The sun generates approximately 1.1 x 10 E20 kilowatt-hours every second. (A kilowatt-hour is the amount of energy needed to power a 100 watt light bulb for ten hours.) The earth’s outer atmosphere intercepts about one two-billionth of the energy generated by the sun, or about 1500 quadrillion (1.5 x 10 E18 ) kilowatt-hours per year. Because of reflection, scattering, and absorption by gases and aerosols in the atmosphere, however, only 47% of this, or approximately 700 quadrillion (7 x 10 E17 ) kilowatt-hours, reaches the surface of the earth.

In the earth’s atmosphere, solar radiation is received directly (direct radiation) and by diffusion in air, dust, water, etc., contained in the atmosphere (diffuse radiation). The sum of the two is referred to as global radiation.
The amount of incident energy per unit area and day depends on a number of factors, e.g.:
local climate
season of the year
inclination of the collecting surface in the direction of the sun.
The solar energy varies because of the relative motion of the sun. This variations depend on the time of day and the season. In general, more solar radiation is present during midday than during either the early morning or late afternoon. At midday, the sun is positioned high in the sky and the path of the sun’s rays through the earth’s atmosphere is shortened. Consequently, less solar radiation is scattered or absorbed, and more solar radiation reaches the earth’s surface.

The amounts of solar energy arriving at the earth’s surface vary over the year, from an average of less than 0,8 kWh/m2 per day during winter in the North of Europe to more than 4 kWh/m2 per day during summer in this region. The difference is decreasing for the regions closer to the equator.
The availability of solar energy varies with geographical location of site and is the highest in regions closest to the equator. Thus the average annual global radiation impinging on a horizontal surface which amounts to approx. 1000 kWh/m2 in Central Europe, Central Asia, and Canada reach approx. 1700 kWh/m2 in the Mediterranian and to approx. 2200 kWh/m2 in most equatorial regions in African, Oriental, and Australian desert areas. In general, seasonal and geographical differences in irradiation are considerable and must be taken into account for all solar energy applications.

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