Motor Efficiency Calculator
Meter the motor — shaft horsepower ÷ efficiency is the input the utility bills, and the losses are heat in the windings.
Motor Efficiency Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Constant efficiency across the load band
- Drive (VFD) losses not modeled
In short: A 10 hp motor at 75% load and 90% efficiency delivers 5.59275 kW of shaft work and draws 6.214167 kW to get it — 0.621417 kW of electricity becomes heat in the windings. Run 8 hours a day and the meter reads 18,145.367 kWh a year, $2,177.44 at $0.12. Move to the 93% efficiency class and the same shaft work saves $70.24 a year; motors pay for their own upgrades.
Formula
input kW = hp × 0.7457 × load% ÷ eff%; losses = input − shaft
One horsepower is 0.7457 kW of shaft. The nameplate is the OUTPUT rating; the utility meters the input, which is output divided by efficiency — and the difference is winding and friction heat. Efficiency is load-dependent (peak near 75–100% load, poor below half), so the load factor rides the arithmetic. The premium classes — 90, 93, 96% — are where motor upgrades pay their own way.
Worked Example
- Enter the nameplate horsepower and the real load factor.
- Enter the efficiency class, the running hours and your rate.
- Read the meter, the losses, and what the next class saves.
Defaults: $2,177.44 a year, $70.24 of it recoverable at the 93% class. Drive to 24 h/day (a pump that never sleeps) and the year reads $6,532.33 — the same three points of efficiency save $210.72.
Strengths & Limits Of This Model
Where this engine is strong
- Losses priced as heat AND dollars — the upgrade case, ready
- The next-class lever computed on your duty, not a brochure
Where it stops
- Real efficiency curves sag at low load; the flat model flatters
Practical Use Cases
Upgrade math
does the premium class pay
Energy audits
the meter behind the nameplate
Heat budgets
what the motor room must remove
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Motor Efficiency Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
Why does a 10 hp motor draw more than 10 hp?
Because 10 hp is the SHAFT rating — the output. The input is output divided by efficiency, and at 90% that is 11% more. The extra 0.6 kW on the default motor is heat: windings, iron, friction.
Why does the load factor matter?
Efficiency peaks near 75–100% load and falls off below half — a motor idling at 25% load can run points worse than its nameplate. A right-sized motor beats a bigger motor loafing.
What is a good efficiency for a motor?
Modern three-phase industrial motors run 85–96% by size class: small motors lower, large ones higher. The premium classes — 90, 93, 96 — are the upgrade ladder; each step is bought once and saves every running hour.
How much does the upgrade actually save?
The lever card prices the next class up on your duty: on the default motor, 90 → 93% saves $70.24 a year at 8 h/day. A continuously running pump triples it. Payback is the upgrade premium divided by that line.
Is power factor on this page?
No — efficiency and power factor are different sins: efficiency bills real kWh, power factor bills apparent kVA and demand charges. The power factor page prices the capacitor bank that fixes the other one.
What about inverter (VFD) drives?
A VFD changes the speed, not this arithmetic — at each speed the motor still has an efficiency, and the drive adds 2–3% of its own losses. Affinity laws on pumps and fans are where VFDs earn their keep.
Where do the losses go?
Almost all of it becomes heat in the motor room: winding copper loss, iron loss, friction and windage. The default motor dumps 0.62 kW — the ventilation load your mechanical designer needs, free of charge from this page.
Should I enter nameplate or measured amps?
Enter the load factor you can defend — from a clamp meter's kW or amps against nameplate. A guessed 100% load overstates both the meter and the savings; motors rarely run at nameplate.