Electric Motors
Motor Efficiency Calculator
Use Motor Efficiency when motor efficiency is the quantity you need. Enter shaft output power and electrical input power, then compare the answer with the equipment or operating limits that apply to your case.
Calculate Motor efficiency
Keep every entry on the units shown below.
Values this page needs
Use nameplate data and measurements from the same load and supply condition. If operating conditions changed between readings, calculate separate cases.
A denominator entry cannot be zero. Use the displayed unit for every field and document conversions separately. For the companion round-trip efficiency calculation, open Battery Round-Trip Efficiency Calculator.
- Shaft output power
- Default example: 11 kW. Enter shaft output power in kW.
- Electrical input power
- Default example: 12.5 kW. Enter electrical input power in kW.
Understanding the equation
Here, Motor efficiency is obtained from efficiency = output ÷ input × 100. Its variables correspond to Shaft output power and Electrical input power.
The initial scenario returns 88.00%. Re-enter field data before using the answer in a design or comparison.
Calculate motor efficiency from shaft output power and electrical input power. The next worksheet may be Motor Power Factor Calculator if motor power factor is needed.
Compare two operating cases
Change Shaft output power from 11 kW to 13.2 kW and leave all other entries untouched. The displayed value changes from 88.00% to 105.60%.
The comparison demonstrates sensitivity, not a guaranteed field response.
Applying the result
Read Motor efficiency as the outcome of this equation, not as automatic equipment approval. Compare it with starting demand, running current, torque, efficiency, slip, and thermal loading.
Store the measurement state and conversion notes with the answer. The broader analysis may also use Motor Slip Calculator.
Where this estimate can fail
Compare values at the same operating point.
Additional checks may be needed for voltage imbalance, drive waveform, transient torque, and bearing load. Resolve material omissions before selecting a standard size or rating.
Record supply arrangement, speed, duty cycle, enclosure, and winding temperature. If you also need shaft torque, continue with Motor Torque Calculator.
Questions about Motor Efficiency
What should I verify beyond the displayed inputs?
Compare values at the same operating point. Also consider starting method, service factor, enclosure, and thermal duty.
Why might measured motor efficiency differ?
First confirm that the measurements match the equation. Then review voltage imbalance, drive waveform, transient torque, and bearing load.
What is the cleanest way to test another scenario?
Use separate labeled cases when more than one condition changes. Record supply arrangement, speed, duty cycle, enclosure, and winding temperature.
Is the result a code-compliant design value?
No. Compare the value with starting demand, running current, torque, efficiency, slip, and thermal loading and apply relevant equipment and installation requirements.
How much precision should I keep?
Retain the raw result for comparison and create a separately rounded reporting value if needed.
Why is zero rejected for one of the fields?
Electrical input power is used in a denominator and must be greater than zero.