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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 approximate average value cited, [1] 1.3608 ± 0.0005 kW/m 2, which is 81.65 kJ/m 2 per minute, is equivalent to approximately 1.951 calories per minute per square centimeter, or 1.951 langleys per minute. Solar output is nearly, but not quite, constant. Variations in total solar irradiance (TSI) were small and difficult to detect ...
Dividing the irradiance of 1,050 W/m 2 by the size of the Sun's disk in steradians gives an average radiance of 15.4 MW per square metre per steradian. (However, the radiance at the center of the sun's disk is somewhat higher than the average over the whole disk due to limb darkening.)
Yield photon flux (YPF) micromoles per square meter per second (μmol·m −2 ·s −1) When measuring the irradiance of PAR, values are expressed using units of energy (W/m 2 ), which is relevant in energy-balance considerations for photosynthetic organisms .
In radiometry, irradiance is the radiant flux received by a surface per unit area. The SI unit of irradiance is the watt per square metre (symbol W⋅m −2 or W/m 2).The CGS unit erg per square centimetre per second (erg⋅cm −2 ⋅s −1) is often used in astronomy.
Radiant intensity is used to characterize the emission of radiation by an antenna: [2], = (), where E e is the irradiance of the antenna;; r is the distance from the antenna.; Unlike power density, radiant intensity does not depend on distance: because radiant intensity is defined as the power through a solid angle, the decreasing power density over distance due to the inverse-square law is ...
The intensity of solar irradiance including all wavelengths is the Total Solar Irradiance (TSI) and on average is the solar constant. It is equal to about 1361 W m −2 at the distance of Earth's annual-mean orbital radius of one astronomical unit and as measured at the top of the atmosphere. [42]
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).