Electrical Power Calculator
P = IV = I²R = V²/R. Choose which two variables you know.
How this is calculated
Whichever pair of voltage, current or resistance you provide, the calculator uses the matching form of the power equation (P = IV, P = V²/R, or P = I²R) and then derives the remaining variable from Ohm's law, so all four values are always shown together.
Power (the instantaneous rate of energy use) is closely related to but distinct from energy consumed over time — see the Home Electricity Consumption Estimator for turning a device's power draw into an estimated monthly kWh usage and cost. For the underlying voltage/current/resistance relationship on its own, without the power calculation layered on top, see the Ohm's Law Calculator.
Frequently asked questions
- Why are there three formulas for power?
- They're all the same relationship (P = IV combined with Ohm's law, V = IR), just algebraically rearranged to use whichever two quantities you already know: P = IV, P = I²R, or P = V²/R all give the same answer for a given circuit.
- What is power measured in?
- Watts (W) — one watt is one joule of energy per second. Voltage is in volts (V), current in amps (A), resistance in ohms (Ω).
- Can I use this in my own app?
- Yes — every calculator on Stupidly Clever has a matching REST API and MCP tool that runs the same underlying logic.
- What is electrical power?
- Electrical power is the rate at which electrical energy is transferred or converted — measured in watts, where one watt equals one joule of energy per second. It tells you how much energy a device consumes (or a source supplies) per unit of time, not the total energy used over a period.
- What is the difference between AC and DC power?
- AC (alternating current) periodically reverses direction, typically 50 or 60 times per second depending on the country — it's what mains electricity delivers to homes. DC (direct current) flows in one constant direction, used by batteries and most electronic components internally. The P = IV formula applies to both, though AC calculations often need to account for power factor as well.
- What is power factor and when does it matter?
- Power factor measures how effectively AC power is being used, accounting for the phase difference between voltage and current caused by inductive or capacitive loads (like motors). It matters for AC circuits with such loads — for simple resistive loads and DC circuits, power factor is effectively 1, which is what this calculator assumes.
- How do I calculate the cost of running an appliance?
- Multiply the appliance's power in kilowatts by the hours it runs to get kilowatt-hours (kWh) consumed, then multiply by your electricity price per kWh. See the Home Electricity Consumption Estimator for a fuller household-level version of this calculation.
- What is the difference between watts, kilowatts, and kilowatt-hours?
- Watts and kilowatts (1,000 watts) measure instantaneous power — the rate of energy use at a moment in time. Kilowatt-hours measure total energy consumed over time — a 1 kW appliance running for 1 hour uses 1 kWh. Electricity bills charge for kWh (energy used), not kW (power draw).