Power Calculator

Any one of four pairs works: voltage and current (P = V x I), current and resistance (P = I squared x R), voltage and resistance (P = V squared / R), or energy and time (P = E / t, with energy in joules and time in seconds). Enter just the pair you have and leave the other fields blank. The calculator picks the matching formula and returns the power in both watts and kilowatts.

For purely resistive loads such as heaters and incandescent lamps, yes: volts times amps gives real watts. For motors, transformers and electronics, volts times amps gives apparent power in VA, and you must multiply by the power factor to get real watts: P = V x I x PF. If you use this calculator on an inductive load without accounting for power factor, the result overstates the real power.

Whenever current flows through a known resistance and you want the heat it generates. The classic application is cable loss: 16 A flowing through a circuit with 0.9 ohms of conductor resistance dissipates 16 x 16 x 0.9 = 230 W as heat along the cable. Because the loss rises with the square of the current, halving the current quarters the loss, which is the engineering case for running distribution at higher voltages.

Divide the energy in joules by the time in seconds: P = E / t. One watt is one joule per second, so 360,000 J released over 3,600 seconds is 100 W. To connect this to billing units, one kilowatt-hour is 3,600,000 J, so a kWh figure multiplied by 3.6 million gives joules, and dividing by the seconds in the period returns the average watts.

Only scale: 1 kW is 1,000 W. Small appliances and electronics are usually quoted in watts, while commercial loads, supplies and tariffs work in kilowatts. This calculator shows both. As a sense check, a 13 A socket on a 230 V supply can deliver at most about 3 kW, a typical commercial kitchen oven is 5 to 15 kW, and a 100 A three-phase supply can carry roughly 60 to 70 kW depending on power factor.