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
| Field | What to enter |
|---|---|
| Energy (or power) input | Units: J, kJ, MJ, kWh, cal, kcal … |
| Useful energy (or power) output | Units: 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 (%) |
|---|---|
| 500 | 70 |
| 750 | 46.6667 |
| 1,000 | 35 |
| 1,250 | 28 |
| 1,500 | 23.3333 |
| 2,000 | 17.5 |
Formula
η = E_out / E_in × 100%, losses = E_in − E_outWorked 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.
Frequently asked questions
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.