Motor efficiency calculator
Find what share of electrical power a motor turns into shaft power: 800 W out of 1000 W drawn is 80 % efficiency, with 200 W lost. A result above 100 % is flagged rather than hidden, because it usually points to mixed units. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Mechanical output
- 800
- Electrical input
- 1,000
- Efficiency
- 0.8 %
- Efficiency percent
- 80
- Power loss
- 200
- Loss percent
- 20
Electric motor efficiency is the share of electrical input power that a machine actually turns into useful mechanical power at the shaft, almost always written as a percentage. At a given speed and load you need two consistent power readings: the shaft output P_out from a brake, dynamometer, or rated mechanical kilowatts, and the electrical input P_in absorbed at the terminals.
How electric motor efficiency is defined from shaft and terminal power
Start from the energy balance of a rotating electrical machine: electrical power enters the stator and, on a DC or wound-rotor machine, the rotor circuit; some of that energy is dissipated as copper loss, iron loss, windage, friction, and stray load loss; and the remainder leaves as mechanical power on the shaft. Motor efficiency is defined as the useful mechanical output divided by the electrical input at the same instant or averaged over the same interval: η equals P_out over P_in.
Textbooks, catalogs, and factory test reports multiply that fraction by one hundred so a 0.92 machine is reported as ninety-two percent. This calculator implements exactly that algebraic definition. It does not reconstruct P_out from torque and angular speed, does not reconstruct P_in from line voltage, current, and power factor, and does not apply IEC 60034-30 IE-class tables that also need rated power and pole count.
Those reconstructions are different search intents and deserve their own landings. Mechanical output may be zero, which yields zero efficiency and is a valid stalled or disconnected-shaft case. Output larger than input yields an efficiency above one hundred percent; the calculator does not clamp that result, because the overflow is often the first sign of mixed units such as horsepower versus watts or a swapped pair of readings.
Where motor efficiency calculations show up in real work
University labs reduce dynamometer torque-speed products and three-phase wattmeter readings to a single η number before they discuss stray-load loss or temperature correction. Plant and maintenance engineers sanity-check nameplate claims by backing out η from logged kilowatts at the motor control center and measured shaft or process power over the same interval. HVAC, pump, fan, and conveyor designers compare candidate motors by converting catalog shaft kilowatts and full-load electrical input into an efficiency so energy-cost models stay honest.
If you later need torque-and-rpm converted to watts, three-phase input as the square root of three times voltage times current times power factor, or an IE-class lookup from rated power and poles, treat those as separate search intents and landings rather than overloading this motor efficiency calculator page. Pair the percent with a power-unit converter when one reading is still in horsepower and the other is in kilowatts; this landing will not invent that conversion for you.
What you can do with it
Homework and exam answer keys
Compute η as a percentage from textbook shaft-output and electrical-input pairs so teachers and auto-graders share one ground truth.
Lab and plant performance checks
Reduce measured dynamometer shaft power and terminal kilowatts to motor efficiency, conversion loss, and a first-law flag for lab reports and dashboards.
Motor selection and energy-cost models
Compare candidate motors by converting catalog shaft power and full-load electrical input into percent efficiency before committing to a purchase.
FAQ
Does this look up IE1, IE2, or IE3 efficiency classes?
No. IEC 60034-30 class tables need rated power and pole count and are a different search intent. This tool only evaluates η from shaft output and electrical input.
What does the motor efficiency calculator cost?
The browser tool is free and runs locally.
How do I calculate the efficiency of an electric motor?
Divide the mechanical power at the shaft by the electrical power drawn at the terminals and multiply by 100. A motor delivering 800 W while drawing 1000 W runs at 80 %, and the remaining 200 W are losses.
Why does my motor show more than 100 % efficiency?
Because one reading is probably wrong. The result is not capped, so an output larger than the input stays visible and is flagged; the usual causes are horsepower mixed with watts or the two readings swapped.
