DHI Diffuse Horizontal Irradiance: Solar Resource Component

Definition

Diffuse Horizontal Irradiance (DHI) is the diffuse component of solar irradiance on a horizontal surface, in W/m². Sum of DHI plus direct beam (projected onto horizontal) = GHI.

DHI vs. GHI and DNI

DHI is one half of the equation that produces GHI:

GHI = DNI × cos(solar zenith angle) + DHI

DNI is the direct beam measured perpendicular to the sun; DHI is everything scattered by clouds, haze, and the atmosphere before it reaches a horizontal surface. A pyranometer with a shading device isolates DHI directly, while an unshaded pyranometer records GHI — subtracting the projected DNI component from GHI gives DHI (or vice versa, depending on which two of the three are measured on site).

Why the Diffuse Component Matters in Design

A site’s DHI fraction is a proxy for how “diffuse” its sky is. Overcast, humid, or hazy locations see a larger share of their annual irradiance arrive as DHI, while clear desert skies are dominated by the direct beam. That split has design consequences:

Fixed-tilt and tracker yield modeling in PVsyst and similar tools consumes hourly DHI, DNI, and GHI series together — a design built only on GHI totals can misjudge the tracker gain or transposition factor if the underlying DHI/DNI split isn’t representative of the site.
Bifacial gain depends on diffuse and albedo-reflected light reaching the rear of the module, so DHI (alongside albedo) is a meaningful input wherever bifacial modules are being specified.
Cloudy-climate sites generally lean more on accurate DHI data, since a larger share of deliverable energy is diffuse rather than direct.

Worked Example

If a horizontal pyranometer records a GHI of 600 W/m² at a given moment, and a shaded (diffuse-only) pyranometer at the same time and location reads a DHI of 100 W/m², then the projected direct-beam component on the horizontal plane is 600 − 100 = 500 W/m². Dividing that by the cosine of the solar zenith angle at that moment recovers the DNI value perpendicular to the sun’s rays — illustrating how the three quantities are always internally consistent.

Data Sources

Meteonorm, NREL’s NSRDB, and Solargis all publish hourly DHI series alongside GHI and DNI, which is the standard input format expected by PVsyst and other yield simulation tools.

Key Takeaways

DHI = diffuse irradiance on horizontal surface.
GHI = DNI projected + DHI.
Higher DHI fraction in cloudy climates.
Standard hourly data source for PVsyst.
Critical for cloudy-climate solar yield modeling.

Because DHI only ever shows up as part of an hourly weather file, it’s best understood alongside how that data actually drives a yield study — see how PVsyst tracker yield studies sequence DHI, DNI, and GHI inputs, and how solar simulation software compares in handling irradiance data quality. Since the diffuse component becomes decisive at lower irradiance angles, it also feeds into P50/P90/P99 yield report confidence levels that lenders scrutinize. For state-wise solar resource data covering the DHI/DNI/GHI split across India, Qbits Energy’s irradiance data guide is a useful reference point.

Frequently Asked Questions

5 commonly searched questions about DHI (Diffuse Horizontal Irradiance).

What is DHI?

Diffuse Horizontal Irradiance — scattered solar radiation on a horizontal surface. Component of GHI: GHI = DNI × cos(zenith) + DHI.

Where is DHI data found?

Meteonorm, NSRDB, Solargis publish hourly DHI alongside GHI and DNI for solar simulation.

Why does the DHI fraction of GHI change from site to site?

The diffuse share of GHI rises with cloud cover, humidity, and aerosol/haze loading, and falls under clear, dry desert skies where most irradiance arrives as direct beam. This is why the same GHI total can produce a different DNI/DHI split in a coastal or monsoon-affected site versus a desert site.

How is DHI measured on site?

DHI is typically measured with a pyranometer fitted with a shading ball or shadowband that blocks the direct beam, so the sensor records only the scattered sky component. GHI is measured with an unshaded pyranometer, and DNI is derived or measured separately with a pyrheliometer.

Does DHI matter for bifacial modules and trackers?

Yes. Bifacial modules and their rear-side gain calculations depend heavily on diffuse and albedo-reflected light reaching the back of the module, so DHI is a direct input to bifacial gain modeling. For trackers, DHI still reaches the module even when the direct beam is partially blocked, which is one reason backtracking logic focuses on protecting the DNI component rather than the diffuse one.