Motor Power and Efficiency Calculator
Works out what a motor actually draws from the supply, given the shaft power on its plate, its efficiency and its power factor.
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The formula
P_input = P_shaft ÷ efficiency I = P_input ÷ (√3 × V × PF) for three-phase I = P_input ÷ (V × PF) for single-phase 1 hp = 745.7 W
The rating plate on a motor states its mechanical output, so the electrical input is always larger — efficiency is the ratio between them, and the difference becomes heat. Power factor then sets how much current is needed to deliver that input power.
What to watch out for
Three things the rating plate does not tell you directly:
- The plate figure is output, not input. A 5.5 kW motor at 89% efficiency draws 6.2 kW from the supply. Sizing a supply on the plate figure underestimates it.
- Starting current is far higher. Direct-on-line starting draws six to eight times the running current for a few seconds. Protection has to tolerate that without tripping.
- Efficiency and power factor both fall at part load. A motor running at a quarter of its rating is markedly worse on both counts, which is why oversizing a motor wastes energy rather than saving it.
Frequently Asked Questions
How do I calculate motor current?
Divide the electrical input power by √3 × voltage × power factor for three-phase, or by voltage × power factor for single-phase. A 5.5 kW, 89% efficient motor on 400 V at 0.86 power factor draws about 10.4 A.
Is the motor rating plate input or output power?
Output — the mechanical power at the shaft. The electrical input is that divided by the efficiency, so it is always higher.
How many kW is 1 hp?
About 0.746 kW, so a 5.5 kW motor is roughly 7.4 hp. Metric horsepower (PS) is slightly different at 0.7355 kW.