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Solar irradiance is the power per unit area received from the Sun in the form of electromagnetic radiation as measured in the wavelength range of the measuring instrument. The solar irradiance is measured in watts per square metre (W/m2) in SI units. Solar irradiance is often integrated over a given time period in order to report the radiant energy emitted into the surrounding environment (joule per square metre, J/m2) during that time period. This integrated solar irradiance is called solar irradiation, solar exposure, solar insolation, or insolation.

Irradiance may be measured in space or at the Earth's surface after atmospheric absorption and scattering. Irradiance in space is a function of distance from the Sun, the solar cycle, and cross-cycle changes.[1] Irradiance on the Earth's surface additionally depends on the tilt of the measuring surface, the height of the sun above the horizon, and atmospheric conditions.[2] Solar irradiance affects plant metabolism and animal behavior.[3]

The study and measurement of solar irradiance have several important applications, including the prediction of energy generation from solar power plants, the heating and cooling loads of buildings, and climate modeling and weather forecasting.

## Types

Global Map of Global Horizontal Radiation [4]
Irradiance may be measured in space or at the Earth's surface after atmospheric absorption and scattering. Irradiance in space is a function of distance from the Sun, the solar cycle, and cross-cycle changes.[1] Irradiance on the Earth's surface additionally depends on the tilt of the measuring surface, the height of the sun above the horizon, and atmospheric conditions.[2] Solar irradiance affects plant metabolism and animal behavior.[3]

The study and measurement of solar irradiance have several important applications, including the prediction of energy generation from solar power plants, the heating and cooling loads of buildings, and climate modeling and weather forecasting.

There are several measured types of solar irradiance.

• Total Solar Irradiance (TSI) is a measure of the solar power over all wavelengths per unit area incident on the Earth's upper atmosphere. It is measured perpendicular to the incoming sunlight.[2] The solar constant is a conventional measure of mean TSI at a distance of one astronomical unit (AU).
• Direct Normal Irradiance (DNI), or beam radiation, is measured at the surface of the Earth at a given location with a surface element perpendicular to the Sun.[5] It excludes diffuse solar radiation (radiation that is scattered or reflected by atmospheric components). Direct irradiance is equal to the extraterrestrial irradiance above the atmosphere minus the atmospheric losses due to absorption and scattering. Losses depend on time of day (length of light's path through the atmosphere depending on the solar elevation angle), cloud cover, moisture content and other contents. The irradiance above the atmosphere also varies with time of year (because the distance to the sun varies), although this effect is generally less significant compared to the effect of losses on DNI.
• Diffuse Horizontal Irradiance (DHI), or Diffuse Sky Radiation is the radiation at the Earth's surface from light scattered by the atmosphere. It is measured on a horizontal surface with radiation coming from all points in the sky excluding circumsolar radiation (radiation coming from the sun disk).[5][6] There would be almost no DHI in the absence of atmosphere.[5]
• Global Horizontal Irradiance (GHI) is the total irradiance from the sun on a horizontal surface on Earth. It is the sum of direct irradiance (after accounting for the solar zenith angle of the sun z) and diffuse horizontal irradiance:[7]

${\displaystyle {\text{GHI}}={\text{DHI}}+{\text{DNI}}\times \cos(z)}$

• Global Tilted Irradiance (GTI) or is the global radiation received on a surface with defined tilt and azimuth, fixed or sun-tracking. GTI can be measured[6] or modeled from GHI, DNI, DHI.[8][9][10] It is often a reference for photovoltaic power plants, while photovolotaic modules are mounted on the fixed or tracking constructions.
• Global Normal Irradiance (GNI) is the total irradiance from the sun at the surface of Earth at a given location with a surface element perpendicular to the Sun.

## Units

The SI unit of irradiance is watt per square metre (W/m2, which may also be written Wm−2).

An alternative 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) per unit time[${\displaystyle {\text{GHI}}={\text{DHI}}+{\text{DNI}}\times \cos(z)}$

## Units

The SI unit of irradiance is watt per square metre (W/m2, which may also be written Wm−2).

An alternative 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) per unit time[citation needed]. The relation to the SI unit is thus:

1 kW/m2 x (24 h/day) = (24 kWh/m2)/day
(24kWh/m2)/day*(365days/year) = (8760 kWh/m2)/year.

## Irradiation at the top of the atmosphere

watt per square metre (W/m2, which may also be written Wm−2).

An alternative 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) per unit time[citation needed]. The relation to the SI unit is thus:

1 kW/m2 x (24 h/day) = (24 kWh/m2)/day
(24kWh/m2)/day*(365days/year) = (8760 kWh/m2)/year.