## 5 Conclusions  
The present study investigated the solar potential all over Greece through the incidence angles on flat-plate solar panels with south-facing tilt angles in the range of 5-60\({}^{\circ}\) in steps of 5\({}^{\circ}\) (\(\upvarphi\) included) in order to find the optimal tilt for the maximum solar energy in a year under all-sky conditions. The solar availability on a horizontal plane was also included, because this parameter was one of the four criteria for defining the TOTEE zones in Greece for energy purposes in buildings. The analysis showed that the 33 Greek sites adopted in this study did not all belong to the declared TOTEE zones, but most of them had to move to another zone. Therefore, four new zones were formed and were adopted for the solar energy \(G_{\rm e}\) received on a horizontal plane during a year, those being the PV\({}_{0}\) zones (PV\({}_{0}\) zone A: \(G_{e}>1775\) kWhm\({}^{-2}\cdot\)year\({}^{-1}\); PV\({}_{0}\) zone B: 1700 kWhm\({}^{-2}\cdot\)year\({}^{-1}\) < \(G_{e}\leq 1775\) kWhm\({}^{-2}\cdot\)year\({}^{-1}\); PV\({}_{0}\) zone C: 1625 kWhm\({}^{-2}\cdot\)year\({}^{-1}\) < \(G_{e}\leq 1700\) kWhm\({}^{-2}\cdot\)year\({}^{-1}\); PV\({}_{0}\) zone D: \(G_{e}\leq 1625\) kWhm\({}^{-2}\cdot\)year\({}^{-1}\)). Apart from this finding, a second innovation of the present work was the use of data in a TMY instead of data in a long period of time. The annual solar energy sum for each site on inclined surfaces in the range 5-60\({}^{\circ}\) S was estimated, and four new energy zones were determined, namely the PV\({}_{\rm cts}\) zones (PV\({}_{\rm cts}\) zone A: \(G_{e}>1900\) kWhm\({}^{-2}\cdot\)year\({}^{-1}\); PV\({}_{\rm cts}\) zone B: 1850 kWhm\({}^{-2}\cdot\)year\({}^{-1}\) < \(G_{e}\leq 1900\) kWhm\({}^{-2}\cdot\)year\({}^{-1}\); PV\({}_{\rm cts}\) zone C: 1800 kWhm\({}^{-2}\cdot\)year\({}^{-1}\) < \(G_{e}\leq 1850\) kWhm\({}^{-2}\cdot\)year\({}^{-1}\); PV\({}_{\rm cts}\) zone D: \(G_{e}\leq 1800\) kWhm\({}^{-2}\cdot\)year\({}^{-1}\)). The analysis showed that the optimum tilt angles for all sites in Greece, in terms of the maximum solar energy received on flat solar convertors oriented to the south, were either 25\({}^{\circ}\) or 30\({}^{\circ}\), depending on the site. These two inclinations were, therefore, adopted throughout all analyses. This is an extremely useful conclusion for the solar industry in Greece, as it gives a standard inclination for the metallic frames that support the solar panels and, thus, it reduces the cost of the supports. Moreover, the definition of four new (PV\({}_{\rm cts}\)) energy zones was done to provide the solar industry, solar system investors, and solar engineers in Greece with a standardization tool, as far as the classification of various regions of the country in terms of the expected solar potential on surfaces inclined to the south is concerned (see Figures 13 and 14). The intra-annual variation of the solar energy sums averaged over the locations belonging to the same PV\({}_{\rm cts}\) zone, as well as to all PV\({}_{\rm cts}\) zones together, for inclinations of 25\({}^{\circ}\) S or 30\({}^{\circ}\) S were estimated. The plots included the best-fit curves to the annual mean ones and provided equations that estimated the solar energy potential per PV\({}_{\rm cts}\) zone (and all PV\({}_{\rm cts}\) zones) with great accuracy (R\({}^{2}>0.99\)). These expressions are very useful tools to architects, civil engineers, solar energy engineers, and solar energy system investors for assessing the solar energy availability at a place in Greece. For sites not included in this study, these equations may serve as a guiding tool for guessing the approximate energy. Nevertheless, this guess may be confirmed by the map in Figures 11a and 13. Another important finding of this study was the confirmation of the perception that the optimum tilt angle in Greece was not \(\upvarphi\) S, but 25\({}^{\circ}\) S or 30\({}^{\circ}\) S instead. Figure 8 provided this sound proof.