Calculator KB-10
Current power calculator
How many watts does an appliance draw if its voltage and current are known?
Result
In brief: Calculate power from voltage and current: active, apparent, and reactive power; power factor; DC, single-phase, and three-phase circuits. Example: Active power P: 1035 W.
Calculation example
| Circuit | AC, single phase |
|---|---|
| Voltage | 230 V |
| Current | 5 A |
| Power factor cos φ | 0.9 |
| Active power P | 1035 W |
|---|---|
| Apparent power S | 1150 VA |
| Reactive power Q | 501.3 var |
| Phase angle φ | 25.8° |
| Daily energy consumption | 24.84 kWh |
How it is calculated
S = U · I (3 phases: S = √3 · U · I)
Q = S · sin φ
In AC circuits, some energy flows back and forth without doing any work. Active power goes into useful work and heat, while apparent power is used to size wires, circuit breakers, and inverters.
What to look for
- For three-phase circuits, enter the line voltage between phases, usually 400 V.
- Choose an inverter or UPS based on apparent power in VA, not the load’s wattage.
Frequently asked questions
What’s the difference between active and apparent power?
Active power P, measured in watts, does useful work and produces heat. Apparent power S, measured in volt-amperes, shows the load on the mains: P = S · cos φ. At 230 V, 5 A, and cos φ 0.9, apparent power is 1150 VA and active power is 1035 W.
How do I calculate the power of a three-phase load?
P = √3 · U · I · cos φ, where U is the line voltage. At 400 V, 10 A, and cos φ 0.85, the result is about 5.9 kW.
How much current does a 2 kW appliance draw from a 230 V outlet?
A heater with cos φ of 1 draws about 8.7 A: I = P / U. A motor draws more current because its cos φ is less than 1.
