Solar Lattice

Concept

Direct normal irradiance

ID
concept:direct-normal-irradiance
IRI
https://solarlattice.org/kb/concept/direct-normal-irradiance
Status
active
Also known as
DNI, beam irradiance, Direct Normal Irradiation, Direct irradiance, direct beam irradiance, Edni
Category
Irradiance
Concept kind
physics

Direct normal irradiance is the solar power received per unit area on a surface that always faces the sun directly. When it is summed over a period as energy, it is called direct normal irradiation. Direct normal irradiation, global horizontal irradiation and diffuse horizontal irradiation are the values normally used to describe the solar resource at a site. DNI matters most for systems that track the sun and for concentrating technologies, because concentrating technologies can only use the direct beam. The irradiance on a PV array is the sum of three components: direct beam, diffuse sky and albedo. DNI is measured with a pyrheliometer. The beam irradiance on the module plane is DNI multiplied by the cosine of the angle of incidence. Weather stations and data loggers often report DNI as a separate data point in watts per square meter. During irradiance transposition, software subtracts the estimated diffuse part and re-angles the remaining direct beam to the module orientation. It then adds the diffuse part back. When the sun is very low, it assumes there is no direct beam.

Claims

related_to

c1 drafted · lead Global horizontal irradiance

DNI, GHI and diffuse horizontal irradiation together usually define a site's solar resource.

c2 verified · fact Diffuse horizontal irradiance

The beam and diffuse components are measured separately in the detailed laboratory approach.

verified by: review-workflowverified on: 2026-10-24

c3 drafted · lead Plane-of-array irradiance

Plane-of-array irradiance is the sum of the direct beam, diffuse sky and albedo components.

review note: support check: partial - POA is stated as the sum of direct, diffuse and albedo components, but the passages do not explicitly link DNI itself to POA (direct beam is not named as DNI).

c4 drafted · lead Irradiance transposition

During transposition, trigonometry is applied to the direct beam part only, after the estimated diffuse part has been removed.

review note: support check: partial - The transposition process is described as subtracting diffuse and applying trigonometry to the direct beam, but it is applied to direct horizontal irradiance, not explicitly DNI.

c5 drafted · lead Clear-sky irradiance model

Clear-sky direct and diffuse irradiance are calculated from the time of day, latitude and longitude.

review note: support check: partial - Clear-sky direct and diffuse horizontal irradiance are computed from time, latitude, longitude, but the link to DNI specifically is not stated.

c6 drafted · lead Albedo

Albedo is the third component of surface irradiance, alongside the direct and diffuse components.

review note: support check: partial - Albedo is stated as the third component alongside direct and diffuse, but no relation to DNI is stated.

c7 drafted · lead Pyrheliometer proposed

DNI is measured with a thermopile pyrheliometer, also called a normal incidence pyrheliometer.

review note: support check: partial - DNI is from a pyrheliometer and a 'normal incidence pyrheliometer' entry exists, but 'thermopile' and 'also called' are only loosely supported.

c8 drafted · lead Air mass

In the Sandia model, an air mass spectral function is applied together with the beam irradiance.

review note: support check: partial - The Sandia equation applies an air mass function f1(AMa) with Eb, but it multiplies the whole bracket rather than only the beam term, and the relation to DNI is only indirect.

Referenced by

Data