Motor Efficiency
η = Pmech / Pelec
Quick answer
This calculator solves η = Pmech / Pelec for Efficiency. With Mechanical Output 10 W, Electrical Input 22 W, the result is 45.45 %. It solves in 3 directions, so you can enter the values you have and get the one you need.
Calculator
Formula
Description
Motor efficiency is the ratio of useful mechanical output power to electrical input power. The difference is lost as heat due to copper losses (I²R in windings), iron losses (hysteresis and eddy currents in the core), friction and windage losses, and stray losses. Efficiency varies with operating point: it is typically low at no load (where iron and friction losses dominate) and at stall (where copper losses dominate), peaking at an intermediate load. Large industrial motors achieve 90-97% efficiency, while small hobby motors may only reach 50-70%.
Variables
- η — Motor efficiency (0 to 1)
- Pmech — Mechanical output power (W)
- Pelec — Electrical input power (W)
Practical Notes
IE efficiency classes for industrial motors: IE1 (standard), IE2 (high), IE3 (premium), IE4 (super-premium). Variable speed drives improve system efficiency by matching motor speed to load requirements instead of throttling. BLDC motors are generally 10-20% more efficient than brushed DC motors at the same power level due to eliminated brush friction and optimized commutation.
Symbols and units
| Symbol | Meaning | Default unit | Accepted units |
|---|---|---|---|
| eta | Efficiency | — | —, % |
| Pmech | Mechanical Output | W | µW, mW, W, kW |
| Pelec | Electrical Input | W | µW, mW, W, kW |
Step by step
- 1. What you know: Mechanical Output 10 W, Electrical Input 22 W
- 2. The relation: η = Pmech / Pelec
- 3. Substitute: Pmech = 10 W, Pelec = 22 W
- 4. Result: Efficiency = 45.45 %
Worked examples
Minimum range
Given: Mechanical Output 10 mW, Electrical Input 22 mW
Result: Efficiency = 45.45 %
Low range
Given: Mechanical Output 100 mW, Electrical Input 0.22 W
Result: Efficiency = 45.45 %
Typical
Given: Mechanical Output 10 W, Electrical Input 22 W
Result: Efficiency = 45.45 %
High range
Given: Mechanical Output 1 kW, Electrical Input 2.2 kW
Result: Efficiency = 45.45 %
Maximum range
Given: Mechanical Output 10 kW, Electrical Input 22 kW
Result: Efficiency = 45.45 %
Solving for each variable
Solve for Efficiency
Mechanical Output 10 mW, Electrical Input 22 mW → 45.45 %
Solve for Mechanical Output
Efficiency 2.5 %, Electrical Input 22 mW → 0.55 mW
Solve for Electrical Input
Efficiency 2.5 %, Mechanical Output 10 mW → 0.4 W
How Efficiency changes with Mechanical Output
| Mechanical Output (input) | Efficiency (result) |
|---|---|
| 1 W | 4.545 % |
| 2.5 W | 11.36 % |
| 5 W | 22.73 % |
| 7.5 W | 34.09 % |
| 10 W | 45.45 % |
| 15 W | 0.6818 |
| 20 W | 0.9091 |
| 40 W | 1.818 |
| 70 W | 3.182 |
| 100 W | 4.545 |
The same result in other units
| Unit | Value |
|---|---|
| 0.4545 | |
| % | 45.45 |
Embed this calculator
Paste this into any page. It keeps working when this page updates, and credits the source.
<iframe src="https://vbatpower.com/embed/motor-efficiency/" width="100%" height="460" style="border:1px solid #1f2a37;border-radius:12px" loading="lazy" title="Motor Efficiency"></iframe>
<p><a href="https://vbatpower.com/formulas/motor-efficiency/">Motor Efficiency</a> Calculator by vbatpower.com</p>Frequently asked questions
How do you calculate Motor Efficiency?
Use η = Pmech / Pelec. With Mechanical Output 10 W, Electrical Input 22 W, Efficiency works out to 45.45 %.
What is the formula for Motor Efficiency?
The formula is η = Pmech / Pelec. Motor efficiency is the ratio of useful mechanical output power to electrical input power.
How do you find Mechanical Output when you know the other values?
Rearrange for Mechanical Output: with Efficiency 2.5 %, Electrical Input 22 mW, Mechanical Output is 0.55 mW.
How do you find Electrical Input when you know the other values?
Rearrange for Electrical Input: with Efficiency 2.5 %, Mechanical Output 10 mW, Electrical Input is 0.4 W.
What units does Motor Efficiency use?
Efficiency is shown in , %. Values are held in SI base units internally, so you can mix prefixes without converting by hand.
Related Concepts
All Motors & Mechanical formulas →Need more features?
Save calculations, import telemetry data, simulate battery discharge, and collaborate with your team.
Try the App