A watt second (also watt-second, symbol W s or W·s) is a derived unit of energy equivalent to the joule. The watt-second is the energy equivalent to the power of one watt sustained for one second. While the watt-second is equivalent to the joule in both units and meaning, there are some contexts in which the term "watt-second" is used instead of "joule".
Although joule and watt-second mean exactly the same thing (they represent the same amount of the same physical quantity, viz. energy), the newton metre is not equivalent to the joule (despite having the same dimensions) as it represents torque rather than energy. (In terms of vectors, torque is a cross product of force and distance, and energy is their dot product.)
In photography, the unit for flashes is the watt-second. A flash can be rated in watt-seconds (e.g. 300 Ws) or in joules (different names for the same thing), but historically the term "watt-second" has been used and continues to be used. An on-camera flash, using a 1000 microfarad capacitor at 300 volts, would be 45 watt-seconds. Studio flashes, using larger capacitors and higher voltages, are in the 200 - 2000 watt-second range.
The energy rating a flash is given is not a reliable benchmark for its light output because there are numerous factors that affect the energy conversion efficiency. For example, the construction of the tube will affect the efficiency, and the use of reflectors and filters will change the usable light output towards the subject. Some companies specify their products in "true" watt-seconds, and some specify their products in "nominal" watt-seconds.
Companies that use watt-seconds to describe their products:
- Guide number
- Kilowatt hour, a scaled unit of the watt-second often used in the electrical energy industry
Taylor, Barry; Thompson, Ambler (2008), The International System of Units (Special publication 330), Washington: National Institute of Standards and Technology