If a piece of electrical equipment is examined it will usually be found to have a label or engraved plate giving the working voltage and the power consumption in watts. For example, an electric lamp may be marked 240 V 60 W, or a fire 240 V 3 kW. The abbreviation kW stands for kilowatt (1 kW = 1000 W). By definition (page 82) a power of 1 watt is a rate of working of 1 joule per second, and therefore power in watts = rate of working in J/s V2t The three expressions, VIt, P Rt and R(as above) represent the work in joules done by the current in time t in seconds. Hence the wattage or power in J/s is found by dividing each expression by t.
The other two expressions may then be derived from this by substituting for either volts or amperes, using the formula
volts = amperes x ohms