## 4 Discussion
The equation for the simplified performance model has the form described by Equation (15):  
\[P(t)=k_{type}k_{const}\cdot k_{tilt}\cdot k_{azin}\cdot k_{tcell}\cdot k_{loss} \cdot f_{G}(t)\cdot P_{inst} \tag{15}\]  
where \(P\) is power of PV system in kW, \(k_{type}\) coefficient for PV module type, \(k_{const}\) is coefficient for PV system construction, \(k_{tilt}\) coefficient for PV module tilt angle, \(k_{azin}\) is coefficient for PV module azimuth angle, \(k_{tcell}\) is coefficient for PV cell temperature, \(k_{loss}\) coefficient for PV system loss, \(f_{G}(t)\) is the function for solar radiation intensity in kW\(\cdot\)m\({}^{-2}\), and \(P_{inst}\) is installed power of PV system in kW\({}_{\rm P}\). The tilt angle of PV panels in relation to the location and azimuth position was investigated experimentally in many studies. Latitude based model for tilt angle optimization for solar systems in the Mediterranean region was detected by [67]. Mentioned article presents also quadratic regression model that allows the prediction of the annual optimal tilt angle. Optimum tilt angle for 1 MW PV system at Sukkur in Pakistan was determined and described in literature [68]. The research results confirmed the tilt angle variations during the year from 0\({}^{\circ}\) to 61.1\({}^{\circ}\) in the northern Pakistan. It was found that optimal tilt angle for that region is 29.5\({}^{\circ}\). An experimental and mathematical investigation of optimal tilt angle and effects of reflectors on PV energy production was presented by authors [69]. Experimental results show that for gain of optimum power output the tilt angle needs to be changed every month. From experimental study located in Nitte (India), it was found that the PV system produced maximum power output in April for tilt angle 0\({}^{\circ}\); in March optimal tilt angle is 13\({}^{\circ}\), for February 22\({}^{\circ}\), tilt angle for January is 33\({}^{\circ}\) and 30\({}^{\circ}\) for December. World estimates of PV optimal tilt angles and ratios of sunlight incident upon tilted and tracked PV panels relative to horizontal panels were described in [70]. One of the best ways to optimize the energy balance of PV panels is to use solar trackers that can optimize their position. The use of solar trackers can increase electricity production by around a third, and some claim by as much as 40% in some regions, compared with modules at a fixed angle. In any solar application, the conversion efficiency is improved when the modules are continually adjusted to the optimum angle as the sun traverses the sky. As improved efficiency means improved yield, use of trackers can make quite a difference to the income from a large plant. This is why utility scale solar installations are increasingly being mounted on tracking systems [71]. One of the best technologies for PV module position optimalization are dual axis trackers. Dual axis trackers typically have modules oriented parallel to the secondary axis of rotation. No matter where the Sun is in the sky, dual axis trackers are able to angle themselves to be in direct contact with the Sun [72]. Very innovative idea is presented in literature [51] where authors described design and performance of a new self-powered LCPV solar trackers using bifacial solar cells and concentrating mirror. All mentioned references confirmed that the orientation of the module with respect to the tracker axis is important when modelling performance. However, when installing solar trackers, there are design and economic limitations that do not allow their use. In this case, it is essential to install a PV system with fixed panels into the best operating position and it is appropriate to use the created mathematical model. All the above mentioned publications confirm the local variability of the PV panels tilt angle and azimuth orientation. It is in accordance with the results presented in the text.