Global radiation
Radiation & sun global horizontal irradiance GHI global solar radiation
Global horizontal irradiance is the total shortwave solar radiation reaching a horizontal surface at ground level. It includes direct sunlight and scattered light from the sky.
“Global” means the combined light here
The name does not mean a total for the whole planet. Global horizontal irradiance combines direct radiation onto the horizontal plane with diffuse sky radiation. Energy therefore still reaches the ground under clouds.
Separate power from energy
- W/m²: Power per area, reported as an instantaneous value or time average depending on the display.
- kWh/m² or MJ/m²: Energy accumulated over a period.
- Conversion: 1 kWh/m² equals 3.6 MJ/m².
For example, an average of 500 W/m² for two hours delivers 1 kWh/m² of radiant energy. Clouds may come and go; the average is what counts.
What changes the value?
A higher sun puts more direct radiation onto a horizontal surface. Clouds, water vapour and aerosols scatter or absorb radiation, so power can change quickly as clouds pass.
It is not electrical output yet
Solar panels convert only part of the radiation to electricity. Roof tilt, orientation, shade, temperature and system losses also matter. Global irradiance is neither a count of sunshine hours nor a measure of sunburn risk.