Diffuse Horizontal Irradiance (DHI)

Diffuse Horizontal Irradiance (DHI) is the portion of solar radiation that reaches the Earth’s surface after being scattered by molecules, aerosols, clouds, and atmospheric particles. Unlike direct sunlight, DHI does not come from a single direction—it arrives from all parts of the sky dome, making it essential for accurate solar energy modeling, especially in shaded, cloudy, or polluted regions.

DHI is a foundational irradiance parameter used in PV modeling tools, energy simulations, and system performance forecasts. Designers rely on DHI when using solar modeling tools, shading platforms like Shadow Analysis, and integrated design environments such as Solar Designing to generate accurate POA irradiance and expected energy yield.

Key Takeaways

Diffuse Horizontal Irradiance (DHI) is the scattered portion of sunlight reaching a horizontal surface.
It is critical for accurate energy modeling, shading analysis, and POA calculations.
DHI heavily influences production in cloudy, humid, or polluted environments.
Professional weather datasets and shaded pyranometers are used to measure it.
Solar designers rely on DHI to optimize system tilt, layout, and expected performance.

What Is Diffuse Horizontal Irradiance (DHI)?

DHI represents the sunlight that reaches a horizontal surface after being scattered in the atmosphere rather than coming directly from the sun’s beam. This scattered light originates from:

Clouds
Air molecules
Water vapor
Dust and pollution
Horizon glow and sky dome brightness

DHI becomes especially important when:

A module is partially shaded
The sun is behind clouds
Urban pollution increases scattering
The installation tilt is not optimal
Calculating POA Irradiance, which uses both direct and diffuse components

Related core concepts include POA Irradiance, Shading Analysis, and Insolation.

How Diffuse Horizontal Irradiance (DHI) Works

1. Sunlight enters the atmosphere

Sunlight interacts with atmospheric gases and particles.

2. Scattering occurs

Two main scattering processes create diffuse light:

Rayleigh scattering (by small molecules — causes blue sky)
Mie scattering (by larger particles — pollution, aerosols, dust)

3. Light arrives from all angles

Diffuse light is non-directional and reaches the module from every part of the sky dome.

4. Horizontal surface receives scattered irradiance

DHI is measured on a horizontal plane, not a tilted surface.

5. Combined into POA Irradiance for PV modeling

POA = Direct Beam + Diffuse (DHI contribution) + Ground Reflected

See Plane of Array Irradiance.

6. Used in energy simulation engines

Solar designers rely on DHI when generating annual energy yield forecasts using tools like SurgePV’s modeling environment.

Types / Variants of Diffuse Irradiance in Solar Modeling

1. Isotropic Diffuse Model

Assumes diffuse irradiance is uniformly distributed across the sky.

2. Anisotropic Diffuse Models

Recognize that some regions of the sky contribute more irradiance, such as:

Horizon brightening
Circumsolar region (near the sun)

Common advanced anisotropic models include:

Hay-Davies
Perez Model
Klucher Model (Klum)

These are widely used in professional simulation tools for better accuracy.

How DHI Is Measured

1. Pyranometer (Shaded)

A shaded pyranometer blocks direct beam sunlight, measuring only diffuse irradiance.

2. Weather Data Sets

Professional datasets include hourly DHI values:

TMY / TMY2 / TMY3
Typical Meteorological Year files
Satellite-derived irradiance models
Ground weather stations

These datasets integrate directly into solar design workflows.

3. POA Decomposition Models

When DHI is missing, tools calculate it using measured GHI and DNI.

Typical Values / Ranges

DHI varies by geography, weather patterns, and atmospheric clarity:

High DHI Regions:

Coastal areas
Cloudy climates
High-pollution urban centers
Humid tropical zones

DHI can exceed 50–70% of total GHI in such locations.

Low DHI Regions:

Dry deserts
High-altitude regions
Areas with clear, stable skies

Here, DHI may contribute only 10–20% of GHI.

Typical Global Ranges

Clear sky: 50–150 W/m²
Partly cloudy: 100–300 W/m²
Overcast sky: 200–600 W/m² (no direct beam, only diffuse)

Practical Guidance for Solar Designers & Installers

1. Use accurate weather files

TMY or satellite-based irradiance dramatically improves modeling accuracy.

2. Combine DHI with shading tools

DHI influences energy production even when direct beam is blocked.

See Shading Analysis.

3. Understand regional DHI behavior

High DHI zones may benefit from:

Lower tilt angles
East–west racking
Higher-density installations

4. Consider bifacial applications

Diffuse irradiance can significantly increase backside gain:

See Bifacial Solar Panel (when available).

5. Evaluate impact on POA Irradiance

DHI is a core input in POA modeling and performance forecasting tools.

6. Use SurgePV for layout + irradiance workflows

SurgePV integrates DHI values into automated layout, modeling, and shade analysis:

Solar Designing

Real-World Examples

1. Cloudy Climate (Germany)

A rooftop system in Munich sees high DHI due to frequent cloud cover.

The strong diffuse component stabilizes production even during partially overcast days.

2. Coastal PV System (San Francisco)

Morning fog increases scattering, making DHI a significant contributor to annual yield.

3. Urban Rooftop (Delhi, India)

Pollution and haze cause elevated DHI values, influencing module orientation and tilt decisions to maximize exposure to diffuse light.