watt per square metre — from formula anatomyThe watt per square metre (W/m²) is the SI unit of irradiance or heat flux density — the rate of energy transfer per unit area. It measures how much power flows through a surface, whether that is sunlight hitting the Earth, heat radiating from a hot object, or sound energy passing through a plane. In physics, it is the natural unit for any quantity that describes energy per time per area, such as the intensity of electromagnetic radiation or the acoustic intensity of a wave.
The unit follows from the adoption of the watt and the metre in the SI system, formalized in the 20th century. The concept of irradiance dates back to the study of radiation in the 19th century, but the specific unit became standard with the SI system in 1960.
The unit is derived from the watt (joule per second) divided by the square metre. For a wave, the intensity in W/m² is proportional to the square of the amplitude (for example, the electric field of light or the pressure variation of sound). For thermal radiation, the Stefan–Boltzmann law states that a blackbody at temperature T emits power per area equal to σT⁴, where σ is the Stefan–Boltzmann constant, so the unit directly connects temperature to radiative flux.
It is used in meteorology to measure solar radiation, in engineering to specify heat transfer in buildings, in astronomy to quantify the luminosity of stars per unit area, and in medicine for dosimetry of laser and ultrasound therapy.
The solar constant, the average irradiance of sunlight at Earth's distance from the Sun, is about 1361 W/m², which means a square metre of the Earth's surface facing the Sun receives roughly the power of a typical household microwave oven.
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