Irradiance
The instantaneous solar power received per unit area, measured in watts per square metre (W/m²).
Irradiance is a power flux: how many watts of solar radiation strike one square metre at a given moment. Its time integral is irradiation, an energy quantity in kWh/m².
- Outside the atmosphere, the solar constant averages about 1,361 W/m² (total solar irradiance, satellite measurements).
- At sea level on a clear day with the sun high, irradiance on a surface facing the sun is roughly 1,000 W/m², which is why STC uses that value.
- Under thick overcast it may fall to 50–200 W/m²; cloud-edge effects can briefly push it above 1,300 W/m².
Irradiance is split into components:
where GHI is global horizontal, DNI direct normal, DHI diffuse horizontal irradiance and θz the solar zenith angle. For PV design what matters is plane-of-array (POA) irradiance, obtained by transposing these components onto the tilted module surface with models such as Perez or Hay-Davies, and adding a ground-reflected term that depends on albedo.
Irradiance is measured with pyranometers (global) and pyrheliometers (direct), classified under ISO 9060, or estimated from satellite imagery.
Related terms
All terms →- Irradiation (insolation)
Solar energy received per unit area over a period, usually expressed in kWh/m² per day or per year; the time integral of irradiance.
- GHI (Global Horizontal Irradiance)
Total shortwave solar radiation received on a horizontal surface, the sum of direct beam and diffuse sky components.
- DNI (Direct Normal Irradiance)
Solar radiation arriving in a straight line from the sun disc, measured on a surface held perpendicular to the sun rays.
- DHI (Diffuse Horizontal Irradiance)
Solar radiation reaching a horizontal surface after being scattered by molecules, aerosols and clouds, excluding the direct beam from the sun disc.
- Standard Test Conditions (STC)
The reference conditions for rating PV modules: 1,000 W/m² irradiance, 25 °C cell temperature and an AM1.5G solar spectrum.
- Albedo
The fraction of incoming solar radiation that a surface reflects, from 0 (absorbs everything) to 1 (reflects everything).