The tilt of the Earth's axis with respect to the Earth's orbit around the Sun generates a change in the seasons, which affects the amount of solar energy that reaches different areas of the Earth during the year. The northern hemisphere receives the most energy in summer and the least in winter. This is due to the change in the angle of the Earth's plane to the direction of the sun's rays and the time of insolation (length of day), in which solar radiation reaches the Earth. The insolation depends not only on your location on the parallel, but also on the cloud cover in your region.

The power of solar radiation thus depends on the time of day, the season, and the geographic location where it will be measured ( Fig. 1 ).

The distance traveled by solar radiation ( Fig. 2 ) in the atmosphere varies with the position of the sun above the horizon. The seasonal dependence is related to the tilt of the Earth's axis of rotation with respect to the plane in which the Earth moves ( Fig. 1 ).
The photovoltaic phenomenon is inextricably linked to the energy source, which is the Sun. The average power delivered by the Sun to the boundary of the Earth's atmosphere is \( 1362 \frac{W}{m^{2}} \) (solar constant) [1], that is, about 175* \( 10 ^{15} \) \( W ^{} \) of solar irradiance power (energy per second) falls on the entire surface of the Earth. This energy, after passing through the atmosphere, is converted in photovoltaic cells into electricity. The amount of energy reaching the Earth's surface obviously varies in different geographical areas. It is usually determined by the amount of insolation, which is the amount of energy of solar radiation falling per unit time on a unit area. The unit of insolation is \( \frac{W}{m^{2}} \) or \( \frac{kWh}{m^{2} { year}} \). The distribution of insolation over the globe is shown in Fig. 3.

Figure 3: Annual average solar irradiance distribution over the surface of the Earth. The impact of the Earth's atmosphere is taken into account. Fig. OpenStreetMap, CC BY-SA 2.0 license, source: OpenStreetMap.

Black dots depicted in Fig. 3 cover an area that, when covered with photovoltaic cells with a conversion efficiency of \( 8\% \), can generate 568 EJ of energy (EJ= \( 10 ^{18} \)J), which covers the entire world electricity demand [2], [3].
Photovoltaic installations are usually mounted in a south-facing tilt. The tilt angle is chosen such that the amount of solar energy reaching the cell surface is maximized. Ideally, the cell surface should follow the movement of the sun, tracking the apparent movement of the Sun across the sky (aligning the plane perpendicular to the incident solar radiation). However, the tracking system generates additional costs associated with the fixation, with the drive of the entire cell system, and with its maintenance. Therefore, this solution is used less frequently than permanent mounting of photovoltaic panels.

Figure 4: Solar energy reaching Europe at an area of 1 square metre in one year. Solar insolation of the surface of Europe taking into account the influence of the Earth's atmosphere (different overcover in different areas). Fig. OpenStreetMap, CC BY-SA 2.0 license, source: OpenStreetMap.

Fig. 4 [2], [3] presents the contour map of Europe, while Fig. 5 shows contours of Poland; they present the average insolation (solar radiation power per unit horizontal area) for Europe (in 1994-2010) and for Poland (in 1994-2013) [2], [3]. The intensity of the color illustrates the amount of insolation, which depends not only on the latitude (it does not coincide exactly with the system of parallels), but also on the cloud cover that is present in a given area.

Figure 5: Solar energy reaching the Polish the surface of a square meter in one year. Fig. OpenStreetMap, CC BY-SA 2.0 license, source: OpenStreetMap.

Fig. 5 presents a map of Poland with the insolation (incident energy per 1 \( m^{2} \) during the year in kWh/ \( m^{2} \)) marked. The map shows that the insolation during the year is the highest in south-eastern Poland. The differences in the amount of energy in different areas reach \( 30\% \), which must be taken into account in the economic calculation, especially for large photovoltaic installations.
The diagrams of the course of the solar disk along the sky are determined individually for each location and can be presented using the PVSol program (e.g., for Krakow in Fig. 6 ) (own work). Thus, in summer the altitude at zenith is about \( 70_{}^{o}\textrm{} \), and in winter about \( 25_{}^{o}\textrm{} \), whereby in summer, the azimuth varies from about \( 60_{}^{o}\textrm{} \) to about \( 300_{}^{o}\textrm{} \). In contrast, the winter Sun rises when the azimuth is about \( 120_{}^{o}\textrm{} \), and occurs when the azimuth is approximately \( 240_{}^{o}\textrm{} \).

In summary, light of H=1.362 \( \frac{kW}{m^{2}} \) (the solar constant) reaches the boundary of the Earth's atmosphere, while about \( 73\% \) of this magnitude reaches the Earth's surface.

Vosti Lublin, Wojewódzki Program Rozwoju Alternatywnych Źródeł Energii dla Województwa Lubelskiego – see Nasłonecznienie w Polsce a panele fotowoltaiczne.