## 5 Conclusions
It is very interesting to know the extreme values of the energy received on a tilted plane. These two extremes occur in the winter (minimum) and summer (maximum) months because of the corresponding intensities of the solar radiation. Considering these two seasons, the seasonal \(G_{e,255}\) or \(G_{e,305}\) sum values were estimated for all stations. Similar graphs were provided for the seasonal \(G_{e,255}\) or \(G_{e,305}\) sum values averaged over each PV\({}_{\rm cts}\) zone. From those graphs, a discrepancy between the winter values in the PV\({}_{\rm cts}\) zones B and C was seen, in the sense that the solar energy in the latter zone was greater than that in the former. On the contrary, the summer \(G_{e}\) averaged sums in all four zones followed an expected decreasing trend from PV\({}_{\rm cts}\) zone A to PV\({}_{\rm cts}\) zone D. Nevertheless, this discrepancy had no effect on the annual energy yield. A comparison between the TMY-PV-derived \(G_{\rm e}\) curves with those using the PV-GIS tool with the SARAH 2005-2016 solar radiation database for the 33 locations showed a remarkable coincidence, one which was demonstrated at four selected sites. An observed overestimation in the annual solar energy yield at two sites might have been due to the different data periods used in both methods and the algorithm utilized for converting thesatellite signals into solar radiation values [68]. Improvement of the algorithm is, therefore, considered necessary. The main conclusion of the analysis in this study is that a solar flat-plate convertor receives its maximum energy all year in Greece with a tilt of 25\({}^{\circ}\) S or 30\({}^{\circ}\) S. As Greece covers latitudes between \(\approx\)35.2\({}^{\circ}\) N and \(\approx\)41.0\({}^{\circ}\) N, the optimum tilt angles, according to [43], lie in the range [\(\approx\)45.8\({}^{\circ}\) S, \(\approx\)53.3\({}^{\circ}\) S], which is different from our result of 25-30\({}^{\circ}\) S. On the contrary, [PERSON] [47] estimated the optimum tilt angles for Athens at 25\({}^{\circ}\) S in the case of flat-plate panels all year round, a finding that quite agrees with ours. The work by [48] for Athens found that a tilt angle equal to the site's \(\varphi\) collected the maximum energy all year round. This conclusion is very similar to ours, if the tilt of \(\varphi^{\circ}\) is included in the 25-30\({}^{\circ}\) S range. The work by [49] for 35 sites in the Mediterranean region found optimum tilt angles with southern orientations for Irakleio, Athens, and Mika of 35.1\({}^{\circ}\), 36.8\({}^{\circ}\), and 38.7\({}^{\circ}\), respectively, values which differ from those in the present study. [PERSON] and [PERSON] [50] estimated the optimum tilt angle for solar applications in Athens to be 29\({}^{\circ}\), quite close to our 30\({}^{\circ}\). In addition, [PERSON] et al. [52] found an optimum tilt angle of 30\({}^{\circ}\) S in Athens, which quite agrees with our results. Conceptualization, methodology, data collection, data analysis, and writing original draft manuscript, H.D.K. ; methodology, mathematical formulation, FORTRAN code development for the calculation of solar radiation on inclined surfaces, review and editing of manuscript, B.E.P. Both authors have read and agreed to the published version of the manuscript. This research was implemented in the frame of the KRIPIS-THESPIA-II project under the grant MIS 5002517, funded by the General Secretariat of Research and Technology in Greece. The authors are thankful to HNMS for the disposition of the meteorological data from 33 stations of the network with the purpose of building the typical meteorological years. The authors declare no conflict of interest.