Motors & Loads calculator

Motor Efficiency Calculator

This motor efficiency calculator is an operating-point screen for three practical questions: measured efficiency from shaft output and input power, electrical input and losses from one efficiency value, and annual energy comparison for two efficiency values at the same shaft output. It is intended for quick engineering review, not as a substitute for a formal motor acceptance or IEEE 112 laboratory test.

Updated April 25, 2026

Quick Answer

At 15 kW of shaft output and 16.3 kW of electrical input, motor efficiency is about 92.0% and losses are about 1.3 kW.

η = Pout ÷ Pin × 100 | If the same 20 HP shaft load moves from 90% to 95% efficiency, input power drops by about 0.873 kW.

Choose measured efficiency, input power, or comparison mode below to screen operating-point efficiency, losses, and annual energy difference.

Calculator Inputs

Quick Presets

Calculation Results

Enter values above to see calculation results

Calculation History

Example Calculations

Measured efficiency example

15 kW shaft output and 16.3 kW electrical input.

Inputs

calculationMode: measured_efficiency
shaftOutputPower: 15
powerUnit: kw
measuredInputPower: 16.3

Comparison example

20 HP shaft output, 90% baseline efficiency, 95% comparison efficiency, 4000 hours per year.

Inputs

calculationMode: efficiency_comparison
shaftOutputPower: 20
powerUnit: hp
currentEfficiency: 90
improvedEfficiency: 95
operatingHours: 4000
electricityRate: 0.12

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.

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.

Example: if the shaft output is 15 kW and the measured electrical input is 16.3 kW, efficiency is about 15 ÷ 16.3 × 100 = 92.0%, and motor losses are about 1.3 kW.

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

Checking measured operating-point motor efficiency from shaft output and electrical input power

Estimating electrical input and losses when shaft output and efficiency are known

Comparing annual energy use for baseline and improved efficiency values at one load point

Building a quick payback screen from annual energy difference and project cost

Reviewing motor operating-point data before deeper current, power-factor, or system studies

Frequently Asked Questions

What is the basic motor efficiency formula?

Motor efficiency is shaft output power divided by electrical input power, multiplied by 100%. If shaft output is 15 kW and input power is 16.3 kW, the operating-point efficiency is about 92.0%.

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.