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Insolation is a calculation of the amount of solar radiation received from the Sun, either at the top of the Earth’s atmosphere or at land level. The term is derived from the phrase INcoming SOLar RadiATION. It is usually defined as watts per meters squared (W/m2) for solar cell power applications, or kilowatt hours per square meter per day (kW-h/m2-day), which is commonly used in meteorology for weather predictions. The entire spectrum of radiation from the Sun is considered, which ranges from a wavelength of about 350 nanometers to 2,350 nanometers, even though the visible spectrum of light for human beings is only between 400 and 700 nanometers.
Both world and regional insolation values have been minutely charted. Since chart values are usually based on what is considered usable insolation rates, the value is often quite small on a daily basis. This is due to the fact that sunlight that reaches the Earth’s surface early or late in a day or through cloudy or rain obscured skies often has limited value for solar power applications or industrial and agricultural processes that rely on it.
On a global insolation map, rates of solar energy are based on optimal tilt towards the Sun, at the lowest radiation level of the year for direct sunlight, and in meters squared. This gives values in hours that range from 6 to 6.9 in equatorial, desert regions, to 1.0 to 1.9 hours in northern or southern latitudes that receive little direct light. European nations range from 3.5 hours per day in Portugal to 0.8 hours per day in Iceland. By contrast, an insolation rate in a region such as California in the US, is typically 5.5 hours per day for every average square meter of land, and Anchorage, Alaska, receives 2.09 hours.
Solar insolation is also directly attenuated by the Earth’s atmosphere in general. At the very top of the atmosphere, insolation rates are more constant around the globe, and are estimated at 1,366 W/m2. Once this light has reached the surface of the Earth, it has been reduced to an average of about 1,000 W/m2, but this, itself, is an exaggeration of the actual radiation level because it’s a value based on direct sunlight only. When averaging insolation rates over an entire 24-hour period including night and sunrise/sunset, as well as obscuring conditions in the atmosphere, insolation levels drop to about 250 W/m2.
Knowing the angle of insolation for the latitude and sea level of a location determines how large a solar array has to be in order to generate a predictable amount of power. Typically, direct sunlight is most often present during the periods of 10 am to 4 pm. While solar cells can generate power under limited light conditions, they will be most efficient during this time period.