In one line
The solar power arriving on a surface per unit area, measured in watts per square metre — the input that determines how much a PV array can produce at any instant.
What it means
Irradiance is instantaneous power density from the sun. Energy professionals distinguish global horizontal irradiance (GHI, measured on a flat horizontal surface), direct normal irradiance (DNI, the beam component perpendicular to the sun), diffuse horizontal irradiance (DHI, scattered sky light), and plane-of-array irradiance (POA, what the tilted module actually sees). Yield models convert GHI and DNI into POA using tilt, azimuth, and a transposition model, then apply module and system losses. Irradiance is power (W/m²); accumulated over time it becomes irradiation or insolation (kWh/m²/day), which is the number used for annual energy estimates.
Worth knowing
- Irradiance is instantaneous power; irradiation (insolation) is energy accumulated over time.
- POA irradiance — not GHI — is what a tilted or tracked array actually converts.
- 1,000 W/m² is the reference used in Standard Test Conditions, not a typical field value.
- Clear-sky peaks can briefly exceed 1,200 W/m² through cloud-edge enhancement.
The four components you will see in a yield report
Any bankable yield study separates the resource into components because each behaves differently with tilt, tracking and weather.
- GHI — total on a horizontal plane; the headline resource number for a site.
- DNI — the beam component; the only component concentrating technologies can use, and the one trackers exploit most.
- DHI — scattered light; dominant on overcast days and the reason arrays still produce in monsoon conditions.
- POA — the transposed value on the actual module plane, after tilt, azimuth, row shading and ground reflection.
Field notes
- 1Compare sites on long-term GHI from a validated dataset, never on a single year of satellite data — interannual variation of ±5% is normal.
- 2Ask which transposition model produced the POA figure; different models can differ by 2–3% on the same GHI input.
- 3Pyranometers need cleaning and recalibration; a drifting sensor makes a healthy plant look underperforming.
Common mistakes
- Quoting GHI when the contract performance guarantee is written against POA — the two differ by 5–20% depending on tilt.
- Treating 1,000 W/m² as an expected operating condition rather than a laboratory reference point.
Frequently asked
What is the difference between irradiance and irradiation?
Irradiance is instantaneous power per unit area, measured in W/m². Irradiation (also called insolation) is irradiance accumulated over time, measured in kWh/m² — typically expressed per day or per year and used for energy yield estimates.
What irradiance do solar panels need to produce power?
Modules begin producing at very low light — around 20–50 W/m² — though inverters usually need a minimum DC voltage before they start exporting. Output scales close to linearly with irradiance across the normal operating range.
Sources
Related terms
Browse every term in the renewable energy glossary, or read the latest analysis on Solar PV.