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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). [11] The langley is an alternative unit of insolation. One langley is one thermochemical calorie per square centimetre or 41,840 J/m 2. [12]
The angular diameter of the Earth as seen from the Sun is approximately 1/11,700 radians (about 18 arcseconds), meaning the solid angle of the Earth as seen from the Sun is approximately 1/175,000,000 of a steradian. Thus the Sun emits about 2.2 billion times the amount of radiation that is caught by Earth, in other words about 3.846×10 26 watts.
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 ...
Multiplying the figure of 1,050 watts per square meter by 93 lumens per watt indicates that bright sunlight provides an illuminance of approximately 98,000 lux (lumens per square meter) on a perpendicular surface at sea level. The illumination of a horizontal surface will be considerably less than this if the Sun is not very high in the sky.
The solar constant is the amount of power that the Sun deposits per unit area that is directly exposed to sunlight. 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]
astro: power per square metre received from the Sun at Mercury's orbit at perihelion: 1.6–3.2 × 10 4 W eco: average photosynthetic power output per square kilometer of land [23] 3 × 10 4 W tech: power generated by the four motors of GEN H-4 one-man helicopter: 4–20 × 10 4 W tech: approximate range of peak power output of typical ...
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 . Irradiance is often called intensity , but this term is avoided in radiometry where such usage leads to confusion with radiant intensity .
Pd (Watts/meter 2) = E × H (Volts/meter × Amperes/meter)where Pd = the power density, E = the RMS electric field strength in volts per meter, H = the RMS magnetic field strength in amperes per meter. [6] The above equation yields units of W/m 2. In the USA the units of mW/cm 2, are more often used when making surveys.