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solar insolation

Brief Summary: In this video we define the term solar insolation, how it is measured, and how it determines the success of a solar installation.

Keywords: solar insolation, angle of incidence, solar constant, Kilowatts , BTUs, and solar conversion efficiency, solar irradiation.

Detailed Description: Solar insolation is a measure of radiation energy received on a given surface on Earth over a given time. It is also called solar irradiation and can be expressed in interchangeable units of MJ/m2 (megajoules per square meter) or J/cm2 (joules per square centimeter), or Wh/m2 (watt-hours per square meter). If this energy is divided by the recording time in hours, it is then a density of power called irradiance, expressed in W/m2 (watts per square meter). Some of the radiation will be absorbed and the remainder will be reflected. The absorbed solar radiation is converted to thermal energy, causing an increasing in the object's temperature. The proportion of radiation reflected or absorbed depends on the object's reflectivity orabsoption.

Insolation is the solar irradiance at a given location on Earth perpendicular to the Sun's rays. It is equal to the solar constant minus the losses due to the atmosphere. The losses depend on the time of day (length of light's path through the atmosphere depending on the Solar azimuth), cloud cover, moisture content, and other air impurities. Over the course of a year the average solar radiation arriving at the top of the Earth's atmosphere at any point in time is roughly 1366 watts per square meter The Sun's rays are attenuated as they pass through the atmosphere, thus reducing the irradiance at the Earth's surface to approximately 1000 Wm2 for a surface perpendicular to the Sun's rays at sea level on a clear day. However, The actual amount varies with the sun's angle at different times of year and the extent haze and cloud cover. Ignoring clouds, the daily average irradiance for the Earth is approximately 250 Wm2 The insolation of the sun can also be expressed in Suns, where one Sun equals 1000 W/m2 , with kWh/(m2·day) displayed as hours/day. When calculating the output of, for example, a solar thermal panel, the angle of the sun relative to the panel needs to be taken into account as well as the insolation. This 'projection effect' is the main reason why the less temperate regions are much colder than equatorial regions on Earth. On an annual average, the poles receive less insolation than does the equator, because at the poles the Earth's surface are angled away from the Sun. The skydome illustration to right demonstrates this.

To see how many hours a day (sun days) a location receives, you can view our sun day chart


availabel soon screen

sky dome illustration plots path of sun over the course of a year

Conversion factor (multiply top row by factor to obtain side column)

W/m2 kW·h/(m2·day) sun hours/day kWh/(m2·y) kWh/(kWp·y)
W/m2 1 41.66666 41.66666 0.1140796 0.1521061
kW·h/(m2·day) 0.024 1 1 0.0027379 0.0036505
sun hours/day 0.024 1 1 0.0027379 0.0036505
kWh/(m2·y) 8.765813 365.2422 365.2422 1 1.333333
kWh/(kWp·y) 6.574360 273.9316 273.9316 0.75 1


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