Motors & Loads calculator
Motor Efficiency Calculator
This motor efficiency calculator is an operating-point screen for shaft output, measured input power, estimated losses, and annual comparison at the same shaft output. Enter the motor data you have, choose the matching mode, and let the calculator produce the efficiency, loss, energy, and cost values for that operating point.
Updated July 10, 2026
Enter shaft output, measured input power, efficiency values, annual hours, and rate to screen motor efficiency, input power, losses, and annual savings for the operating point.
Use measured mode for shaft output plus input power, input/loss mode for one efficiency value, or comparison mode for two efficiency values at the same shaft load.
Choose measured efficiency, input power, or comparison mode below to screen operating-point efficiency, losses, and annual energy difference.
Example Calculations
How to Use
How to use the motor efficiency calculator
The page works from the basic operating-point relationships efficiency = output power ÷ input power × 100%, input power = output power ÷ efficiency, and losses = input power - output power. In comparison mode, the calculator assumes the same shaft output in both cases.
1. Choose the screen that matches your data
- Measured Efficiency is for a known shaft output and a measured electrical input.
- Input Power And Losses is for a known shaft output and one efficiency value.
- Efficiency Comparison is for two efficiency values at the same shaft output.
2. Enter shaft output at the actual operating point
- Use the mechanical shaft output that matches the point being reviewed, not just the nameplate rating unless the motor is actually operating there.
- Enter horsepower or kilowatts as needed. The page converts between them automatically.
3. Add annual hours and utility rate when you want yearly results
- Annual operating hours convert kW into annual kWh.
- Utility rate converts annual kWh into annual cost.
- Upgrade cost is optional and is used only for a simple payback screen.
4. Read the output as an operating-point screen
- Measured efficiency reflects the entered shaft output and input power only.
- Input power and losses are based on the entered efficiency value and shaft output.
- Annual comparison assumes both efficiency cases deliver the same shaft output for the same number of hours.
Run the calculator before treating any sample motor data as a result. Sample inputs are included to show the workflow, but the useful answer depends on your shaft output, measured input power, operating hours, and utility rate.
Core equations
| Question | Relationship | Use |
|---|---|---|
| Measured efficiency | η = Pout / Pin × 100% |
Use when you know shaft output and measured electrical input. |
| Estimated input power | Pin = Pout / η |
Use when you know shaft output and one efficiency value. |
| Motor losses | Losses = Pin - Pout |
Use to screen heat and wasted input power at one operating point. |
| Annual energy difference | ΔkWh = (Pin1 - Pin2) × hours |
Use only when both cases deliver the same shaft output. |
For a measured-input review, enter the shaft output and measured electrical input above, then calculate before using the efficiency or loss values in a report. For a savings review, use comparison mode so the baseline and improved efficiency values are evaluated at the same shaft output and hours.
For current and power-factor follow-up, pair this page with the three phase motor calculator, motor power calculator, and electricity cost calculator.
Common Applications
More applications. Open to review 2 additional use cases.
Frequently Asked Questions
What is the basic motor efficiency formula?
Motor efficiency is shaft output power divided by electrical input power, multiplied by 100%. Use the calculator with your shaft output and measured input power so the result matches the operating point you are reviewing.
Why does this page ask for shaft output instead of just nameplate horsepower?
Efficiency is an operating-point relationship. A motor can run below its nameplate output, so using actual shaft output gives a more honest screen for input power, losses, and annual energy use.
Can I use the comparison mode for annual savings?
Yes, as long as both efficiency values are being compared at the same shaft output and the same annual operating hours. The page calculates the input-power difference first, then converts it into annual kWh and annual cost if you enter hours and rate.
Does this page replace a formal efficiency test?
No. It is a quick operating-point screen. Formal acceptance or laboratory testing may require measured shaft output and electrical input under the applicable published test method and test conditions.
What is the difference between losses and annual savings?
Losses are the wasted kilowatts at one operating point. Annual savings depend on how many hours the motor operates and, if you enter it, the electricity rate used to convert kWh into dollars.
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