The rate at which electrical energy is dissipated into other forms of energy is called electrical power, i.e.,
On electrical appliances (bulbs, heater, etc.), wattage, voltage, etc, are printed called rated values. For example, if suppose we have a bulb of 40 W, 220 V then rated power (PR) = 40 W while rated voltage (VR) = 220 V. It means that on operating the bulb at 220 V, the power dissipated will be 40 W or in other words 40 J of electrical energy will be converted into heat and light per second.
Resistance of electrical appliance
If variation of resistance with temperature is neglected then resistance of any electrical appliance can be calculated by rated power and rated voltage, i.e., by using
An electrical appliance (bulb, heater, etc.) consumes rated power (PR) only if the applied voltage (VA) is equal to the rated voltage (VR), i.e., if VA = VR. So Pconsumed = PR.
If VA < VR, then . Also, we have So