GHI, DNI and DHI: the three solar radiation components

Definitions

• DNI, direct normal irradiance (beam radiation): the radiation arriving straight from the sun's disc, on a surface held perpendicular to the sun's rays.
• DHI, diffuse horizontal irradiance: the radiation scattered by the atmosphere and clouds that reaches a horizontal surface from the rest of the sky, excluding the sun's disc and a small circumsolar ring around it. Without an atmosphere there would be almost none.
• GHI, global horizontal irradiance: the total reaching a horizontal surface, direct plus diffuse. Radiation reflected from the ground also exists, but on a horizontal surface it is usually insignificant.
• In-plane irradiance: the total on a tilted or vertical surface, such as a PV module or a façade, made of beam, sky diffuse and ground-reflected parts.

The closure relation

The three components are tied together by geometry: GHI = DHI + DNI × cos(Z), where Z is the solar zenith angle, 90° minus the sun's altitude. The cosine projects the beam, measured facing the sun, onto the horizontal. With the sun 30° above the horizon, cos(60°) = 0.5, so a DNI of 800 W/m² adds 400 W/m² to a horizontal surface.

The relation lets a dataset derive any one component from the other two, and it is used to check and calibrate instruments against each other. It also explains why GHI can exceed DNI: under overcast skies DNI is close to zero while GHI, made up of diffuse radiation, is not.

Units: irradiance and irradiation

Irradiance is a rate, power per unit area, in W/m². Irradiation is the energy received over a period, in Wh/m² or kWh/m², often summed per day, month or year. The solar constant, the irradiance at the top of the atmosphere at the mean Earth-Sun distance, is about 1361 W/m² (older references, such as NREL's solar glossary, give 1366 W/m²); the atmosphere reduces what reaches the ground.

Weather files mix the two ideas. EnergyPlus EPW files store GHI, DNI and DHI as the Wh/m² received during the preceding hour, which numerically equals the average irradiance in W/m² over that hour. EnergyPlus itself works from the direct and diffuse fields and does not use the GHI field in its calculations.

How they are measured and modelled

At ground stations, a pyranometer with a hemispherical view measures GHI, and the same kind of instrument with a shading disc or band blocking the sun measures DHI. A pyrheliometer on a sun tracker, with a narrow field of view, measures DNI. Even the best methods carry 1 to 3% uncertainty, and careless ones 10 to 30%.

High-quality stations are sparse, so most long records come from satellites, which estimate the effect of clouds from geostationary images at a resolution of a few kilometres, and from reanalysis such as ERA5, which covers the whole globe at around 30 km but is generally less accurate for solar radiation than satellite data where both exist. PVGIS, the European Commission's free tool, combines satellite-based data, the US NSRDB and ERA5.

Using GHI, DNI and DHI in design

• Façades and windows: a vertical surface receives the beam at an angle set by the sun's position and the façade orientation, part of the sky's diffuse radiation and ground-reflected radiation. The beam is pure geometry; the diffuse part needs a sky model. The simplest, the isotropic model, gives a surface tilted at angle β a sky diffuse irradiance of DHI × (1 + cos β) / 2, so a vertical wall sees half the DHI. Anisotropic models such as Perez, Reindl or Muneer add a brighter region around the sun and, in some models, a brighter band near the horizon.
• Shading: DNI is what an overhang or fin can block. Diffuse radiation still reaches a shaded window from the visible sky, so an overhang or fin reduces solar gains but does not remove them.
• PV: yield estimates transpose GHI, DNI and DHI onto the module plane before applying module and system losses, which is why both the annual total and the split between direct and diffuse matter.
• Passive solar heating: look at monthly irradiation on each façade orientation in the heating season, not at the annual horizontal total.

How C4B shows solar radiation

C4B charts direct normal, diffuse horizontal and global horizontal radiation for the site, with a compass to read the radiation reaching vertical surfaces facing any direction. Its yearly PV potential uses the open-source pvlib library with an isotropic sky model, sweeping panel tilt and azimuth with a 19% module efficiency to find the optimal tilt. It is an early-stage estimate, not a detailed PV system design.