# Efficiency Calculator

URL: https://speedycalculator.com/calculators/physics/efficiency/
Last updated: 2026-10-30

> The efficiency calculator finds the efficiency of a machine or process from its useful energy (or power) output and the energy input, plus the energy lost.

## Inputs

- Energy (or power) input (J, kJ, MJ, kWh, cal, kcal, BTU)
- Useful energy (or power) output (J, kJ, MJ, kWh, cal, kcal, BTU)

## Results

- Efficiency
- Energy lost

## Formula

```
η = E_out / E_in × 100%,  losses = E_in − E_out
```

## Explanation

No real machine converts all its input into useful output. LED lamps are about 40% efficient at making light, electric motors 85–95%, car engines 25–40%. The rest is lost, mostly as heat.

### How to use the Efficiency Calculator

- **Energy (or power) input**
- **Useful energy (or power) output**

Results update as you type, every field has its own unit menu, and everything is calculated in your browser.

### How it works

Enter the useful output and the input in the same units.

### Tips

- Use absolute temperature (K) in gas laws and efficiency formulas.
- Temperature differences are the same in K and °C.
- Heat = mass × specific heat × temperature change when no phase change occurs.
- Phase changes need latent heat at a constant temperature.

### Common mistakes to avoid

- Using °C in pV = nRT.
- Forgetting latent heat during melting or boiling.
- Confusing heat capacity (J/K) with specific heat (J/(kg·K)).

### Inputs explained

FieldWhat to enterEnergy (or power) inputUnits: J, kJ, MJ, kWh, cal, kcal …Useful energy (or power) outputUnits: J, kJ, MJ, kWh, cal, kcal …

### What each result means

- **Efficiency** = [Useful energy (or power) output]÷[Energy (or power) input]×100
- **Energy lost** = [Energy (or power) input] - [Useful energy (or power) output]

### Example values

How the main result changes when **Energy** changes and everything else stays at the example values above:

Energy (or power) input (J)Efficiency (%)5007075046.66671,000351,250281,50023.33332,00017.5

## Worked example

Inputs — Energy (or power) input: 1,000 J; Useful energy (or power) output: 350 J.

Results — **Efficiency = 35 %**; Energy lost = 650 J.

## Key terms

- **Efficiency**: Useful output divided by input.

## FAQ

**Can efficiency exceed 100%?**
Not for energy conversion; heat pumps use COP instead.

**Where does lost energy go?**
Mostly into heat, sound and friction.

**How efficient is a petrol engine?**
About 25–35%.

**Should power or energy be used?**
Either, as long as both are in the same units.

**What formula does the Efficiency Calculator use?**
η = E_out / E_in × 100%,  losses = E_in − E_out.


## Sources

- [OpenStax – University Physics Volume 2](https://openstax.org/details/books/university-physics-volume-2)
- [HyperPhysics (Georgia State University)](http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html)