Design, installation and performance
Irradiance
Also known as: W/m², radiation power.
Definition
Irradiance is the power of solar radiation falling on a surface at a given instant, measured in watts per square metre (W/m²). Accumulated over time, it determines the energy available to generate electricity.
The distinction from accumulated radiation is the same as the one between power and energy. Irradiance is an instantaneous snapshot — how much power is arriving right now, in W/m²; irradiation is that snapshot integrated over the day, in kWh/m². The first explains why the inverter shows one figure or another at any moment; the second explains how much energy the system produced by the end of the day.
The value of 1,000 W/m² is the sector's universal reference: it is the irradiance of the standard test conditions under which any panel's peak power is measured. On a clear day with the sun high, a well-oriented surface can approach that value; under heavy cloud it falls to a fraction of it.
A counter-intuitive detail: at specific moments irradiance can exceed 1,000 W/m² because of the cloud edge effect, which adds direct and reflected radiation. Inverters are designed to handle those brief peaks without difficulty.
The essentials of Irradiance
- It is measured in W/m²: it is instantaneous power, not accumulated energy.
- 1,000 W/m² is the reference value for standard test conditions.
- Integrated over time, it gives daily radiation in kWh/m².
- Cloud edge peaks can briefly exceed 1,000 W/m².
Related terms
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Frequently asked questions about Irradiance
What is the difference between irradiance and irradiation?
Irradiance is instantaneous power (W/m²) and irradiation is that power accumulated over a period (kWh/m² per day). In everyday use they get confused, but only the second is useful for estimating how much energy a system will produce in a day or a year.
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