How the watts calculator works
In a DC circuit power is simply voltage times current. In AC circuits with motors or electronics, current and voltage can be out of step, and only part of the apparent power does useful work. The power factor (PF) is that share.
Real power P (W) = V × I × PF
Reactive power Q (VAR) = √(S² − P²)
Impedance Z = V ÷ I (Ohm's law)
Heaters, incandescent bulbs and DC circuits have a power factor of 1, so watts equal VA. Motors typically run at 0.8 to 0.9. Generators and UPS units are rated in VA or kVA because the wiring has to carry the full apparent current. For three-phase systems use the three-phase power calculator.
Worked example
A 120 V circuit carrying 15 A with a power factor of 0.9 draws an apparent power of 1,800 VA and real power of 1,800 × 0.9 = 1,620 W. Reactive power is √(1,800² − 1,620²) = 784.6 VAR. The same current through a resistive heater would deliver the full 1,800 W.
Frequently asked questions
How do I convert amps to watts?
Multiply amps by volts, and by the power factor for AC. 10 A at 120 V is 1,200 W for a resistive load.
What is the difference between watts and VA?
Watts are real power doing work; VA is the product of voltage and current, including reactive power.
What power factor should I use?
1 for DC and resistive loads, about 0.8 to 0.95 for motors and modern electronics with power factor correction.
How many watts can a 15 A, 120 V circuit handle?
1,800 W peak, and about 1,440 W for continuous loads under the 80% rule.
Updated October 2026. Results are estimates for education, not financial or medical advice. All math runs in your browser; nothing you type is sent anywhere.