Calculate electrical power in watts from voltage and current, or find the missing voltage or current value.
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Electrical power in watts (W) measures the rate of electrical energy consumption, and in simple circuits (DC or single-phase AC with a purely resistive load) it's calculated by multiplying voltage (V) by current (I): P = V × I. This calculator lets you find any one of the three quantities (power, voltage, or current) if the other two are known. For AC circuits whose load contains non-purely-resistive elements (such as motors and transformers), a "power factor" (PF) value is added to the equation (P = V × I × PF), because part of the current doesn't produce real, usable power; the default value of 1 suits purely resistive loads like heaters and incandescent bulbs, while motors, transformers, and other inductive loads typically have a power factor noticeably below 1, meaning their actual real power consumption is lower than a simple voltage-times-current calculation would suggest. This is why electricians and engineers distinguish between real power, reactive power, and apparent power when designing circuits for inductive loads like motors.
The relationship between watts, volts, and amps (P = V × I) is one of the first equations anyone learning basic electricity encounters, and its everyday usefulness — from understanding an appliance's power consumption to sizing a circuit breaker correctly — makes it worth understanding thoroughly rather than just memorizing.
Voltage represents the electrical 'pressure' pushing current through a circuit, while current represents the actual rate of charge flow, and power represents the rate at which electrical energy is actually being converted into other forms (heat, light, motion) by whatever device is connected. Multiplying voltage by current gives power precisely because power is fundamentally the rate of energy transfer, and voltage times current captures exactly that rate for simple resistive circuits.
This basic relationship is why every electrical device's power rating, voltage requirement, and current draw are all mathematically linked: knowing any two of the three values for a given device lets you calculate the third, which is genuinely useful for practical tasks like determining whether a device will overload a circuit, or figuring out how much current a device with a known power rating will draw from a known voltage supply.
The complication for AC circuits with motors, transformers, and similar equipment comes from power factor, a value between 0 and 1 that accounts for the fact that these inductive loads draw current that isn't perfectly in phase with voltage, meaning some of that current doesn't contribute to real, usable power output. A motor with a power factor of 0.8, for instance, actually consumes less real power than a simple voltage-times-current calculation would suggest, because 20% of the apparent power isn't converted into useful mechanical work.
This is exactly why electrical equipment nameplates for motors and similar devices often list both a power rating in watts (real power, what the device actually converts into useful work) and separately specify voltage and current ratings that, when multiplied together, would give a larger 'apparent power' figure in volt-amperes — the gap between these two numbers is a direct measure of that equipment's power factor, and understanding this distinction prevents a common calculation error when estimating a motor's real energy consumption from its voltage and current specifications alone.
Watts measure real (active) power actually consumed, while volt-amps measure apparent power (voltage × current without accounting for power factor); they're equal only when the power factor is 1, as with purely resistive loads.
Only for AC circuits with inductive or capacitive loads (like motors or fluorescent ballasts), where power factor is typically less than 1. For DC circuits or simple resistive AC loads (heaters, incandescent bulbs), leave it at 1.
Yes — use the Power Factor Calculator on this site if you know the real and apparent (or reactive) power values.