Electric Power Calculator (P = VI)
Calculate electric power from any two of voltage, current and resistance (P = VI = I²R = V²/R), plus the missing value and energy used over time.
Electric power is the rate at which a circuit turns electrical energy into heat, light or motion, measured in watts. If you know any two of voltage, current and resistance, Ohm's law gives the third and the power follows directly. This calculator does both and also shows how many kilowatt-hours the load uses over a running time you choose.
How to use this tool
- Choose which pair of values you know.
- Enter the voltage in volts, the current in amps or the resistance in ohms. The unused field is ignored.
- Optionally enter how many hours the device runs.
- Read the power in watts, kilowatts and horsepower, the missing electrical value, and the energy used.
Formula
P = V × I = I² × R = V² / R E = P × t
P is power in watts, V voltage in volts, I current in amps and R resistance in ohms. E is energy: watts × hours ÷ 1,000 gives kilowatt-hours, and watts × seconds gives joules. All three power forms are the same law rearranged with V = I × R.
Example
A heater draws 5 A on a 120 V circuit. P = 120 × 5 = 600 W (0.6 kW, about 0.80 hp), and its resistance is 120 ÷ 5 = 24 Ω. Running for 1 hour it uses 600 × 1 ÷ 1,000 = 0.6 kWh, which is 2.16 million joules.
Frequently asked questions
Why does doubling the voltage quadruple the power?
With a fixed resistance, doubling the voltage also doubles the current, and P = V × I multiplies the two. That is why P = V²/R has the voltage squared — a 120 V heater plugged into 240 V would try to draw four times its rated power.
Which formula loses the most power in wires?
Use P = I²R with the wire's resistance and the current through it. Because the current is squared, halving the current cuts wire losses to a quarter, which is why power lines use high voltage.
Does this work for AC?
For resistive loads like kettles and heaters, yes, using RMS voltage and current. Motors, chargers and LED drivers can have power factors below 1; use the actual factor to find their real power.
Last updated: 2026-10-30