AC Power Calculator - Watts from Volts and Amps

AC Power and Wattage

In alternating current circuits, real power (wattage) depends on voltage, current, and power factor. Reactive loads like motors and transformers draw current out of phase with voltage, reducing real power delivered.

AC Wattage Formula

$$P = V \times I \times PF$$

where $P$ is real power in watts (W), $V$ is RMS voltage (V), $I$ is RMS current (A), and $PF$ is power factor (0 to 1). A 120 V circuit at 5 A with PF = 0.9 delivers $P = 120 \times 5 \times 0.9 = 540$ W.

Power Factor

Power factor is the cosine of the phase angle between voltage and current. Purely resistive loads (incandescent lamps, heaters) have PF ≈ 1. Inductive motors often have PF between 0.7 and 0.9 without correction capacitors.

Frequently Asked Questions

What is the difference between real and apparent power?

Real power (watts) does useful work. Apparent power (volt-amperes, VA) is $S = VI$. Real power $P = S \times PF$.

Should I use RMS values?

Yes. For AC circuits, always use RMS voltage and current. Peak values will overestimate power by a factor of 2 for sinusoidal waveforms.

What power factor should I use?

Resistive loads: PF = 1. Typical motors: 0.7–0.9. Check equipment nameplates or utility bills for actual values.

Does this work for three-phase power?

This calculator is for single-phase AC. Three-phase real power is $P = \sqrt{3} \times V_{line} \times I_{line} \times PF$.

How do I reduce my electricity bill?

Lowering current draw, improving power factor, and using efficient equipment reduce real power consumption measured in kilowatt-hours (kWh).

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AC Wattage Calculator

AC Power and Wattage

AC Wattage Formula

Power Factor

Frequently Asked Questions

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