Electrical power calculator
Calculate power or current for DC, single-phase and three-phase circuits – power factor included.
Mode
Calculate
Real power P: 2.3 kW
Apparent power S: 2.3 kVA
Reactive power Q: 0 var
How the calculator works
For DC, P = U × I. With alternating current the power factor comes into play: P = U × I × cos φ for single phase and P = √3 × U × I × cos φ for three-phase, where U is the line-to-line voltage (typically 400 V). The calculator works both ways – power from current or current from power, which helps when sizing protective devices.
Besides real power P (W) it also shows apparent power S (VA) and reactive power Q (var). A purely resistive load (heater, incandescent bulb) has cos φ = 1; motors and older luminaires typically 0.7 – 0.9.
Worked examples
| Input | Result |
|---|---|
| DC: 12 V, 5 A | P = 60 W |
| 1-phase: 230 V, 10 A, cos φ = 1 | P = 2,300 W |
| 1-phase: 230 V, 10 A, cos φ = 0.8 | P = 1,840 W; S = 2,300 VA |
| 3-phase: 400 V, 16 A, cos φ = 0.8 | P ≈ 8,868 W |
Frequently asked questions
How do I calculate current from power?
For single phase I = P / (U × cos φ), for three-phase I = P / (√3 × U × cos φ). For example, a 3 kW single-phase appliance with cos φ = 1 draws 13 A at 230 V.
What is the power factor cos φ?
The ratio of real to apparent power. It expresses what share of the drawn current performs useful work. Resistive loads have cos φ = 1, motors typically 0.7 – 0.9.
What is the difference between W and VA?
Watts express real power (actually consumed), volt-amperes apparent power (the product of voltage and current). P = S × cos φ – at cos φ = 1 they are equal.
Why is 400 V used for three-phase?
400 V is the line-to-line voltage between two phases. Between a phase and neutral the phase voltage is 230 V; the relationship is 400 = 230 × √3.
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Last updated: 16 July 2026