Quick answer
Solar irradiance is incident solar power per area, measured in W/m². Its surface and time basis matter when modeling PV output.
Read the units before comparing the numbers
Irradiance tells you how much solar power reaches a surface. Irradiation tells you how much solar energy accumulated there over a stated period. Neither is the electricity delivered by a PV system.
What solar irradiance measures
Solar irradiance is incoming solar power per unit area, expressed in watts per square metre (W/m²). Its meaning depends on the receiving surface and the time basis. A horizontal weather-station reading, a measurement normal to the sun and an array-plane value describe different quantities, even when their units match.
Sandia’s PV Performance Modeling Collaborative distinguishes irradiance from insolation, the energy accumulated over time. That distinction matters when a dataset mixes instantaneous readings, interval averages and daily totals. Preserve the original units and timestamps when matching weather observations with inverter measurements.
Horizontal, direct and array-plane values
The quantity label identifies the measurement plane or component. Use this table to decide which weather input a model expects; it does not convert one quantity into another.
| Quantity | Receiving plane or component | Design question |
|---|---|---|
| GHI | Direct and diffuse irradiance on a horizontal surface | Does the weather input describe a horizontal plane? |
| DNI | Direct beam measured normal to the solar beam | Has the beam been projected onto the intended surface? |
| DHI | Diffuse irradiance on a horizontal surface | Which diffuse model and input does the simulation use? |
| POA | Irradiance on the array plane | Are tilt, orientation and reflected-light assumptions documented? |
See Sandia’s GHI definition and POA modeling overview. The horizontal direct contribution depends on solar geometry. Dividing DNI by GHI therefore does not, by itself, give the direct-light fraction received by a tilted panel.
The GHI glossary explains that projection. The POA glossary follows the quantity used at the module plane.
From irradiance to accumulated energy
For a constant irradiance, multiply power per area by elapsed time. For a changing time series, integrate the intervals according to the dataset’s sampling convention. A single instantaneous reading cannot stand in for the rest of the day.
Find the radiant energy received per square metre during an assumed three-hour interval. These inputs are illustrative, not measured weather.
| Parameter | Symbol | Value | Unit | Input basis |
|---|---|---|---|---|
| Constant irradiance | G | 600 | W/m² | Hypothetical input |
| Duration | Δt | 3 | h | Hypothetical input |
- Formula
- H = G × Δt; 1 kWh = 1,000 Wh
- Calculation
- 600 W/m² × 3 h = 1,800 Wh/m²; 1,800 ÷ 1,000 = 1.8 kWh/m²
- Result
- 1.8 kWh/m² over the assumed three hours
- Interpretation
- This is incident radiant energy per area. It is not PV electricity in kWh.
- Assumptions and limitations
- The example assumes constant irradiance on one specified surface. Real weather requires interval integration; electrical production requires a PV model.
- Basis
- Sandia irradiance/insolation definitions; hypothetical inputs and independently scripted arithmetic
For the related accumulated-energy quantity, continue with insolation. Keep the period next to the value: an annual total and a daily total are not comparable without a clearly defined normalization.
Inspect a dataset before comparing locations
A resource comparison should be reproducible from the export. Save coordinates, database and version, period, interval, units, receiving surface and timestamp convention. Check how the provider treats missing records and whether a reported interval is an average or an accumulated total.
The European Commission’s PVGIS user manual documents resource and PV-output options. Its distinction between radiation data and calculated PV production is useful when a report contains both. Retain the selected database, geometry and horizon settings with the output.
Compare the same period and plane across providers. If one export is a historical year and another is a typical meteorological year, identify that difference before attributing a discrepancy to model quality. A typical year is a constructed weather representation, not an observed customer operating year.
How resource data become a production estimate
Irradiance is an input to a production model. Equipment behavior, temperature, shading, optical and electrical losses, and operating conditions affect the electrical output. The following process describes the information boundary; it does not certify any software implementation.
A qualitative modeling sequence, with no assumed loss percentages.
- Identify the resource
Retain the database, period, components, units and time convention.
- Document the array plane
Specify geometry, horizon and the conversion from horizontal components when required.
- Model the equipment and losses
Record the module/inverter selection and thermal, shading and operating assumptions.
- Report electrical output
Name the AC or DC measurement boundary and preserve the model revision.
Sandia PVPMC resource and POA modeling guides
This is a conceptual workflow, not a yield forecast, equipment design or verified product-capability list.
A resource number cannot establish customer savings. The financial analysis also needs consumption, tariff, export treatment, capital costs and other project assumptions. Ask for those inputs when comparing two proposals that use the same weather source but show different savings.
Uncertainty and scenario reporting
State the production boundary, time horizon and uncertainty method before presenting P50 or P90. Do not assign a universal gap between them or label a typical-year result P50 without the analysis supporting that interpretation.
Keep hypothetical calculations separate from measured results. A tracker gain, lender decision or customer outcome needs retained project evidence before it can be described as observed.
Practical evaluation
For a design review, request the weather export and report revision together. For an installation check, distinguish the sensor’s plane and interval from the inverter measurement being compared. For a sales discussion, label the output as modeled and explain which assumptions may change.
Inspect the available inputs and outputs in the solar simulation workflow on a representative project. Confirm what the selected product configuration supplies instead of assuming worldwide automatic coverage or finance-grade acceptance.
Related Glossary Terms
About the Contributors
Co-Founder · SurgePV
Akash Hirpara is identified by SurgePV as a company co-founder. His SurgePV author page lists only role information that can be tied to the public profile below; education, certifications, project totals, financial results, speaking engagements, and media appearances are not asserted without retained evidence.
Editorial contributor · SurgePV
Rainer Neumann is credited as an editorial contributor on SurgePV content. This profile does not assert engineering credentials, project totals, software-testing experience, education, speaking engagements, or media citations because independent verification evidence is not retained in the publication record.
