## 5 Conclusions
The average values of the daily clearness index range between 0.497 in November to 0.692 in July. The annual number of cloudy days is 17 (4.66%), the respective number of partially cloudy days is 137 (37.53%) and 211 days are classified as clear days representing the 57.81% of the annual number of days. As can be seen, the summer months are classified mainly as clear days. The annual average of the direct normal beam radiation on clear days is 27.3 MJ/m\({}^{2}\), while the respective value of direct horizontal is 17.7 MJ/m\({}^{2}\). The annual average of diffuse radiation is 4.2 MJ/m\({}^{2}\), while the global radiation is 23.9 MJ/m\({}^{2}\). Finally, the annual mean of daily sunshine duration on clear days is 11.5 h. The major difference between day types according to \(K_{T}\), is the significant reduction in the beam radiation which also affects the global irradiation level. For example, the annual average daily direct normal radiation available for energy conversion on partially cloudy days is reduced by a factor of 0.61 relative to that available on clear days. The respective reduction of global radiation is 0.65. Generally, the summer season is characterized by relatively small variations in the global radiation, horizontal beam radiation and their corresponding ratios. This means that a relatively constant solar energy source is available for energy conversion systems during this period. Regarding the estimation of solar energy on incline surfaces, it was found that the optimum slope angles during the winter-time range between 50 and 60\({}^{\circ}\), while during summer the slopes are close to zero. In spring the optimum slope angle is 15\({}^{\circ}\), in summer is 0\({}^{\circ}\), in autumn is 45\({}^{\circ}\) and in winter is 55\({}^{\circ}\). The annual optimum angle is about 26\({}^{\circ}\) which is about 10\({}^{\circ}\) less than the latitude of the location. For different optimum slopes for each month of the year, the annual amount of energy is equal of 2221 kWh/m\({}^{2}\), while the measured total horizontal energy is 1938 kWh/m\({}^{2}\). Therefore, the percentage increase of energy is 14.6%. For the case of a permanent slope angle of 26\({}^{\circ}\) throughout the year the inclined panel receives 2097 kWh/m\({}^{2}\) with the increase of about 8.2% from the horizontal level. For the case the slope \(\beta=\varphi\) (35\({}^{\circ}\)) we have high energy production during the months of March and September. The annual increase in energy with respect to the horizontal level is 7.3%. Therefore, higher total energy can be obtained if we use two different slopes one for the summer and one for winter period. For instance, if we use a slope of 10\({}^{\circ}\) for the summer period (April-September), then the total energy for this period is 1313 kWh/m\({}^{2}\), while if we use a slope of 50\({}^{\circ}\) for the winter period (October-March), then the total energy for this period is 878 kWh/m\({}^{2}\). Therefore, the annual total by using the two slopes during the year is 2191 kW/m\({}^{2}\), which is higher by about 95 kWh/m\({}^{2}\) from the case of an annual permanent slope of 26\({}^{\circ}\). As a result, the case with two slopes (\(\beta=10^{\circ}\) for summer and \(\beta=50^{\circ}\) for winter) could have an increase of total annual energy by 13% with respect to the horizontal one. The study has shown that the above methodology of analysis gives valuable information concerning the application of solar radiation in renewable energy resources projects. Investigation, S.P. ; Methodology, A.P. ; Resources, S.P. ; Supervision, S.A.K. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable. These are confidential from the Meteorological Department. The authors would like to thank the Meteorological Department of Cyprus for providing the meteorological data. The authors declare no conflict of interest.