Sola irradiance

Solar irradiance is a very important quantity that is studied extensively especially in the design of solar technologies. Understanding and measuring solar irradiance has important implications, such as the heating and cooling loads of buildings, the prediction of energy generation from solar power plants, and weather forecasting.

In this NEW series, we introduce you to solar irradiance. In the first instalment, we touch on what affects irradiance, how it is measured and the types of irradiance. The second article in the series delves into solar potential mapping and applications of solar irradiance. SUBSCRIBE to get these informative articles straight to your inbox!

What is solar irradiance?

Solar irradiance is the power per unit area received from the Sun in the form of electromagnetic radiation. This power or energy is a combination of the strength/intensity of the sunlight received, and the hours the sunlight falls at a particular place.

Solar panels are designed with a quoted solar irradiance of 1000W/m2. This is not an average, it's only a chosen value so that standard comparisons can be performed. There are only two types of solar irradiance which are diffuse and beam irradiance.

What affects solar irradiance?

Solar irradiance is not uniform throughout. It differs from place to place and from time to time. The main factors that affect the amount of irradiance at a place are:

  • Ozone layer thickness
  • Haze (dust and vapour)
  • Zenith angle
  • Extend of cloud cover
  • Distance travelled through the atmosphere
  • Seasons

What are the Units of Solar Irradiation Measurement?

The solar irradiance is measured in watt 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.

The solar energy industry uses watt-hour per square metre (Wh/m2) per unit time, to measure irradiance.

How is Solar Irradiance measured?

The solar irradiance at a place can be measured using a pyranometer or a pyrheliometer. These instruments differ in their principle of operation. They're normally mounted together with solar panels to monitor and assess the solar irradiance and the electricity generated.

What types of irradiance are measured?

There are several measured types of solar irradiance:

1. Total Solar Irradiance (TSI) measures the overall wavelengths per unit area incident on the Earth’s upper atmosphere.

2. Diffuse Horizontal Irradiance (DHI) is the radiation at the surface of the Earth, from light scattered by the atmosphere.

3. Direct Normal Irradiance (DNI) is a measure at the Earth’s surface at a given location, with a surface element perpendicular to the Sun.

4. Global Horizontal Irradiance (GHI) is the total irradiance from the Sun on a horizontal surface on Earth. GHI = DHI + DNI x cos(ʐ)

5. Global Tilted Irradiance (GTI) is the total radiation received on a surface with defined tilt and azimuth, fixed or sun-tracking. 

6. 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.


Look out for the second instalment of the series which covers solar potential mapping and applications of solar irradiance. SUBSCRIBE to get these informative articles straight to your inbox!

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