Electrical basics
Motor input power calculator
Enter the shaft (output) power and the efficiency of a motor to see the approximate electrical input power and power loss. You can carry the result over to the three-phase current calculator.
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How is it calculated?
The rated power on a motor nameplate is usually the mechanical power at the shaft. The electrical power drawn from the grid is that power divided by the efficiency.
Input power
P_in = P_out / η
P_out: shaft power (kW), η: efficiency (0–1], P_in: electrical input power (kW).
Losses
P_loss = P_in − P_out
Losses turn into heat (kW).
Example
- Inputs: P_out = 7.5 kW, η = 0.9 (example efficiency; check the nameplate).
- Input power: 7.5 / 0.9 ≈ 8.33 kW; losses ≈ 0.83 kW.
The example inputs are for illustration only; they are not verified defaults. Results are approximate and do not replace engineering advice.
Frequently asked questions
Where do I find the efficiency?
On the motor nameplate or in the manufacturer’s catalogue. Efficiency changes with load and is typically lower at part load.
How do I get the motor current?
Enter the input power, voltage and the motor’s power factor in the three-phase current calculator. The link in the result card carries the input power over.
Is the starting current included?
No. This gives the approximate running power; the current at start-up can be much higher.