Watt

The Watt (named after James Watt, unit symbol W) is the physical unit of power. In respect of lamps, Watt is often used as a dimensional unit for the brightness, although actually, it can only be used to describe the total power consumption of the lamp. But since a part of the consumed power is not radiated as visible light, but in the form of heat, these losses have to be taken into account. Hence, the use of Lumen, Candela and Lux is far more suitable to describe the brightness of a lamp.