Circuit Power Calculator
AC Power Splits Into Three Parts
In an AC circuit with any inductive or capacitive load — a motor, a transformer, a fluorescent ballast — voltage and current fall slightly out of step with each other, and that phase difference means not all the power delivered actually does useful work. Utilities and electricians describe this with three related quantities: apparent, real, and reactive power. This calculator computes all three from voltage, current, and power factor.
The Formulas
Real Power P = S × PF
Reactive Power Q = √(S² − P²)
Apparent power (measured in volt-amps) is what the voltage and current alone suggest. Real power (in watts) is what's actually converted to work, heat, or light. Reactive power (in VARs) is the portion that sloshes back and forth without doing useful work, driven by the phase offset that power factor quantifies.
A Worked Example
A 240 V circuit drawing 15 A at a 0.9 power factor:
| Quantity | Value |
|---|---|
| Apparent Power (S) | 3600.0 VA |
| Real Power (P) | 3240.0 W |
| Reactive Power (Q) | 1569.2 VAR |
Typical Power Factors
| Load Type | Typical Power Factor |
|---|---|
| Resistive heating elements, incandescent lighting | ~1.0 |
| Modern electronics with power factor correction | 0.95–0.99 |
| Fully loaded induction motor | 0.8–0.9 |
| Lightly loaded induction motor | 0.5–0.7 |
| Fluorescent lighting with magnetic ballast | 0.5–0.6 |
A power factor of 1.0 means voltage and current are perfectly in phase, so real power equals apparent power with no reactive component.
Where This Calculation Matters
- Utility billing — some commercial and industrial rate structures include a power factor penalty when it drops too low.
- Generator and UPS sizing — these are rated in VA (apparent power), which can be significantly higher than the real power (watts) a load actually consumes.
- Motor and equipment specification — matching real power delivered against a motor's rated output.
- Power factor correction projects — quantifying reactive power is the first step in sizing correction capacitors.
How to Use This Calculator
- Enter the circuit Voltage.
- Enter the circuit Current.
- Optionally enter a Power Factor between 0 and 1 (defaults to 1 if left blank).
- Select Calculate to see apparent, real, and reactive power.
Related Calculations
For simple DC or purely resistive loads, the Wattage Calculator covers the same ground without the power factor step, and the Transformer Rating Calculator uses apparent power (VA) directly.