The watt (W) is the SI unit of power, i.e., a unit for an energy rate, such as the rate that energy is being produced, transmitted or consumed. A watt is equal to a joule per second.
The watt is familiar for being commonly used in specifying the energy consumption of light bulbs. The energy output of the Sun (its luminosity) can be cited in watts (3.828 × 1026 watts). Solar luminosity (LSun) is a convenient larger unit, e.g., for stars. Horsepower is another common unit of power, which has different definitions, 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.