Electrical Power Calculator
Electrical power (P) is the rate at which electrical energy is transferred or consumed, measured in watts (W) or kilowatts (kW). The fundamental formula is P = V × I, where V is voltage (volts) and I is current (amperes). Combined with Ohm's Law (V = I × R), this gives a family of formulas for any combination of P, V, I, and R. For AC circuits, true power also depends on the power factor (cos φ) — the ratio of real power to apparent power — because inductors and capacitors cause voltage and current to be out of phase. Three-phase systems multiply power by √3 for line voltage calculations.
input Known Values
Enter any two values — the rest will be calculated.
bolt Results
Real Power (P)
1,200 W
1.200 kW
Apparent Power (S)
1,200 VA
Voltage (V)
120 V
Current (A)
10 A
Resistance (Ω)
12 Ω
Power Triangle
lightbulb Tips
- •P = V × I (DC) or V × I × PF (AC)
- •Three-phase power = √3 × V × I × PF ≈ 1.732×
- •Size breakers at 125% of continuous load (NEC)
- •kW = kVA × Power Factor
How to Use This Calculator
Select Circuit Type
Choose DC, AC Single-Phase, or AC Three-Phase to match your circuit. For AC circuits, a power factor field will appear to account for reactive loads.
Choose What to Solve For
Select whether you want to calculate Power (W), Voltage (V), Current (A), or Resistance (Ω). The input fields will update to ask for the two values you already know.
Enter Known Values
Type in the known electrical values such as voltage and current. For AC circuits, enter the power factor (0–1) to get accurate real power results.
Read Your Results
Instant results show real power (W), apparent power (VA), reactive power (VAR), and the solved unknown. Use these values to size wiring, breakers, or generators correctly.
The Formula
In DC circuits, P = V × I is exact. In AC circuits, voltage and current may be out of phase by angle φ, so real power P = V × I × cos(φ) where cos(φ) is the power factor. Apparent power (S, in VA) is the product V × I regardless of phase. Reactive power (Q, in VAR) represents energy stored and released by inductors/capacitors. These three form the power triangle: S² = P² + Q².
P = V × I = I² × R = V² / R | AC: P = V × I × PF | 3φ: P = √3 × V_L × I_L × PF
lightbulb Variables Explained
- P Real power in watts (W) or kilowatts (kW)
- V Voltage in volts (V)
- I Current in amperes (A)
- R Resistance in ohms (Ω)
- PF Power factor (0–1) — ratio of real power to apparent power (AC only)
- S Apparent power in volt-amperes (VA) = V × I
- Q Reactive power in VAR = √(S² − P²)
tips_and_updates Pro Tips
A power factor below 0.9 on industrial equipment often incurs utility surcharges — consider power factor correction capacitors.
For safety, always size circuit breakers at 125% of the continuous load (NEC 210.20).
Three-phase systems carry √3 ≈ 1.732 times more power than single-phase at the same voltage and current.
kW × 1000 = W; kVA × 1000 = VA — watch units when mixing large appliances.
Efficiency = Output Power ÷ Input Power × 100%. A 90% efficient motor drawing 1000 W delivers only 900 W of mechanical work.
Frequently Asked Questions
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Data sourced from trusted institutions
All formulas verified against official standards.