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The SI unit of irradiance is watts per square metre (W/m 2 = Wm −2). The unit of insolation often used in the solar power industry is kilowatt hours per square metre (kWh/m 2). [12] The Langley is an alternative unit of insolation. One Langley is one thermochemical calorie per square centimetre or 41,840 J/m 2. [13]
The solar constant includes radiation over the entire electromagnetic spectrum. It is measured by satellite as being 1.361 kilo watts per square meter (kW/m 2) at solar minimum (the time in the 11-year solar cycle when the number of sunspots is minimal) and approximately 0.1% greater (roughly 1.362 kW/m 2) at solar maximum. [1]
If the extraterrestrial solar radiation is 1,367 watts per square meter (the value when the Earth–Sun distance is 1 astronomical unit), then the direct sunlight at Earth's surface when the Sun is at the zenith is about 1,050 W/m 2, but the total amount (direct and indirect from the atmosphere) hitting the ground is around 1,120 W/m 2. [6]
Of the ~340 W/m 2 of solar radiation received by the Earth, an average of ~77 W/m 2 is reflected back to space by clouds and the atmosphere and ~23 W/m 2 is reflected by the surface albedo, leaving ~240 W/m 2 of solar energy input to the Earth's energy budget. This amount is called the absorbed solar radiation (ASR).
For example, the intensity of radiation from the Sun is 9126 watts per square meter at the distance of Mercury (0.387 AU); but only 1367 watts per square meter at the distance of Earth (1 AU)—an approximate threefold increase in distance results in an approximate ninefold decrease in intensity of radiation.
The solar constant is equal to approximately 1,368 W/m 2 (watts per square meter) at a distance of one astronomical unit (AU) from the Sun (that is, at or near Earth's orbit). [99] Sunlight on the surface of Earth is attenuated by Earth's atmosphere , so that less power arrives at the surface (closer to 1,000 W/m 2 ) in clear conditions when ...
Direct insolation is measured in (W/m 2) or kilowatt-hours per square meter per day (kW·h/(m 2 ·day)). 1 kW·h/(m 2 ·day) = 1,000 W · 1 hour / ( 1 m 2 · 24 hours) = 41.67 W/m 2 In the case of photovoltaics, average direct insolation is commonly measured in terms of peak direct insolation as kWh/(kWp·y) (kilowatt hours per year per ...
The intensity of electromagnetic radiation can be expressed in W/m 2.An example of such a quantity is the solar constant.; Wind turbines are often compared using a specific power measuring watts per square meter of turbine disk area, which is , where r is the length of a blade.