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What are the units of irradiance?

Vaisala provides various solar products for numerous purposes. The different physical quantities represented and the units used in these products can be somewhat confusing.

You can change the units displayed in the Solar Prospecting Tools from W/m² to kWh/m²/day under the "Account" menu, located at the upper right of the screen. In that menu under "Preferences" you can change the unit type displayed.

Irradiance is a measurement of solar power and is defined as the rate at which solar energy falls onto a surface. The unit of power is the Watt (abbreviated W). In the case of solar irradiance, we usually measure the power per unit area, so irradiance is typically quoted as W/m², that is, Watts per square meter. The irradiance falling on a surface can and does vary from moment to moment, which is why it is important to remember that irradiance is a measure of power - the rate that energy is received, not the total amount of energy.

The total amount of solar energy that falls over a given time is called the insolation. Insolation is a measure of energy. It is the power from the sun added up over some time period.

Now here comes the confusing part. If the sun shines at a constant 1000 W/m² for one hour, we say it has delivered 1 kWh/m² of energy. The amount of power is the product of the power (1000 W/m²) times the length of time (1 hour), so that the unit of energy is the kWh. Insolation (measured in kWh) is not the same as power (measured in kW) in the same way that miles per hour is not the same as miles.

Another commonly used term is “peak sun hours,” which reflects the energy received during total daylight hours as defined by the equivalent number of hours it would take to reach that total energy value had solar irradiance averaged 1000 W/m². Although "peak sun hours" has the unit of hours, because of the assumptions behind its definition, the value is interchangeable with kWh/m²/day.

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