Efficiency calculator
Efficiency is how much of the power going in comes out as useful work. Pick what you want to find, enter the other two, and choose watts, kilowatts or horsepower for each (1 hp = 746 W, the electrical horsepower on motor plates).
Real power (watts) in and out. 1 hp = 746 W. On an AC load, volt-amps are not watts — use the power factor to get watts first.
Keep these results
We send this one email through our mail provider and don't keep your address unless you tick the box.
We send this one email through our mail provider and don't keep your address.
How it works
- η = out ÷ in × 100efficiency, %
- in = out ÷ (η ÷ 100)input needed
- out = in × (η ÷ 100)output you get
Efficiency is a percentage like any other: the part (output) over the whole (input). Everything has to be in the same unit first, which is why the calculator converts kW and hp to watts before it divides. Losses are simply input minus output.
Worked examples
A 10 hp motor at 90 %
- Output at the shaft: 10 × 746 = 7,460 W.
- Input needed: 7,460 ÷ 0.90 = 8,289 W ≈ 8.29 kW.
- Losses: 8,289 − 7,460 = 829 W of heat the frame and fan have to shed.
A power supply
- Measured 250 W in, rated 200 W out at full load.
- Efficiency: 200 ÷ 250 × 100 = 80 %.
- Losses: 250 − 200 = 50 W — about what a small soldering iron puts out.
Questions people ask
Where does the lost power go?
Almost all of it becomes heat — in a motor’s windings, iron and bearings, in a transformer’s core and coils, in a power supply’s electronics. That is why an inefficient machine runs hot, and why the losses figure here is worth knowing: it is the heat something has to get rid of.
Is a motor’s horsepower the input or the output?
The output — the mechanical power at the shaft. The electrical power it draws is more than that, by its efficiency. A 10 hp motor at 90 % efficiency draws about 8.29 kW of real power at full load. The current that works out to also depends on the voltage, the phases and the power factor.
Can efficiency be over 100 %?
No. Output can never be more than input; if your numbers say otherwise, one of them is wrong — a nameplate figure read in the wrong unit, or an input measured as volt-amps instead of watts. The calculator flags it.
Related
- Trade math refresher — the lesson behind this tool.
- Magnetism and motors — the lesson behind this tool.
- Percent in the trade — the arithmetic behind the percentage.
- Reading a motor nameplate — where the hp and efficiency come from.
- Ohm’s law & power calculator
- Power triangle & power factor calculator
Spot something wrong on this page? Report it
Thanks — your report was sent.
That didn’t send. Please try again later.
A heads-up on tools that fit — by email, optional
Leave your email and tick the box for a short heads-up, at most every two weeks, about tools, drills and diagrams that fit what you've been working through — here and on bentright.com and fieldcalc.net. One list across the three, one click to leave, and nothing is sent until you confirm from your inbox.