Electrical Power Calculator
Calculate electrical power from voltage, current and resistance using P = VI, I²R and V²/R.
How it works
P = V·I · P = I²·R · P = V²/R
Electrical power is the rate at which a circuit converts electrical energy into heat, light or motion, measured in watts. It can be computed three equivalent ways depending on which two quantities you know, all linked by Ohm’s law. Picking I²R or V²/R is handy when you know the resistance rather than both voltage and current — for a fixed resistor, power rises with the square of whatever you increase.
Worked example
A 120 V appliance drawing 5 A uses P = 120 × 5 = 600 W, which is 0.6 kW or about 0.8 hp. Running it for an hour consumes 0.6 kWh of energy.
Sources: Electric power — Wikipedia
Frequently asked questions
What is the difference between watts and watt-hours?
Watts measure power — the instantaneous rate of energy use. Watt-hours (and kilowatt-hours) measure energy — power multiplied by time. A 600 W device running for two hours uses 1.2 kWh, which is what your electricity bill charges for.
Why are there three formulas?
They are algebraically identical, derived by substituting Ohm’s law (V = IR) into P = VI. Use whichever matches the two quantities you actually know so you don’t have to compute the third first.
Does this apply to AC circuits?
For resistive loads, yes, using RMS voltage and current. Reactive AC loads (motors, transformers) also have a power factor, so real power is P = V·I·cosφ — lower than the V·I product.
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