Power Calculator
This power calculator finds electrical power using any two of voltage (V), current (I), or resistance (R). Choose the mode that matches your known values and get power in watts, milliwatts, and kilowatts — plus all four circuit quantities — instantly.
Known values
Select which two quantities you know.
The three power formulas
P = V × I (voltage × current)
P = I² × R (current squared × resistance)
P = V² ÷ R (voltage squared ÷ resistance)
All three are derived from each other using Ohm's Law (V = IR). In practice: use P = VI when you know supply voltage and load current; use P = I²R when sizing resistors or measuring resistive heating; use P = V²/R when voltage is fixed and you're computing load power. This calculator computes all four circuit quantities (V, I, R, P) from whichever two you enter.
Frequently asked questions
- What is electrical power?
- Electrical power is the rate at which electrical energy is transferred or consumed in a circuit. It is measured in watts (W), where 1 watt = 1 joule per second. In a DC circuit, power equals voltage multiplied by current: P = V × I. For an AC circuit, the formula becomes more complex (involving power factor), but for purely resistive loads the same relationship holds.
- What are the three formulas for electrical power?
- Starting from P = V × I and substituting Ohm's Law (V = I × R) gives three equivalent forms: P = V × I (power from voltage and current), P = I² × R (power from current and resistance), and P = V² ÷ R (power from voltage and resistance). All three are fully equivalent for resistive (ohmic) components — use whichever matches the two values you know.
- What is the difference between watts, kilowatts, and kilowatt-hours?
- Watts (W) and kilowatts (kW) measure power — the rate of energy use at a moment in time. A kilowatt-hour (kWh) measures energy — the total work done over time. A 1 kW heater running for 1 hour uses 1 kWh of energy. Your electricity bill charges you per kWh. On the other hand, your circuit breaker trips based on current (amps), not energy.
- How do I calculate power dissipated by a resistor?
- Use P = I² × R if you know the current, or P = V² ÷ R if you know the voltage across the resistor. For example, a 100 Ω resistor carrying 50 mA dissipates P = (0.05)² × 100 = 0.25 W. This tells you the minimum power rating the resistor must have — choose a resistor rated at 0.5 W or more to give a 2× safety margin.
- What is power factor and when does it matter?
- Power factor (PF) is the ratio of real power (watts, which does useful work) to apparent power (volt-amperes, VA). In purely resistive circuits, PF = 1. In circuits with capacitors or inductors (motors, transformers, fluorescent lighting), current and voltage are phase-shifted, so PF < 1. Utility companies penalise industrial consumers for low power factor. This calculator assumes PF = 1 (purely resistive loads).
- How much does it cost to run an electrical appliance?
- Cost = power (kW) × time (hours) × price per kWh. A 2000 W (2 kW) electric kettle run for 0.05 hours (3 minutes) uses 2 × 0.05 = 0.1 kWh. At £0.28/kWh that costs £0.028 (2.8p) per boil. A 100 W light bulb left on for 10 hours uses 1 kWh, costing about £0.28 at typical UK electricity prices.
- What power rating should I specify for a resistor or component?
- Calculate the power the component will actually dissipate (P = I²R or V²/R), then select a component rated at least twice that value — this is standard engineering practice to ensure the component runs cool and has a long service life. Standard resistor power ratings are ⅛ W, ¼ W, ½ W, 1 W, 2 W, 5 W, and higher. Always check the datasheet for the derating curve at high temperatures.