A watt second (also watt-second, symbol W s or W·s) is a derived unit of energy equivalent to the joule.{harv|Taylor|Thompson|2008| pp = 39–40, 53} 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 whi...