Find Power

Imagine a garden hose: voltage is the water pressure, current is how fast water flows, and power is how much work the water can do per second. Crimp the hose (add resistance) and you reduce flow — less power delivered even at the same pressure. Open the hose wide (low resistance) and flow increases, delivering more power. Electrical circuits work the same way, and the relationship is just as predictable.

Power is the rate at which electrical energy is converted into heat, light, motion, or any other form. A 1,000 W microwave running for one hour has consumed 1.02 kWh of electricity — that is the number on your utility bill. The physical formula P = V x I connects all three quantities: volts, amps, and watts. Know any two, derive the third.

Ohm's Law (V = I x R) extends this to resistance. Combined with the power formula, you get three equivalent expressions: P = VI, P = I²R, and P = V² / R. This calculator uses whichever pair of inputs you provide and solves for the rest. The practical payoff: you can cross-check a device's nameplate rating against your actual supply voltage, or work out the resistance of a heating element from its published wattage.

Use this calculator when you are adding a new appliance and want to confirm the circuit can handle the load, when you are sizing a replacement fuse or circuit breaker, or when you want to estimate the operating cost of a device before buying it. It is also useful when reading a manufacturer spec sheet that lists only two of the three quantities (V, A, or W) and you need the third.

It is equally useful in reverse: if you know a resistor's resistance and the voltage across it, this tool tells you how much power it dissipates — important for choosing a resistor with a high enough wattage rating in electronics projects. Underrated resistors overheat and fail.

Do not use this calculator for AC circuits with significant reactive loads — induction motors, large transformers, fluorescent lighting with magnetic ballasts — without also applying a power factor correction. In those cases the product of nameplate voltage and nameplate current gives apparent power (VA), not real power (W). The difference can be 10 to 30 percent, enough to affect breaker sizing. For any installation involving new wiring, panels, or high-power equipment, a licensed electrician should verify the calculation.

Confusing VA with watts. Appliance nameplates sometimes show VA (volt-amps) rather than watts. For resistive loads these are equal. For motors and transformers they are not — a device rated 500 VA might only consume 400 W of real power if its power factor is 0.8. Using VA as watts overstates the load, which is conservative but can mislead when planning circuit capacity.

Ignoring startup current. The calculator computes steady-state power from steady-state voltage and current. Electric motors, refrigerators, and air conditioners draw a large inrush current at startup — sometimes three to seven times the running value. A motor showing a comfortable running current might pull several times that for the first half-second. Fuse and breaker sizing must account for this, not just the calculated running power.

Assuming voltage is exactly nominal. Entering 120 V for a US outlet is accurate as a nominal value, but actual outlet voltage varies between roughly 114 V and 126 V depending on load and utility conditions. If you are calculating whether a device will exceed a breaker rating, measure the actual voltage with a multimeter rather than relying on the nominal figure.

The core formula is P = V × I, where P is power in watts, V is voltage in volts, and I is current in amps. For the example of 120 V at 8.5 A, the result is 1,020 W.

When only voltage and resistance are known, Ohm's Law substitutes current: I = V / R. Plugging into the power formula gives P = V² / R. When only current and resistance are known, voltage substitutes as V = I x R, giving P = I² × R. All three forms are mathematically equivalent — choose based on which quantities you have.

Energy over time follows from power directly: Energy (Wh) = P × time (hours). Dividing by 1.02 kWh per hour is the energy the example load consumes in one hour of continuous operation. Multiply by your utility rate per kWh to get operating cost.

The formula P = VI assumes voltage and current are in phase — true for DC and pure resistive AC. In reactive circuits, voltage and current are phase-shifted; the true real power is P = VI cos(θ), where θ is the phase angle and cos(θ) is the power factor. At a power factor of 0.85 (typical for a lightly loaded motor), this calculator overstates real power by about 18 percent. The overstatement is safe for thermal design but causes errors in energy cost projections. For three-phase systems, the formula extends to P = √3 × V_line × I_line × PF — a single-phase tool gives one-third the correct answer if applied naively to a three-phase load.