DHI

What is DHI

Diffuse Horizontal Irradiance (DHI) is the solar radiation received on a horizontal surface from all directions of the sky except the direct beam from the sun. It is the scattered light produced by atmospheric scattering of sunlight by air molecules, clouds, aerosols and water vapour.

Measured in W/m² as irradiance and kWh/m² as integrated irradiation, DHI complements the direct beam (DNI) and together with the projected direct beam makes up the total Global Horizontal Irradiance (GHI):

For Indian solar, DHI ranges from 600 to 950 kWh/m²/year. Humid coastal regions and monsoon periods have proportionally higher DHI. DHI is critical for accurate yield modelling, bifacial gain calculation and building daylighting design.

Why DHI matters

PV yield modelling. Diffuse light contributes 15 to 30 percent of plane of array irradiance for fixed tilt arrays, which is why bankable PV yield simulation software models GHI, DNI and DHI separately rather than relying on GHI alone.

Bifacial yield. DHI determines sky diffuse contribution to rear-side gain, alongside ground albedo.

Cloudy day generation. Most generation under clouds is from DHI, a pattern documented across the monsoon solar prep checklist QBits publishes for Indian installers.

Building daylighting. North-facing windows in northern hemisphere rely on DHI.

Resource decomposition. Bankable yield models need DNI and DHI separately, not just GHI.

DHI measurement

DHI can be measured directly or computed indirectly:

Technical deep dive

Indian DHI by region (annual, kWh/m²)

Diffuse fraction (DHI/GHI)

Plane-of-array contribution

For a south-facing tilted module: POA irradiance = direct beam + diffuse on tilt + ground reflected

The diffuse-on-tilt component uses sky models (isotropic, Hay, Perez):

Diffuse on tilt = DHI × f(tilt, sky model)

For most Indian sites, diffuse contributes 20 to 30 percent of POA on annual basis, which is why peak sun hours calculated from GHI alone can understate the diffuse contribution on a tilted roof.

Real-world applications

Yield modelling for utility solar plants, typically run through PV simulation software that separates DNI and DHI inputs.

Bifacial yield projection.

Building integrated PV design, often coordinated with rooftop detailed engineering design teams.

Architectural daylighting (BIPV facades).

Cloudy-day generation estimation, an important input during pre-monsoon inspection planning for existing rooftop systems.

Common DHI problems

Soiled pyranometer underreading DHI.

Shadow ball misalignment.

Satellite-derived DHI errors under heavy aerosols.

Decomposition errors between GHI and DNI.

Sky model assumptions affecting POA diffuse calculation.

Best practices

Use shadow-ball pyranometer for direct DHI measurement when available.

Apply validated decomposition models (DIRINT, DISC) for satellite data.

Document sky model used in POA calculation.

Compare DHI estimates across data sources for validation.

Apply local aerosol corrections in dust-heavy regions.

Indian market context

NIWE limited DHI ground stations; most plants use derived or satellite-based DHI.

Solargis and NSRDB provide Indian DHI from satellite-derived methods.

Bifacial yield modelling for SECI tender bids uses DHI as input for rear-side calculation.

PM Surya Ghar residential systems do not require DHI-specific data; GHI is sufficient.

Standards and certifications

Key takeaways

DHI is the diffuse sky-scattered solar irradiance, complementing DNI to make up GHI. Indian DHI ranges from 600 in dry regions to 950 in humid and monsoon-affected regions. DHI contributes 20 to 30 percent of POA irradiance for tilted PV. Bifacial yield, cloudy-day generation and daylighting design all depend on accurate DHI. Modern bankable yield modelling requires separate GHI, DNI and DHI inputs.

Further reading

For how DHI plays out in real projects, these guides go deeper:

• Solar Irradiance Data India, Peak Sun Hours
• Solar Inverter ki Life Kitni Hoti Hai: Poori Hindi Guide (2026)
• Solar Subsidy in Gujarat 2026, Surya Gujarat
• Punjab Dairy Farm Solar, Replacing Diesel

Sources

• NIWE Solar Resource Atlas.
• ISO 9060, Pyranometer classification.
• IEC 61724-1, Performance monitoring.
• Solargis Solar Resource Database.
• NSRDB National Solar Radiation Database, NREL.
• Meteonorm Global Solar Radiation Database.
• WMO Guide to Meteorological Instruments.
• Perez R., et al., Modeling daylight availability and irradiance components from direct and global irradiance, Solar Energy, 1990.
• Duffie J. A., Beckman W. A., Solar Engineering of Thermal Processes, Wiley.