DHI — Diffuse Horizontal Irradiance
Diffuse Horizontal Irradiance (DHI) is the solar radiation received on a horizontal surface that has been scattered by the atmosphere — clouds, aerosols, water vapor, and air molecules — and arrives from all directions in the sky rather than directly from the solar disc. It excludes the direct beam component.
On a clear day, DHI represents about 10–15% of total GHI. On a heavily overcast day, DHI represents nearly 100% of GHI, as direct beam sunlight is blocked by clouds and all arriving radiation has been scattered.
Importance for PV Systems
Fixed-tilt PV panels capture both direct and diffuse radiation. Unlike concentrating systems, flat PV panels receive diffuse radiation from the entire sky dome, making them effective even on cloudy days. This is why solar PV is economically viable in Germany, the UK, and the Pacific Northwest despite relatively low average irradiance.
High-DHI locations — characterized by frequent cloud cover — tend to have lower temperature-related performance losses, which partially compensates for lower total irradiance. The cool, cloudy climate of the Netherlands, for example, results in lower thermal derating of panels.
DHI in SolarScope
SolarScope retrieves DHI values from NASA POWER (ALLSKY_SFC_SW_DIFF parameter) and displays them in the project analysis dashboard. DHI is particularly relevant when evaluating bifacial panels (which capture reflected diffuse radiation from the ground) or vertical façade installations that see a larger share of diffuse sky radiation.