Efficiency Calculator
Work out efficiency as output divided by input, times 100. Enter the useful output and the total input in the same unit — watts, horsepower, joules, BTU, or kWh — and get the efficiency percentage and the amount lost.
Example: with Useful output 750 · Total input 1000 · Unit (for the loss figure) watts (W) → Efficiency: 75.00%.
- Lost to waste250 W lost
- RatingGood
Computed by the calculator below using its default values. Change any input to see your own numbers.
Efficiency is the fraction of what you put in that comes out useful: output ÷ input × 100. The rest leaves as waste heat, friction, or noise.
What efficiency really measures
Efficiency is the simplest performance ratio there is: the useful output divided by the total input, expressed as a percentage. Whatever does not come out as useful work or energy is lost — usually as heat from friction and electrical resistance, sometimes as sound or vibration. Because output and input share the same unit, efficiency is dimensionless, which is why the same formula works for a motor in watts, an engine in horsepower, or a furnace in BTU.
The one hard rule is that ordinary efficiency cannot exceed 100%: you never get more useful energy out than you put in. Devices that seem to break that rule, like heat pumps with a coefficient of performance above 1, are moving existing heat rather than creating energy, so they are rated differently. If your numbers give more than 100%, the input is understated or the output is overstated.
How it’s calculated
Efficiency = (output / input) × 100. Loss = input − output, reported in the unit you select (the unit is a label; output and input must already share it). Works for energy or power since the ratio is dimensionless.
Assumes output and input are in the same unit and measured over the same basis (both power, or both energy over the same interval). Does not apply to heat-pump COP, which can exceed 1 by moving heat.
Typical efficiencies
| Device | Typical efficiency | Main loss |
|---|---|---|
| LED bulb | 80–90% | Heat |
| Electric motor (large) | 90–97% | Heat, friction |
| Gas furnace | 80–98% | Flue gases |
| Car gasoline engine | 20–35% | Exhaust, heat |
| Solar panel | 15–22% | Reflection, heat |
| Incandescent bulb | 2–5% | Heat |
Source: typical published ranges (DOE, manufacturer data); representative figures.
Common mistakes
- Using output and input in different units — convert both to the same unit before dividing.
- Comparing power to energy; use watts with watts, or kWh with kWh, not one of each.
- Getting over 100% and trusting it — that signals a measurement error, not a super-efficient machine.
- Confusing energy efficiency with a heat pump's COP, which is a ratio of heat moved to work in.
Frequently asked questions
What is the efficiency formula?
Efficiency = (useful output ÷ total input) × 100, with output and input in the same unit. A motor drawing 1000 W and delivering 750 W of shaft power is 75% efficient.
Can efficiency be more than 100 percent?
Not for ordinary energy conversion — you cannot get more useful energy out than you put in. A result above 100% means the input is undercounted or the output overcounted.
Why do heat pumps seem over 100 percent efficient?
They move existing heat rather than generate it, so their coefficient of performance can be 3 or 4. That is a different ratio from conversion efficiency and is not capped at 100%.
How much energy is lost?
Loss equals input minus output. At 75% efficiency, a quarter of the input is lost, usually as waste heat from friction and electrical resistance.