What if the shaft output is zero?
The efficiency is 0 %, which is a valid result for a stalled motor or a disconnected shaft. Only an electrical input of zero is refused, because the ratio cannot be divided by nothing.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/eng/motor-efficiency \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"mechanical_output":800,"electrical_input":1000}'const res = await fetch("https://api.kit.forhosting.com/eng/motor-efficiency", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"mechanical_output": 800,
"electrical_input": 1000
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/eng/motor-efficiency",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"mechanical_output": 800,
"electrical_input": 1000
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/eng/motor-efficiency", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"mechanical_output":800,"electrical_input":1000}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"mechanical_output":800,"electrical_input":1000}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/eng/motor-efficiency", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"mechanical_output": 800,
"electrical_input": 1000
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "eng.motor_efficiency",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Limits
max_abs_power | 1000000000000000 |
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
Their ratio is η = P_out / P_in; multiplying by one hundred gives the percent used on nameplates, IEEE 112 and IEC 60034 test sheets, and plant dashboards. This motor efficiency calculator takes those two numbers, rejects a zero electrical input that would divide by zero, and returns the percent, the dimensionless fraction, conversion losses, and a first-law flag when output exceeds input.
How electric motor efficiency is defined from shaft and terminal power
Division by zero is the critical edge: if electrical_input is exactly zero or vanishes within floating-point epsilon, the ratio is undefined, so the request fails with invalid_input instead of returning Infinity or NaN.
What you send and what you get back
Classic textbook checks include eight hundred watts of shaft power from one thousand watts of electrical input for eighty percent; a ten-horsepower motor delivering 7460 W while drawing 8500 W for about 87.76 percent; and a zero-output case that correctly reports zero percent. Keep both powers in matching units so the ratio is dimensionless before the percent scale is applied. Send two finite numbers under the fields mechanical_output and electrical_input. mechanical_output is the shaft power P_out in watts or any power unit you prefer, and must be non-negative. electrical_input is P_in in the same power unit and must be strictly positive; both fields are required. Aliases output_power, p_out, p_mech, and shaft_power are accepted for mechanical_output; aliases input_power, p_in, p_elec, and electric_power are accepted for electrical_input so spreadsheets and typed forms can reuse the same contract. Missing fields, non-numeric strings, NaN, Infinity, a negative shaft power, and a zero or negative electrical input fail with invalid_input and are not charged. When electrical_input is zero the error message states that motor efficiency cannot be computed because division by zero would occur in η equals P_out over P_in, so clients can fix the energy balance rather than guessing. On success the response echoes cleaned inputs, efficiency as the dimensionless ratio, efficiency_percent as the percentage, power_loss as P_in minus P_out, loss_percent as the complementary share of input that did not reach the shaft, first_law_ok as true when output does not exceed input, a unit string of percent, the formula string, and method motor_efficiency. Pricing for successful API calls is the published base of $0.002 per item; validation failures do not consume prepaid balance.
Where motor efficiency calculations show up in real work
Secondary-school and college electrical-machines courses constantly assign problems that give shaft power and electrical input and ask for efficiency as a percent, often before they introduce equivalent-circuit models or IEEE 112 segregation of losses. Because this capability is a pure parse engine, API pricing is the published base of $0.002 per successful item and failed validation does not consume balance.
Technical questions
What formula does this motor efficiency calculator use?
η = (P_out / P_in) × 100, where P_out is mechanical_output (shaft power) and P_in is electrical_input in matching power units. The response also includes the fraction P_out/P_in, power_loss = P_in − P_out, and loss_percent.
What happens if electrical input is zero?
The request fails with invalid_input. Motor efficiency is undefined when P_in = 0 because the defining ratio would divide by zero.
Can efficiency exceed 100%?
Mathematically yes if mechanical_output is larger than electrical_input. That usually signals mixed units (hp vs W) or swapped readings; the calculator does not clamp the result and sets first_law_ok to false.