## 3 Results
### Intra-Annual Energy Yields
It shows the monthly solar energy sum averaged over all sites belonging to the same PV\({}_{\rm ctts}\) zone together with the \(\pm 1\sigma\) (standard deviation) from the mean (Figure 7a-d) and the monthly solar energy sum averaged over all sites (without discrimination of the PV\({}_{\rm ctts}\) zone) together with the \(\pm 1\sigma\) (standard deviation) from the mean (Figure 7e). The graphs also contain the best-fit curve to the monthly means together with their sixth-order polynomial equation and R\({}^{2}\), which are repeated in Table 6 for clarity. It is interesting to observe that the best-fit curves vary between the \(\pm 1\sigma\) band and are accompanied by high R\({}^{2}\) values, which are greater than 0.99. These equations are useful to solar engineers and solar entrepreneurs to estimate the expected energy received by inclined solar convertors inclined 25\({}^{\circ}\) S or 30\({}^{\circ}\) S per month in almost any location in Greece. These estimations provided high accuracy because of the high R\({}^{2}\) values. \begin{table}
\begin{tabular}{c c c} \hline \multicolumn{1}{c}{**PV\({}_{\rm ctts}\) Zone**} & \multicolumn{1}{c}{**Equation**} & \multicolumn{1}{c}{**R\({}^{2}\)**} \\ \hline A & \(G_{e,\beta S}=0.0065\,t^{6}-0.2119\,t^{5}+2.6031\,t^{4}-15.6520\,t^{3}+\) & 0.9911 \\  & \(46.4010\,t^{2}-17.3060\,t+340.0000\) & \\ B & \(G_{e,\beta S}=0.0024\,t^{6}-0.0718\,t^{5}+0.8320\,t^{4}-5.5786\,t^{3}+\) & 0.9916 \\  & \(21.7070\,t^{2}-11.0270\,t+78.1620\) & \\ C & \(G_{e,\beta S}=-0.0007\,t^{6}+0.0531\,t^{5}-1.0940\,t^{4}+8.9273\,t^{3}\) & 0.9948 \\  & \(-33.2980\,t^{2}+84.4560\,t+23.5210\) & \\ D & \(G_{e,\beta S}=0.0033\,t^{6}-0.1047\,t^{5}+1.3230\,t^{4}-8.8159\,t^{3}+\) & 0.9922 \\  & \(29.7600\,t^{2}-10.9440\,t+61.1720\) & \\ All & \(G_{e,\beta S}=0.0052\,t^{6}-0.1390\,t^{5}+1.0938\,t^{4}-1.0223\,t^{3}-\) & 0.9938 \\  & \(23.9260\,t^{2}+139.6400\,t+206.1700\) & \\ \hline \end{tabular}
\end{table} 
Table 6: Regression equations of the best-fit curves to the monthly mean \(G_{e,\beta S}\) sums, averaged over all sites together with their R\({}^{2}\) values, where \(t\) is the month in the range 1–12 and \(\beta=25^{\circ}\) or 30\({}^{\circ}\). Figure 6: Intra-annual variation of \(G_{e,\beta S}\) (\(\beta=25^{\circ}\) or \(\beta=30^{\circ}\)) at all sites belonging to the four PV\({}_{\rm ctts}\) zones. The black bold curve represents the intra-annual variation of \(G_{e,\beta S}\) averaged over all sites. Figure 7: Intra-annual variation of the \(G_{e,255}\) or \(G_{e,305}\) sums for all sites in \(\mathrm{PV_{cts}}\) zone A (**a**), \(\mathrm{PV_{cts}}\) zone B (**b**), \(\mathrm{PV_{cts}}\) zone C (**c**), \(\mathrm{PV_{cts}}\) zone D (**d**), and all \(\mathrm{PV_{cts}}\) zones (**e**). The black solid curve represents the average monthly \(G_{e}\) values in every \(\mathrm{PV_{cts}}\) zone or all \(\mathrm{PV_{cts}}\) zones, while the green solid line shows the best-fit curve to the mean \(G_{e}\) values. The dashed lines correspond to +1\(\sigma\) (red curve) and \(-\)1\(\sigma\) (blue curve). The best-fit equations are functions of \(\mathrm{y}=G_{e,255}\) or \(G_{e,305}\) in terms of \(\mathrm{x}=\) month of the year (1,..., 12). R\({}^{2}\) is also given. One might wonder why a tilt angle of \(\varphi^{\circ}\) has not been shown to be optimal in the analysis. To clarify this question, Figure 8 presents the variation of the monthly mean \(G_{e,255-305}\) and \(G_{e,q5}\) sum values together with their difference \(\Delta G_{e}\). This graph unambiguously confirms the main conclusion of the present work: the choice of the optimum tilt angles of \(25^{\circ}\) S or \(30^{\circ}\) S for the maximum solar energy availability on inclined solar convertors in Greece provided a higher yield than the believed yield at a tilt angle of \(\beta=\varphi^{\circ}\). Further proof of that are the insignificant monthly \(\Delta G_{e}\) values, which were negative for most of the year.