Efficiency calculator
This efficiency calculator divides useful output by input: 42 kWh of electricity from 100 kWh of coal energy is 42 %. It also finds output or input from the efficiency, and the overall efficiency of several stages.
Example: Calculate: Efficiency from output and input · Unit: kWh · Useful output 42 · Energy input 100 → Result: Efficiency 42 %. Source: OpenStax College Physics 2e, section 7.6, Eq. 7.68: Efficiency = useful energy or work output / total energy input = W_out / E_in; Table 7.2 typical efficiencies (read 13.10.2026). Updated: .
How it is calculated
Efficiency η = useful output ÷ total input (useful energy or work out, divided by the total energy put in; OpenStax College Physics). Multiply by 100 for percent. Real machines are always below 100 %.
Examples
A power plant delivers 42 kWh of electricity from 100 kWh of fuel energy: η = 42 ÷ 100 = 42 %, and 58 kWh is lost. A household gas heater at 90 % turns 50 kWh of gas into 45 kWh of heat (5 kWh lost). To obtain 700 kWh of useful energy at 35 %, a nuclear plant needs 700 ÷ 0.35 = 2000 kWh of input.
Several stages in a row
In a chain the efficiencies multiply. A coal plant (42 %) feeding an electric motor (98 %) gives 0.42 · 0.98 = 41.16 %.
Typical efficiencies
| Device | Efficiency in % |
|---|---|
| Coal power plant | 42 |
| Nuclear power plant | 35 |
| Diesel engine | 35 |
| Gasoline engine | 30 |
| Steam engine | 17 |
| Gas heater (household) | 90 |
| Electric motor | 98 |
| Solar cell | 10 |
| Compact fluorescent light | 20 |
Values from Table 7.2 of OpenStax College Physics; actual devices differ. The theoretical ceiling for a heat engine is the Carnot efficiency; energy units are converted by the energy converter.
Frequently asked questions
How do you calculate efficiency?
Divide useful output by input and multiply by 100 for percent. Example: 42 kWh ÷ 100 kWh = 42 %.
Can efficiency be above 100 %?
No, useful energy cannot exceed the energy supplied; real devices stay below.
How do I find the overall efficiency?
Multiply the stage efficiencies as fractions: 42 % · 98 % = 41.16 %.
How do I find the input from the efficiency?
Divide the output by the efficiency: 700 kWh ÷ 0.35 = 2000 kWh.
Where does the lost energy go?
Mostly into the surroundings as heat; it equals input minus output.
Sources and legal basis
As of: · Editorial policy
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