconductors used in the studied modules were different: 1.1 eV for polycrystalline silicon, the material with a crystalline fraction of over 95%, and 1.45-1.5 eV in case of cadmium telluride [36; 37; 38]. Copper indium gallium selenide (CIGS) is a solid solution characterized by a higher bandgap value of around 1.04-1.68 eV, which depends on the exact composition of the material [39]. In the presented investigations, the role of the bandgap value is thus visible since the use of wider bandgap semiconductors extend their operational temperature [40]. The broader bandgap of CIGS may result in better resistance of this material to temperature increase.  
The comparison of the presented results with studies conducted by other authors is rather difficult, even taking into account investigations performed at similar latitude, since the technical parameters of the modules and other devices differ, as do weather conditions in summer and winter. Exemplary studies performed in the UK show better performance of CIGS in winter due to the spectrum [11]. The investigations focused on the assessment of the spectral impact on different PV technologies,  
Figure 11: Comparison of calculated monthly performance ratio (PR) of each technology of the modules.  
performed in Germany [31], indicate spectral gains of 2.4% for CdTe, 1.1% for c-Si (that can be compared with pc-Si), and 0.6% for CIGS during the entire year. Temperature coefficients of power estimated in the Netherlands for different PV technologies are higher than technical data for CdTe and CIGS and similar for poly-Si [20]. Considering the results of the investigations in the mentioned countries, it is worth remembering that the weather conditions in both winter and summer differ from those in Poland in spite of similar latitude.  
In general, all kinds of modules in the analyzed solar plant of the whole nominal power of 1.4 MWp produced 1530.23 MWh in the entire 2015 year. This achieved result is remarkable, considering the role of the climatic zone in which the PV plant is situated, however, further long-term measurements are necessary. The degradation process of modules has to be observed and taken into account in long term comprehensive assessment. All kinds of modules are sensitive to some external factors, such as humidity that leads to the failure of electric parts, snow, dust and other pollutions that cause hot spot appearance, and even damage of the modules [41; 42]. However, it is also worth emphasizing that external conditions influence particular types of modules in different ways, since the construction of the traditional and thin film modules is not the same. Internal processes in various semiconductor materials also contribute to the degradation.