GHI (Global Horizontal Irradiance)
Total shortwave solar radiation received on a horizontal surface, the sum of direct beam and diffuse sky components.
GHI is the most widely measured and mapped solar resource parameter and the standard input for estimating PV output. It combines the direct beam projected onto the horizontal and the diffuse radiation scattered from the whole sky dome.
Here DNI is direct normal irradiance, DHI diffuse horizontal irradiance and θz the solar zenith angle. GHI is measured with a thermopile pyranometer facing upward (ISO 9060 Class A for bankable stations) or derived from geostationary satellite images, which is how global datasets such as Solargis, NASA POWER, NSRDB (NREL) and PVGIS (JRC) are built.
Annual GHI ranges from below 900 kWh/m² in north-western Europe to about 2,500–2,700 kWh/m² in the Atacama and parts of the Sahara and Arabian Peninsula. A clear-sky noon value around 1,000 W/m² is typical in summer at mid-latitudes.
Because fixed PV is tilted, GHI must be transposed to plane-of-array irradiance; at 35–50° latitude an optimally tilted south-facing surface receives roughly 10–20% more annual energy than the horizontal. Sites with high diffuse fractions gain less from tilt and from tracking. GHI is the base layer of most solar potential maps, including the one on this site.
Related terms
All terms →- 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.
- Irradiance
The instantaneous solar power received per unit area, measured in watts per square metre (W/m²).
- 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.
- Peak sun hours
Daily solar irradiation expressed as the equivalent number of hours at 1,000 W/m²; numerically equal to kWh/m² per day.