Wm2

Wm2, often written as W/m^2 or W m^-2, is the SI-derived unit for radiant flux density: the amount of radiant power (watts) per unit area. It is used to express both irradiance (radiant flux incident on a surface) and, in some contexts, radiant exitance (emitted power per unit area from a surface). Although they describe energy flow in opposite directions, they share the same unit.

In practice, W/m^2 appears in a variety of fields. In solar energy, it measures the solar irradiance reaching a surface, which informs the design and performance of photovoltaic panels and solar collectors. In climatology and atmospheric science, it is used to quantify the transfer of heat and radiation through the Earth’s atmosphere. In astronomy, it describes the flux of energy received from stars and other celestial bodies. The unit is also used in radiative heat transfer and radiometry more broadly.

Measurement and related concepts: irradiance is measured with radiometers or pyranometers and is typically calibrated to SI standards. Spectral irradiance, which resolves irradiance by wavelength, is expressed in units such as W m^-2 nm^-1. The simple unit W/m^2 can be converted to other energy units, for example 1 W/m^2 equals 1000 erg s^-1 cm^-2.

A common reference value is the solar constant, about 1361 W/m^2, representing the solar power per unit area received at the top of Earth’s atmosphere when the Sun’s rays are perpendicular to the surface. The actual irradiance at the surface varies with time of day, latitude, weather, and atmospheric conditions.