Quick answer
Global horizontal irradiance (GHI) is incident solar power per unit area on a horizontal surface, measured in W/m². It combines diffuse horizontal irradiance with the horizontal projection of direct normal irradiance. Integrating it over time gives irradiation in Wh/m² or kWh/m². Check the dataset’s units and time basis before using it in a yield model.
What does GHI measure?
Sandia’s PVPMC definition describes irradiance on a horizontal surface. GHI includes the direct beam projected onto that surface and radiation arriving diffusely from the sky. It is not the irradiance automatically received by every tilted module.
Irradiance and irradiation use different units
| Quantity | Unit | Meaning |
|---|---|---|
| Irradiance | W/m² | Incident power per area |
| Irradiation | Wh/m² or kWh/m² | Incident energy per area accumulated over a period |
Solar datasets sometimes retain the label “GHI” for accumulated daily or annual values. Read the units and documentation rather than assuming the acronym identifies the time basis. A value of 5 kWh/m²/day is an energy total; 500 W/m² is a power density.
As an illustrative calculation, constant irradiance of 500 W/m² for two hours contributes 1,000 Wh/m², or 1 kWh/m². Real irradiance varies during the interval, so measured time-series data must be integrated using the correct sampling convention.
How GHI relates to DNI and DHI
PVPMC gives the relationship:
GHI = DHI + DNI × cos(solar zenith angle)Use consistent units, timestamps and angle conventions. DNI is normal to the direct beam; it cannot simply be added to DHI without projecting it onto the horizontal surface. See DNI and DHI for those distinct measurement planes.
From horizontal data to module-plane irradiance
A tilted PV array requires a plane-of-array assessment. PVPMC’s POA modeling overview describes models using the standard irradiance components. Array geometry and the treatment of direct, diffuse and reflected light must be specified.
Do not substitute an annual horizontal total for the array’s incident energy without documenting the model. Also avoid treating a daily average as a literal number of clock hours of sunshine; equivalent peak-sun-hours are a normalization at a reference irradiance.
Dataset checks before simulation
Record location, data provider, study period, time resolution, units, timestamp convention and missing-data treatment. Distinguish a historical time series from a synthetic typical year. Compare like periods when evaluating two data sources.
For design decisions, inspect the actual surface orientation and obstructions along with the resource. A single “good GHI” threshold is not sufficient to establish financial viability.
Continue with plane-of-array irradiance and evaluate the data inputs in the solar simulation workflow. Retain the dataset and model assumptions with the resulting report.
Related Glossary Terms
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