Appliances are always rated by giving two information i.e power and voltage. e.g, 2500 W and 230 V. This allows the resistance of appliance to be calculated . When the applicance is connected to the power supply , the voltage and hence power may be slightly different but the resistance is a constant value. If the new voltage or current is known then new power rating is easily obtained by using I2R or V2/R. Hence the three power formula ought to be used with utmost caution.