A watt is the standard SI unit of power, representing 1 Joule of energy per second. It is named after a Scottish engineer by the name of James Watt. To give an idea of how much energy this is, lightbulbs in your house typically use about 25 to 100 watts (new types of lightbulbs are doing a bit better than this, actually). For comparison, if you are doing manual labor over the course of a long day, your body will average somewhere in the range of 75 watts of power output.