Enter three known single-phase AC values to calculate real power in watts, current, voltage, or power factor.
Real power is the power that performs useful work or becomes heat.
This is the same real-power result in kilowatts.
This calculated load current is not a complete wire, breaker, or protection-device recommendation.
Confirm the actual supply and equipment voltage ratings separately.
A valid power factor here is greater than 0 and no more than 1.
VA is voltage times current before power factor is applied.
This is apparent power expressed in kilovolt-amperes.
This is the part of the power triangle that does not appear as real watts.
This uses the single-phase AC relationship. It is not a three-phase calculation or a complete wire, breaker, protection, or electrical-code sizing method.
Table of contents
How to use our Single Phase Power Calculator
- Choose the value you need from "Calculate."
- Enter the three fields shown for that task. Use RMS voltage and RMS current from a nameplate, equipment specification, panel document, or meter.
- For "Power factor (0 to 1)," enter a documented or measured value. Do not assume 1 for a motor or electronic load.
- Select "Calculate." The requested value appears first, followed by real power, apparent power, and reactive power.
- Check the units before using the answer: W is real power, while VA is apparent power. When power factor is below 1, VA is greater than W.
Definitions
Real power (W): Power that does useful work or becomes heat. It is measured in watts.
Apparent power (VA): RMS voltage multiplied by RMS current, measured in volt-amperes. It can be greater than real power.
Power factor: Real power divided by apparent power. For this calculator, it must be greater than 0 and no more than 1.
Reactive power (VAr): The part of AC power associated with energy moving back and forth between the source and reactive parts of a load.
RMS: A way to state AC voltage or current that has the same heating effect as an equivalent DC value.
Common mistakes and quick fixes
Mistake: Entering a VA or kVA rating as "Known real power (W)."
Fix: Enter a value labeled W or kW for real power. VA and kVA are apparent-power ratings.
Mistake: Using 1 as the power factor for a motor or electronic device without checking.
Fix: Use the documented or measured power factor. Use 1 only when the load is known to be effectively resistive.
Mistake: Entering a three-phase line voltage into this single-phase calculation.
Fix: Use the RMS voltage across the single-phase load. Use a three-phase calculation for a three-phase circuit.
Mistake: Treating calculated current as a wire or breaker size.
Fix: Use it only as a load-current result. Equipment ratings, continuous-load rules, installation conditions, and local electrical code also affect sizing.
Limitations & Key Assumptions / Boundary Conditions
- This calculator uses a single-phase AC relationship with RMS values. Do not use it for a three-phase circuit.
- Nameplate values, supply specifications, and live meter readings can differ because voltage and load can change during operation.
- The calculation does not size wires, breakers, fuses, panels, generators, or inverters.
- Power factor must be greater than 0 and no more than 1. A calculated value above 1 means the entered watts, volts, or amps do not fit this model.
- Reactive power is calculated from apparent-power and real-power magnitudes. It does not show whether a load is inductive or capacitive.
Methodology
Single-phase calculation
The calculator uses RMS voltage, RMS current, and power factor to find real AC power. It rearranges the same relationship when you select current, voltage, or power factor.
P (real power in W) = V (voltage in V) x I (current in A) x PF (power factor)
Apparent power is voltage multiplied by current before power factor is applied.
S (apparent power in VA) = V (voltage in V) x I (current in A)
Reactive power is calculated only after the calculator confirms that real power does not exceed apparent power.
Q (reactive power in VAr) = sqrt(S^2 - P^2)
Reverse calculations
To find current, divide real power by voltage times power factor. To find voltage, divide real power by current times power factor. To find power factor, divide real power by apparent power. The calculator rejects zero or negative active values because they cannot be used in these divisions.
I (current in A) = P / (V x PF)
V (voltage in V) = P / (I x PF)
PF (power factor) = P / (V x I)
Worked example
With 120 V, 10 A, and power factor 0.8, apparent power is 120 x 10 = 1,200 VA. Real power is 120 x 10 x 0.8 = 960 W, or 0.960 kW. Reactive power is sqrt(1,200^2 - 960^2) = 720 VAr. The calculator converts W to kW and VA to kVA by dividing by 1,000 without rounding the stored value.