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Irradiation

The sun is continuously emitting enormous amounts of energy. A fraction of that energy reaches the earth. The fraction of the energy from the sun that reaches the earth in just one day is still more than enough to cover the energy use of the world in a whole year. However, not all the energy of the sun that reaches the earth can be used effectively. Part of the sunlight is absorbed in the earth's atmosphere or reflected back into space.

The intensity of the sunlight that reaches the earth varies with time of the day and year, location, and the weather conditions. The total energy on a daily or annual basis is called irradiation and indicates the strength of the sunshine. Irradiation is expressed in Wh/m² per day or for instance kWh/m² per day.

To simplify calculations with irradiation data solar energy is expressed in equivalents of hour's bright sun light. Bright sun light corresponds with a power of about 1,000 W/m² so one hour of bright sunlight corresponds with an amount of energy of 1 kWh/m².

This is approximately the solar energy when the sun shines on a cloudless day in the summer on a surface of one square meter perpendicular to the sun.



The irradiation varies with time of day but it can also vary considerably from location to location, especially in mountainous areas. Irradiation various from an average of 1,000 kWh/m² per year for northern European countries such as Germany, to 2,000 to 2,500 kWh/m² per year for desert areas. Weather conditions and the difference in the relative position of the sun (solar elevation) in the sky, which depend on the latitude of each place on earth, cause these variations (see also orientation and tilt angle). Source of the map: BP-Solarex.

Example:

A full sun corresponds with a power of about 1,000 W/m². This full sun shining perpendicular on a surface of 1 m² for one hour corresponds with an energy of 1000 Wh or 1 kWh. (Energy = Power multiplied by time (E = P x t)). Similarly, an average daily irradiation of 5 kWh/m²/day corresponds to 5 hours of full sunshine per day.

Information from http://www.mysolar.com and other sources were used, hardly edited by our engineers.

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