watt

The watt (W) is the SI unit of power, i.e., a unit for an energy rate. As such, it can be used to quantify the rate of production of energy, transmission or conversion of energy, or consumption of energy. A watt is equal to a joule (of energy) per second. The watt is familiar for being commonly used in specifying the energy consumption rate of light bulbs and other familiar appliances. The energy output rate of the Sun (its luminosity) can be cited in watts (3.828 × 10²⁶ watts). Solar luminosity (L_Sun) is a convenient larger power unit, e.g., for other stars. Horsepower is another common unit of power, which has various defined values, generally in the vicinity of 730 watts.

Electromagnetic radiation's density in flight (flux) and its density striking a surface (irradiance) can be expressed as watts per unit area, e.g., watts per square meter. The solar constant (flux of sunlight as it reaches Earth) hovers around 1362 watts per square meter.