Quick Facts
What DHI is
Diffuse Horizontal Irradiance (DHI) is the solar irradiance from scattered sky light on a horizontal surface, excluding the direct beam from the sun’s disk. It includes:
Light scattered by atmospheric particles (dust, aerosols, water vapour).
Light scattered by clouds (when present).
Light from the bright sky dome.
Light from the circumsolar region (close to the sun disk but not the disk itself).
DHI is what makes the sky bright and provides illumination even when the sun is not directly visible. On overcast days, DHI is essentially all the available solar resource.
For solar plant design, DHI is one of three components (with DNI and GHI) used in detailed irradiance modelling. Most solar plants use GHI as the primary input, but DHI separately matters for:
Tracker design decisions (high DHI fraction reduces tracker benefit).
Bifacial design (bifacial captures rear-side light that often includes DHI component).
Concentrating systems (cannot use DHI).
Performance forecasting in monsoon-affected regions.
DHI versus other irradiance components
The relationship:
GHI = DNI × cos(zenith angle) + DHIDNI: Direct beam, measured perpendicular to sun direction.
GHI: Total irradiance on horizontal surface.
DHI: Diffuse component on horizontal surface.
For typical conditions:
Clear desert noon: DNI 850, DHI 100, GHI 750 to 800 W per sq m.
Hazy noon: DNI 500, DHI 250, GHI 600 W per sq m.
Cloudy noon: DNI 100, DHI 400, GHI 500 W per sq m.
Overcast noon: DNI 0, DHI 250, GHI 250 W per sq m.
The DHI fraction varies from low (clear desert) to high (overcast). Annual DHI fraction in India ranges from 15% (Western Rajasthan dry sites) to 65% (Northeast monsoon-heavy sites).
DHI in Indian regions
| Region | Annual DHI (kWh per sq m per year) | DHI Fraction of GHI |
|---|---|---|
| Western Rajasthan, Kutch | 700 to 850 | 15% to 25% |
| Inland Gujarat, Andhra Pradesh | 900 to 1,100 | 25% to 35% |
| Maharashtra inland | 1,000 to 1,200 | 30% to 40% |
| Karnataka, Tamil Nadu | 1,100 to 1,300 | 35% to 45% |
| Coastal Mumbai, Goa | 1,200 to 1,400 | 40% to 50% |
| Delhi, Punjab, Haryana | 1,000 to 1,200 | 40% to 50% |
| Bihar, Jharkhand | 1,100 to 1,300 | 50% to 60% |
| West Bengal, Northeast | 1,300 to 1,500 | 55% to 65% |
The pattern: dry desert regions have low DHI fraction (most light is direct); monsoon and humidity-affected regions have high DHI fraction (most light is diffuse).
How DHI is measured
By a pyranometer with a shadow band or shadow ball:
A shadow band is a metal arc or strip positioned to cast a shadow on the pyranometer sensor for the direct beam.
The shadow tracks the sun through the day (manually adjusted or motor-driven).
The shadow band blocks DNI from reaching the sensor.
The pyranometer measures only the diffuse component.
The measurement requires correction for the shadow band’s blocking of nearby sky regions (which would otherwise contribute to DHI). The correction factor is typically 5% to 15%.
Alternative: a pyranometer with a rotating shadow band system. The rotating shadow alternately blocks and exposes the sensor, providing both GHI (unblocked) and DHI (blocked) measurements from a single instrument.
For utility-scale projects with met stations, DHI measurement is typically included for comprehensive resource characterisation.
DHI in solar plant design
For solar plant design, DHI matters in several contexts:
Tracker decisions: Trackers improve energy capture by following the sun. The benefit depends on the DNI fraction. In high-DNI (low-DHI) sites, trackers add 15% to 25% to annual energy. In high-DHI sites, the benefit is smaller (8% to 15%).
Bifacial designs: Bifacial modules absorb light reflected from the ground and surrounding surfaces. Much of this rear-side light is diffuse. High-DHI sites can still benefit from bifacial designs.
Concentrating systems: CSP and CPV cannot use diffuse light. They require high DNI to be viable. High-DHI sites are not suitable for concentrating applications.
Tilt optimisation: For tilted modules, the diffuse component is treated differently in radiation models. POA calculation involves separate handling of DNI, DHI, and ground-reflected components.
For Indian designs:
Western Indian sites (low DHI fraction): Tracker plus bifacial is the optimal configuration.
Monsoon-affected sites (high DHI fraction): Fixed-tilt with bifacial may be optimal, with trackers adding less benefit.
Coastal humid sites: Generally fixed-tilt; tracker benefit modest.
Diffuse light models
For solar plant simulation, several diffuse radiation models exist:
Isotropic model: Assumes diffuse light is uniform across the sky. Simple, slightly inaccurate.
Anisotropic models (Hay-Davies, Reindl, Perez): Account for the brighter circumsolar and horizon regions. More accurate.
Perez model: The most widely used in PVsyst and SAM. Highly accurate for typical conditions.
Each model translates DHI on a horizontal surface to DHI on a tilted module surface, accounting for the sky distribution and ground reflectance.
For Indian conditions, the Perez model is the standard choice in modelling software.
Common DHI mistakes
Ignoring DHI in tracker decisions. Tracker benefit depends on the DNI fraction, which equals 1 minus DHI fraction.
Treating diffuse light as negligible. In monsoon-affected Indian regions, DHI can dominate annual GHI.
Using simple isotropic models for high-precision projections. Anisotropic models are more accurate.
Mismatching DHI estimates with reality. Local conditions can vary from satellite-derived data.
Confusing DHI with circumsolar irradiance. Circumsolar is a small fraction of DHI close to the sun.
Best practices
For solar plant design in monsoon-affected India, use site-specific DHI data alongside GHI and DNI.
For tracker decisions, evaluate the DNI fraction (not just GHI). High-DHI sites get less tracker benefit.
For bifacial designs, account for the diffuse light captured by the rear face.
For utility-scale projects with met stations, install diffuse pyranometers for measured data.
For lender-grade projections, use anisotropic diffuse models (Perez) in PVsyst or SAM.
Standards and references
DHI measurement follows WMO Guide to Meteorological Instruments. Shadow band corrections follow standard meteorological procedures. Diffuse radiation models (Perez, Hay-Davies, Reindl) are documented in solar engineering literature.
Related glossary terms
- Solar Irradiance
- Global Horizontal Irradiance
- Direct Normal Irradiance
- Peak Sun Hours
- Insolation
- Pyranometer
- Bifacial Solar Panel
- Tilt Angle
Key takeaways
Diffuse Horizontal Irradiance (DHI) is the solar irradiance from scattered sky light on a horizontal surface, excluding the direct sun beam. DHI is significant in monsoon-affected and humid Indian regions, where it can be 50% to 65% of GHI annually. DHI is measured by pyranometers with shadow bands that block the direct beam. For solar plant design, DHI matters in tracker decisions (high-DNI sites benefit more from trackers), bifacial designs (some rear-side light is diffuse), and CSP feasibility (cannot use DHI). DHI is one of three components (with DNI and GHI) used in detailed solar resource modelling.