DHI (Diffuse Horizontal Irradiance)

Answer: Diffuse horizontal irradiance (DHI) is sunlight scattered by the atmosphere that reaches a horizontal surface from the sky, excluding the direct solar beam. It is measured in W/m². Together with the horizontal projection of direct normal irradiance, it forms global horizontal irradiance. Solar designers use these components to estimate irradiance on tilted modules; DHI alone does not determine panel output.

How DHI relates to GHI and DNI

For the sun above the horizon, using synchronized components on compatible measurement boundaries:

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

DNI is measured on a surface normal to the solar beam. The cosine projects that beam onto a horizontal surface. DHI already refers to the horizontal surface, so it does not receive the same projection. Sandia’s GHI guidance explains this relationship and the use of decomposition models when only GHI is available.

Hypothetical calculation: DNI = 800 W/m², zenith = 60° and DHI = 100 W/m² give GHI = 800 × 0.5 + 100 = 500 W/m². The diffuse fraction is 100/500 = 20% for this example, not a climate-wide rule.

Clouds, aerosols and atmospheric conditions change the direct/diffuse split. Under some cloudy conditions diffuse light dominates, while total irradiance can still be low. A high diffuse fraction does not mean high energy production.

Irradiance and irradiation use different units

Irradiance is incoming power per area, in W/m². Irradiation is energy per area integrated over time, commonly Wh/m² or kWh/m². If the example’s GHI remained at 500 W/m² for 15 minutes, the irradiation would be 125 Wh/m², or 0.125 kWh/m². Actual integration needs the recorded interval values and their timestamp convention.

Do not label a daily kWh/m² value as instantaneous DHI. Do not convert either quantity directly into system kWh without module area, conversion behavior and the remaining system model.

From horizontal sky light to tilted panels

A tilted array receives direct beam, sky-diffuse and ground-reflected irradiance. Sandia’s plane-of-array guidance treats these as separate components. Transposition converts horizontal weather components into the array’s orientation; it is different from decomposing GHI into direct and diffuse components.

An isotropic sky model distributes diffuse light uniformly over the sky. More detailed models represent directional features such as circumsolar and horizon brightness. Choose and document the model rather than assigning a universal annual gain to one method. Trees, buildings and rows can obstruct the sky view as well as the direct beam. Ground reflection depends on albedo, geometry and the visible surrounding surface.

Measurement and weather-data checks

DHI can be measured with a pyranometer shaded from the direct sun. The shading arrangement, tracking, calibration, cleanliness and correction method affect the result. A sensor’s location and maintenance record matter; purchasing a DHI sensor does not establish that a dataset is representative of another site.

Before accepting measured or modeled DHI, record:

• Dataset provider, version, coordinates and elevation.
• Measured versus modeled components and any decomposition method.
• Time zone, interval length and timestamp convention.
• Actual meteorological year versus a typical meteorological year.
• Missing-value handling, component consistency and sensor-quality flags.

For an operating-period comparison, use weather representative of that period. A typical year supports long-term modeling but does not reproduce a particular month’s clouds. Compare multiple defensible weather sources where uncertainty matters; proximity alone does not guarantee representativeness.

Practical design implications

DHI helps explain why direct-beam shading does not necessarily eliminate all incoming light. It also helps distinguish climate effects from changes in tilt, orientation or sky obstruction. Keep the weather source consistent when comparing layouts, and review the complete plane-of-array and electrical loss model before interpreting an energy difference. Start with global horizontal irradiance for the total horizontal resource and solar irradiance for the broader terminology.

Common questions

Is DHI the same as GHI?

No. DHI is the diffuse sky component on a horizontal surface. GHI also includes the direct beam projected onto that surface.

Can DHI be estimated without a diffuse sensor?

Yes, a documented decomposition model can estimate components from available inputs. The estimate has model and input limitations and should not be presented as a direct measurement.

Do shaded panels still receive diffuse light?

Often they receive some sky light, but obstructions can reduce the visible sky. Electrical output also depends on module and system behavior.

Does more DHI always increase production?

No. The direct/diffuse split, total resource, orientation, temperature and losses must be considered together. A larger diffuse fraction can occur when total irradiance falls.