## 2 Materials and Methods
### Data Analysis  
The data sets mentioned in the previous section were obtained from the HNMS in the frame of the \"Development of synergistic and integrated methods and tools for monitoring, management and forecasting of environmental parameters and pressures\" (KRIPIS-THESPIA-II) project, during which the atmospheric research team of the National Observatory of Athens derived the TMYs-PV (as well as TMYs for four other applications; see the end of Section 1) for 33 sites in Greece. Each TMY-PV at any of the 33 locations in Greece consisted of hourly values of the ambient temperature (in \({}^{\circ}\)C), relative humidity (in %), and global horizontal and direct horizontal solar irradiances (in \(\mathrm{Wm}^{-2}\)). All mentioned parameters referred to the station's altitude and went through quality control tests during the generation process of the TMYs (see [54]). Figure 1: Distribution of the 33 selected HNMS stations (blue triangles) across the 4 climatic zones of Greece for energy purposes (orange = climatic zone A, green = climatic zone B, blue = climatic zone C, and purple = climatic zone D). The numbers on the map inside the triangles refer to the HNMS stations. The original Sun's azimuth and elevation (SUNAE) routine, first introduced by [PERSON] [59], together with its modifications [59; 60; 61; 62; 63] ran with the geographical coordinates of the 33 stations in local standard time (LST = UTC + 2 h) to estimate the solar altitude \(\gamma\). The SUNAE algorithm computed \(\gamma\) every 30 min prior to the hour, and this value was assigned to the next hour afterward (e.g., computation of \(\gamma\) at 08:30 LST means accommodation of the value at 09:00 LST). The SUNAE algorithm was applied to each TMM of the site's TMY and for the specific year the TMM belonged to (see the example in Table 1). Only values of \(\gamma\) greater than or equal to 5 degrees were considered (to avoid the cosine effect). This criterion affected all subsequent calculations. The diffuse horizontal irradiance \(D_{e}\) was estimated by the difference \(G_{e}-B_{e}\), where \(B_{e}\) is the direct horizontal irradiance and \(G_{e}\) the global horizontal one, both provided in the TMYs-PV. For estimating the global irradiance on an inclined plane facing south, the isotropic model of [PERSON] [64] was adopted for estimating the reflected radiation component from the surrounding surface \(R_{e,\beta S}\) (in Wm\({}^{-2}\)) received by the solar collector. This model proved to be as efficient at providing the tilted total radiation in Greece as other more sophisticated ones [44]. For a south-facing surface inclined at \(\beta\) degrees with respect to the local horizon, the received total solar radiation \(G_{e,\beta S}\) is given by the following equation [65]:  
\[G_{e,\beta S}=B_{e,\beta S}+D_{e,\beta S}+R_{e,\beta S} \tag{1}\]  
where the subscript \(S\) denotes the southern orientation of the solar collector at an inclination of \(\beta\) degrees with respect to the local horizon. According to [PERSON] and [PERSON] [64], the following are true:  
\[D_{e,\beta S}=D_{e}\cdot R_{di} \tag{2}\]  
\[R_{e,\beta S}=G_{e}\cdot R_{r}\cdot\rho_{g} \tag{3}\]  
\[R_{di}=0.5\cdot(1+\cos\beta) \tag{4}\]  
\[R_{r}=0.5\cdot(1-\cos\beta) \tag{5}\]  
\[B_{e,\beta S}=B_{e}\cdot\frac{\cos\theta}{\sin\gamma} \tag{6}\]  
\[\cos\theta=\sin\beta\cdot\cos\gamma\cdot\cos(\psi-\psi^{\prime})+\cos\beta \cdot\sin\gamma \tag{7}\]  
where \(\theta\) is the incidence angle and \(\psi\) and \(\psi^{\prime}\) are the solar azimuths of the Sun and the inclined plane, respectively (see Figure 2 for definition). The former two solar parameters were estimated through the SUNAE algorithm, while \(\psi^{\prime}\) was always equal to 180 degrees. \(R_{di}\) is called the isotropic sky configuration factor, \(R_{r}\) is the configuration factor between the ground and the receiver plane, and \(\rho_{g}=0.2\) (isotropic albedo).