Solar Lattice

Concept

Diffuse horizontal irradiance

ID
concept:diffuse-irradiance
IRI
https://solarlattice.org/kb/concept/diffuse-irradiance
Status
active
Also known as
DHI, diffuse irradiance, Diffuse Horizontal Irradiation, DFI, diffuse sky
Category
Irradiance
Concept kind
physics

Diffuse horizontal irradiance (DHI) is the sunlight that reaches a horizontal surface from every part of the sky after the atmosphere or the surroundings have scattered it. Together with the direct beam, it makes up global horizontal irradiance, and PV plants can use both the diffuse and the beam components. The irradiance landing on a module can be split into three parts: direct beam, diffuse sky and ground-reflected albedo. A pyranometer measures the total from the whole hemisphere unless special equipment is used to measure each part separately. When irradiance is transposed to the module plane, the estimated diffuse part is subtracted first. Only the remaining direct beam is corrected for angles, and then the diffuse part is added back. PowerTrack estimates diffuse light from a model based on site location and sun position, because diffuse light changes fairly little with cloud cover. If the measured irradiance is lower than the modeled diffuse value, the measured value is used instead. How much diffuse light a module can collect depends on its tilt. A horizontal module sees the whole sky, while a vertical module sees only half of it, so it gets about 50 percent.

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Claims

part_of

c1 drafted · lead Global horizontal irradiance

GHI includes both the direct beam and the scattered diffuse component received on a horizontal surface.

c2 drafted · lead Plane-of-array irradiance

Irradiance on the modules is broken into direct beam, diffuse sky and ground-reflected albedo components.

related_to

c3 drafted · lead Direct normal irradiance

DNI, GHI and DHI together are the usual way to describe the solar resource at a site.

c4 drafted · lead Irradiance transposition

Transposition removes the estimated diffuse part, applies trigonometry to the direct beam, then adds the diffuse part back.

c5 drafted · lead Clear-sky irradiance model

PowerTrack estimates diffuse horizontal irradiance from a clear-sky model based on site location and sun position.

c6 drafted · lead Albedo

PowerTrack combines diffuse light and albedo weights using the diffuse factor.

c7 drafted · lead Expected energy model

Finding POA, direct and diffuse irradiance is the first step in the PowerTrack expected-production calculation.

c8 verified · fact AlsoEnergy weather station (digitizer and sensor kit)

The Standard IO map has a Diffuse Horizontal Irradiance point (DHI), stored as an average in W/m².

verified by: review-workflowverified on: 2026-10-24review note: support check: partial - The register row shows DHI stored as an average in W/m², but nothing ties it to a specific AlsoEnergy weather station product or the Standard IO map by name.

Referenced by

Data