# How to calculate my motor input power

## Main Question or Discussion Point

Hello,
I have a question about calculating input power.

Does the following formula work?

Shaft Power (Supply) / Motor Efficiency? We are trying to calculate what an existing fan is using but do not have assess to site. We have the shaft power and motor efficiency from the manufacturers data sheets.
Fan shaft power = 3.46KW
Motor Efficiency = 87%
Rated Power 7.5KW

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russ_watters
Mentor
Does the following formula work?

Shaft Power (Supply) / Motor Efficiency?
Yes, unless there is an additional transmission loss from pulleys and a belt. Maybe add another 5-10%.

cjl
Note that just because it is rated at a particular level doesn't mean that's what it is actually drawing. Efficiency at 3.46kW is unlikely to be the same as at 7.5kW, and the fan won't even necessarily draw 3.46 all the time - it'll depend on airflow, backpressure, altitude, temperature, etc. You won't be able to get a very good estimate without actually measuring, or at least having much more data than you've provided here.

CWatters