Motor Efficiency Calculator

Motor efficiency calculator for input power vs output power (and torque to power)

Motor efficiency is the simplest reality check you can do when you have a measured electrical input and a reasonable estimate of mechanical output. If the efficiency is unexpectedly low, it usually means one of three things: the motor is lightly loaded, the input or output measurement is wrong, or the motor and drive system has a genuine loss problem. This calculator is built for the common case where you want a fast, defensible efficiency estimate without turning the page into a full electrical engineering workbook.

At its core, efficiency is just output divided by input. Output is the useful mechanical power delivered to the shaft. Input is the electrical power drawn by the motor. The result is a percentage that should always be between 0% and 100%. Alongside efficiency, power loss matters because it is the part of the input power that turns into heat, noise, and other losses rather than useful work. Knowing the loss in kW helps you interpret what is happening and whether the numbers pass a sanity check.

To use the calculator, start with the electrical input power in kW. This is usually taken from a power meter, VFD readout, or a measurement system that already accounts for power factor and phase. Then choose how you will provide mechanical output. If you already have output power in kW (from a dynamometer, test bench, rated point with known load, or a trusted estimate), enter it directly. If you do not have output power but you have torque and speed, switch the method to torque and RPM. The calculator will convert torque and rotational speed into shaft power and then compute efficiency and losses from there.

Assumptions and how to use this calculator

• Electrical input power is real power in kW (not kVA). If you only have voltage and current, you need a proper real power reading that includes power factor.
• Mechanical output power must represent shaft output at the same operating point as the input reading (same load, same speed, same time window).
• When using torque and RPM, torque is assumed to be shaft torque (not motor nameplate torque unless the motor is at that point under load).
• Efficiency is capped by physics: output cannot exceed input. If your numbers imply output above input, the measurements are inconsistent.
• The optional cost estimate is a simple annualized view and ignores demand charges, time of use pricing, and other tariff structures.

Common questions

Why does the calculator ask for input power in kW instead of voltage and current?

Because voltage and current alone are not enough to compute real power on AC systems. Power factor and phase matter, and getting those wrong produces nonsense efficiency numbers. This tool assumes you already have a real power reading in kW from a meter, VFD, or logged measurement. If you only have basic clamp meter readings, treat the result as unreliable unless you also have power factor and a proper method to calculate real power.

What is a reasonable motor efficiency range?

It depends heavily on motor size and loading. Small motors can be noticeably less efficient than larger motors, and any motor running far below its rated load will often look inefficient because fixed losses become a bigger fraction of input. If your result is extremely low, the first thing to check is whether the motor is lightly loaded or whether the output estimate is too low for the actual load.

My efficiency is over 100%. What does that mean?

It means the inputs do not describe the same operating reality. Either the input power is understated, the output power is overstated, or the torque or RPM values do not match the input measurement point. It can also happen if input power is entered as apparent power (kVA) or if a reading is taken during transient conditions. Fix the measurement method and re-check at steady state.

How accurate is the torque and RPM method for output power?

The torque and RPM conversion is mathematically straightforward, but the accuracy depends on the torque value. Measured torque from a torque sensor is the best case. Torque inferred from nameplate ratings or from indirect estimates can be wrong by a lot. If torque is estimated, treat the efficiency as a rough indicator rather than a test-grade result.

What should I do if I only know the motor nameplate kW?

Nameplate kW is a rated output at specific conditions, not your current operating output. Using it as output power will often overstate output and inflate efficiency if the motor is not fully loaded. If you do not know the actual output, use torque and RPM if you can measure torque, or use a realistic load estimate. If neither is possible, the calculator can still run, but the result will be more about your assumptions than the motor.