(D) As a form of energy, heath as the unit joule (J) in the International System of Units(SI). However, in many applied fields in engineering the British thermal unit (BTU), calorie and kilocalorie are often used. Watt is the unit of power (symbol:W). The watt unit is named after James Watt, the inventor of the steam engine. One watt is defined as the energy consumption rate of one joule per second.