Solar radiation
Solar irradiance is the power per unit area received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Irradiance may be measured in space or at the Earth's surface after atmospheric absorption and scattering. It is measured perpendicular to the incoming sunlight.Total solar irradiance (TSI), is a measure of the solar power over all wavelengths per unit area incident on the Earth's upper atmosphere. The solar constant is a conventional measure of mean TSI at a distance of one astronomical Unit (AU). Irradiance is a function of distance from the Sun, the solar cycle, and cross-cycle changes. Irradiance on Earth is also measured perpendicular to the incoming sunlight. Insolation is the power received on Earth per unit area on a horizontal surface. It depends on the height of the Sun above the horizon.
The solar irradiance integrated over time is called solar irradiation, solar exposure or insolation. However, insolation is often used interchangeably with irradiance in practice.
The SI unit of irradiance is watt per square meter (W/m2).
An alternate unit of measure is the Langley (1 thermochemical calorie per square centimeter or 41,840 J/m2) per unit time.
The solar energy industry uses watt-hour per square metre (Wh/m2) divided by the recording time. 1 kW/m2 = 24 kWh/(m2 day).
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