Quick answer
Irradiance is radiant power received per unit area on a specified surface, commonly measured in W/m². In solar analysis, identify the surface orientation and the time basis: a horizontal measurement is different from a panel-plane measurement, and an interval average is different from an instantaneous reading. Integrating irradiance over time gives insolation, an energy-per-area quantity.
Power per area, not accumulated energy
Sandia’s irradiance and insolation guidance distinguishes irradiance, a power-per-area quantity, from energy accumulated over time. A reading of 800 W/m² does not establish a daily total or a PV system’s electrical output.
Datasets may contain instantaneous measurements or averages over an interval. Record the sampling interval and timestamp convention before combining a weather series with a load or production series. See insolation for the energy integral.
Which surface does the value describe?
| Quantity | Reference surface or direction |
|---|---|
| Global horizontal irradiance (GHI) | Total irradiance on a horizontal surface |
| Direct normal irradiance (DNI) | Direct-beam irradiance on a surface normal to the solar beam |
| Diffuse horizontal irradiance (DHI) | Diffuse component on a horizontal surface |
| Plane-of-array irradiance (POA) | Irradiance incident on the array plane |
The definitions of GHI, DNI and DHI include different reference geometry. Do not divide DNI by GHI and call the result the fraction of sunlight arriving directly on a panel. The beam must be projected onto the relevant plane.
Sandia’s POA guidance identifies determining irradiance on the array plane as a fundamental performance-modeling step. Preserve orientation, tilt and the chosen transposition method when converting weather inputs for an array.
A quantity check with explicit units
For a hypothetical constant irradiance of 800 W/m² over two hours, the accumulated radiant energy is:
800 W/m² × 2 h = 1,600 Wh/m² = 1.6 kWh/m².
This is an illustration of integration. It does not establish a site’s weather or a module’s electrical energy. Real irradiance varies, so integrate the represented intervals rather than multiplying one midday reading by the whole day.
Irradiance does not uniquely determine PV output
Module temperature, spectral response, incidence angle, equipment behavior and losses also affect electrical output. The standard-test-condition definition explains the reference used for module ratings. A nameplate rating is not a promise of the same output whenever a weather sensor reads the reference irradiance.
Sandia’s effective-irradiance documentation distinguishes POA irradiance from an adjusted input accounting for incidence-angle losses, soiling and spectral mismatch. Use the definition associated with the selected model; do not treat every column labeled irradiance as interchangeable.
Check a resource dataset before using it
Record the location, source, version, period, temporal resolution, surface basis and units. Determine whether the data represent measured weather, satellite-derived estimates or a typical meteorological year. Review missing intervals and the provider’s quality information.
A longer record alone does not prove that one source is more accurate for a site. Likewise, a regional average does not establish the usable resource on a shaded roof. Compare datasets on compatible periods and surfaces, and preserve differences rather than silently mixing them.
Use irradiance in design and troubleshooting
For a design, evaluate production and losses with equipment and operating assumptions. Do not choose the inverter ratio or recommend tracking from an irradiance label alone. For troubleshooting, compare measured and expected output using compatible timestamps, sensor placement, temperatures and operating conditions. This page does not define a universal five-percent fault threshold or a guaranteed cleaning gain.
Continue with plane-of-array irradiance or inspect the resource inputs in the solar simulation workflow. Ask which input quantities and assumptions the report actually uses.
Reviewed September 29, 2026 using the linked Sandia primary documentation. Worked values are illustrative; no retained field trial or observed customer outcome is claimed.
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