## 4 Conclusions and Discussion
Nevertheless, this universality remains to be confirmed at other locations in the world with different climate and terrain characteristics. A graph of CF as a function of \(\rho_{\rm r}\) was prepared for different values of the tilt angle in the range 5\({}^{\circ}\)-55\({}^{\circ}\). Best-fit curves to the data points were estimated and were found to be linear, with decreasing slopes in proportion to decreasing tilt angles. These curves are also assumed to be nomograms, since similar graphs in [1] had the same shape. Nevertheless, this kind of nomogram has to be confirmed at other sites worldwide with different climate and terrain characteristics. Three innovations appeared in the present study: (i) For the first time, solar maps for Saudi Arabia of the maximum energy received on optimally-inclined flat surfaces mounted on a system with a vertical axis that rotates and tracks the Sun were derived; (ii) for the first time, the three energy zones identified in [1] were used here and the same sites asin [1] were accommodated in the same zones; (iii) universal curves (nomograms) of CF in relation to \(\beta\) and \(\rho_{\rm r}\) were derived for this case, too. As far as the utilisation of the results presented here is concerned, this can be summarised as follows. The solar industry has now a new rule for the inclined supporting frames mounted on single-axis solar systems; the inclined frame must have an angle of 40\({}^{\circ}\), 45\({}^{\circ}\), or 50\({}^{\circ}\) to the local horizon, depending on the SEZ to be installed. The same utilisation guidelines given by [1] for the static solar systems apply to this case, too. This last paragraph of the work is devoted to the performance efficiency and operation cost of a static (fixed-tilt) and a single-axis solar PV system. [PERSON] et al. [26] examined the levelised cost of electricity (LCOE, in \(\ell\)kWh\({}^{-1}\)) for the two systems at five sites (two in Spain, one in Saudi Arabia, one in Brazil, and one in the USA) and found it varying according to site and type of the PV system. Overall, the LCOE for the single-axis PV system is a little bit higher than that for a fixed-tilt one. Nevertheless, the difference is not big and varies as +4.4%, 0%, 0%, +8.6%, and +1% for the five sites, respectively. Another study [27] concluded that the amount of electricity generation by a single-axis PV system in the USA is about 12-25% higher than that for a fixed-tilt one. A study for Jordan [28] showed that a dual-axis PV system generates 31.29% more annual electricity than a fixed-tilt one. For Europe, it has been found [29] that a single-axis PV system produces an annual energy yield of 30% higher in southern, and 25% in central Europe and up to 50% in northern Scandinavia. Data collection, data analysis, writing--review and editing, A.F. ; Conceptualisation, methodology, data collection, data analysis, writing--original draft preparation, H.D.K. ; writing--review and editing, M.A. ; writing--review and editing, M.A.O. All authors have read and agreed to the published version of the manuscript. Researchers would like to acknowledge the support provided by the Deanship of Scientific Research (DSR) at the King Fahd University of Petroleum and Minerals (KFUPM) for funding this work through project no. DF181010. Not applicable. The solar radiation data and the ground-albedo ones for Saudi Arabia are publicly available and were downloaded from the PV-GIS platform ([[https://ec.europa.eu/jrc/en/pvgis](https://ec.europa.eu/jrc/en/pvgis)]([https://ec.europa.eu/jrc/en/pvgis](https://ec.europa.eu/jrc/en/pvgis)) (accessed between 19 December 2020 and 16 January 2021)) and the Giovanni website ([[https://giovanni.gsfc.nasa.gov/giovanni/](https://giovanni.gsfc.nasa.gov/giovanni/)]([https://giovanni.gsfc.nasa.gov/giovanni/](https://giovanni.gsfc.nasa.gov/giovanni/)) (accessed on between 6 and 30 May 2021)), respectively. The AANOVA solar radiation data are available on request, but they were used on a self-evident permission to the second author as ex.