## 2 Materials and Methods  
The definitions of azimuth angle and tilt angle mentioned in literature differ. The tilt angle is the angle of the photovoltaic modules from the horizontal plane for a fixed (non-tracking) mounting [27; 54]. Generally, it is recommended that photovoltaic system should be installed with a tilt angle which is equal to the latitude of the site [55; 56]. The visualization of tilt angle meaning is in Figure 1 and in detail it is described by Figure 2.  
Figure 1: Illustration to the definition of the tilt angle, the solar azimuth angle. and the azimuth orientation. Figure 2: Experimental PV power plant with PV modules PHONOSOLAR PS240P\(-\)20/U. The azimuth angle indicates the position of the PV modules relative to the South; \(-90^{\circ}\) is East, \(0^{\circ}\) is South and \(+90^{\circ}\) is West [57; 58]. The azimuth angle should be South for Northern Hemisphere and North for Southern Hemisphere [59; 60]. The next way of azimuth angle definition is at the point of observation. The angle measured between the horizontal plane of the north point and a point on the circle of the horizon intersected by the arc of a vertical plane passing through the zenith and the sun's position at that time [4]. The position of the Sun in the sky at any moment can be defined by two very important angles, the first is the solar altitude angle and the next is the solar azimuth angle. These angles are physical parameters of the position of the Sun with respect to a given place on Earth. Therefore, they are independent of the inclination and orientation of the surface. On Figure 1 the Solar altitude angle \(\alpha_{A}\) is the angle between the horizontal and the line to the Sun (\(0^{\circ}\leq\alpha_{A}\leq 90^{\circ}\)). The complement of this angle is the zenith angle (\(\alpha_{Z}\)). It is defined by the vertical and the line to the Sun (i.e., the angle of incidence of beam radiation on a horizontal surface); \(\beta\) is the tilt angle and \(\gamma\) is the azimuth angle which determine the azimuth orientation of the solar module. Solar azimuth angle \(\alpha_{S}\): angular displacement from South of the projection of beam radiation on the horizontal plane. (In general, \(\alpha_{S}=0\) is South, \(\alpha_{S}<0\) is East, and \(\alpha_{S}>0\) is West). Once these two angles are established, it will help to define exactly the solar reaching the point on Earth where the solar system is going to be erected. The position of the Sun can be determined also mathematically through equations for the solar altitude angle and the solar azimuth angle (Equations (1) and (2), where \(\alpha_{L}\) is the latitude, \(\delta\) is the declination and \(\omega\) is the hour angle. \[\alpha_{A}=\arcsin(sina_{L}\cdot sin\delta+cos\alpha_{L}\cdot cos\delta\cdot cos \omega) \tag{1}\]  
\[\alpha_{S}=\arcsin\left(\frac{cos\delta\cdot cos\omega-sina_{A}\cdot cos\alpha _{L}}{cos\alpha_{A}\cdot sin\alpha_{L}}\right) \tag{2}\]  
The next way for their identification is the solar charts. Presented facts are very valuable from the theoretical point of view, but in practice we usually know only orientation represented by azimuth angle \(\gamma\). The literature has equations for very precise prediction of the energy and power balance. For example, Equation (3) represents percentage of output power drop (\(D\)) and \(\beta\) is the PV module tilt angle shift (\({}^{\circ}\)). If we know the output power in selected time range, we can calculate the total energy production of PV system for different value of tilt angle. Equation (3) was obtained for very specific operating conditions described in literature [61]. \[ \begin{array}{l}D=1.416674\cdot 10^{-9}\cdot\beta^{6}-3.59548024\cdot 10^{-9} \cdot\beta^{5}-5.57767010\cdot 10^{-6}\cdot\beta^{4}+\\ +1.64493758\cdot 10^{-5}\cdot\beta^{3}-1.724028491\cdot 10^{-4}\cdot\beta^{2}-7.114 35953\cdot 10^{-3}\cdot\beta+\\ +99.37322857\end{array}