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.
DHI is the sky radiation that comes from all directions rather than straight from the sun. It is measured with a pyranometer whose view of the sun is blocked by a tracking shading ball or a shadow band, or computed as the difference between GHI and the horizontal projection of DNI.
The diffuse fraction (DHI/GHI) characterises a climate. On an annual basis it is typically:
- 45–60% in north-western Europe and other cloudy temperate regions;
- 30–40% in the Mediterranean and humid subtropics;
- 15–25% in clear desert climates.
Under full overcast the diffuse fraction approaches 100%. Diffuse light matters for PV design because it is spread across the sky: high diffuse fractions reduce the benefit of optimum tilt and of trackers, while improving the relative output of steep or vertical surfaces and the rear side of bifacial modules. Transposition models (isotropic, Hay-Davies, Perez) differ mainly in how they distribute diffuse radiation across the sky dome, and the choice can shift modelled plane-of-array energy by a few percent.
CSP plants cannot use diffuse light at all, which is why they are restricted to low-DHI, high-DNI regions.
Related terms
All terms →- 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.
- Irradiance
The instantaneous solar power received per unit area, measured in watts per square metre (W/m²).
- Bifacial module
A PV module that collects light on both its front and rear faces, using a transparent back sheet or glass to harvest ground-reflected and diffuse light.
- Solar tracker
A motorised mounting structure that rotates PV modules or mirrors to follow the sun, raising the irradiance they receive over the day.