How it works
Real power in watts depends on voltage, current and, for AC, the power factor. For DC and purely resistive AC loads, watts are simply volts times amps.
Motors, compressors and electronic power supplies draw current that is out of step with the voltage, so the real power is smaller than volts × amps. The ratio is the power factor. Apparent power (VA or kVA) is what wiring and transformers must carry.
Three-phase circuits deliver √3 (about 1.732) times more power than a single phase at the same line-to-line voltage and line current, which is why large equipment runs on three-phase.
Formula
DC: P = V × I AC single-phase: P = V × I × PF AC three-phase: P = √3 × V(L-L) × I × PF
Questions people ask
How many amps is 1,500 watts at 120 volts?
1,500 ÷ 120 = 12.5 A for a resistive load such as a space heater. That is within a 15 A circuit, but the NEC treats heaters as continuous loads, so keep them at or below 80% (12 A) of the breaker rating.
What power factor should I use?
Use 1.0 for heaters, incandescent lights and other resistive loads. Induction motors typically run 0.8–0.9 at full load and lower when lightly loaded. Check the nameplate when you can.
Why does three-phase use √3?
In a balanced three-phase system the line-to-line voltage is √3 times the phase voltage, and total power is three times the per-phase power. Combining those gives P = √3 × V(L-L) × I × PF.
What is the difference between watts and VA?
Watts are real power that does work or makes heat. VA (volt-amperes) is apparent power, the product of RMS voltage and current. They are equal only when the power factor is 1.