AC Power Calculator
Real, apparent and reactive power for single-phase or balanced three-phase AC from volts, amps and power factor, or the current a load of known watts draws.
Result
Real power: 960 W
Arithmetic for sinusoidal supplies only. It sizes no wire, breaker or equipment; that is for an electrician and the local code.
- Current
- 10 A
- Apparent power, VA
- 1,200
- Reactive power, var
- 720
- Phase angle
- 36.8699 °
https://calcgallery.com/electronics/ac-power-calculator/
Results come from the published equations applied to the values you enter. They are not electrical design, conductor sizing, laboratory safety or medical guidance. Weather indices describe a standard person in standard conditions and are not a forecast. Where codes apply, local rules vary and must be checked. Read the full disclaimer
How to use the AC Power Calculator
- Choose single phase or balanced three phase, and whether you know the current or the real power in watts.
- Enter the voltage, the current or the power, and the power factor from the equipment's nameplate or a meter.
- Read the real, apparent and reactive power, the current, and the phase angle.
The formulas
single phase: S = V × Ithree phase: S = √3 × V(line to line) × I(line)real power P = S × PFreactive power Q = √(S² − P²)phase angle φ = arccos(PF)current from power: I = P ÷ (V × PF), or P ÷ (√3 × V × PF) for three phaseVoltage and current are RMS values, the figures a meter and a nameplate give.
Limits
These relationships hold for sinusoidal voltage and current and, for three phase, a balanced load. Electronics that draw current in pulses add harmonics, for which apparent power and power factor need the fuller definitions of IEEE 1459, and an unbalanced three-phase load needs each phase worked separately. The reactive power is shown as a size only: whether the load draws it leading or lagging is not asked.
With your numbers
- S = V × I = 120 V × 10 A = 1,200 VA
- P = S × PF = 1,200 × 0.8 = 960 W
- Q = √(S² − P²) = 720 var
Quick reference
Arithmetic for sinusoidal supplies only. It sizes no wire, breaker or equipment; that is for an electrician and the local code.
| Example | Result |
|---|---|
| 120 V, 10 A, power factor 0.8 | Real power: 960 W |
| 230 V, 5 A, power factor 1 | Real power: 1,150 W |
| Three phase 480 V, 50 A, 0.9 | Real power: 37,412 W |
| Current for 1,500 W at 120 V | Current: 12.5 A |
| Current for 2 kW at 230 V, 0.8 | Current: 10.87 A |
| Three phase 10 kW at 400 V, 0.85 | Current: 16.98 A |
Common questions about the AC Power Calculator
- What is the difference between watts and volt-amperes?
- Volt-amperes, VA, are volts times amps: the apparent power the supply has to carry. Watts are the real power the load turns into heat, light or work. They are equal only when current and voltage rise and fall together. A motor at 120 V and 10 A draws 1,200 VA, but at a power factor of 0.8 only 960 W of that is real power.
- What is power factor?
- The share of the apparent power that is real power, from 0 to 1. For a sinusoidal supply it is the cosine of the phase angle between voltage and current. Heaters and incandescent lamps are close to 1. Motors and some electronics are lower, because part of the current flows back and forth without delivering energy: the reactive power, measured in var.
- How is three phase different?
- A balanced three-phase load draws the same current in each of three lines. With V as the voltage between two lines, the total apparent power is √3 × V × I rather than V × I. At 480 V and 50 A that is about 41,570 VA, and at a power factor of 0.9 about 37,410 W.
- How many amps does a load draw?
- Divide its real power by the voltage and the power factor, and by √3 as well for three phase. A 1,500 W heater at 120 V and a power factor of 1 draws 12.5 A. This is the running current of the load itself; the page does not size wires, breakers or circuits, which depend on the local electrical code.