About the Efficiency Calculator
This efficiency calculator finds how much of the energy or power going into a device comes out as something useful. Enter the input and the useful output and it returns the efficiency as a percentage plus the energy wasted (usually as heat). Switch modes to find the useful output from a known input and efficiency, or the input you need to deliver a given output.
Input and output can use different units — for example an electric motor drawing kilowatts and delivering horsepower, or a boiler burning BTU of gas and delivering kilowatt-hours of heat — and the calculator converts them before dividing. It suits physics students working on energy transfer, engineers comparing motors, pumps or generators, and anyone checking the claims on an appliance label.
Efficiency compares like with like: both sides must be power (energy per second) or both must be energy over the same period. A result above 100% means the figures are wrong or something else is supplying energy; heat pumps are described with a coefficient of performance instead.
How to use the efficiency calculator
- 1Choose whether to solve for efficiency, useful output or required input.
- 2Pick power or energy, then the unit for each side.
- 3Enter the known values (input, output and/or efficiency).
- 4Read the answer and the energy wasted.
- 5Use the pie chart to see the useful-versus-wasted split.
Formula and method
Efficiency (η) is the ratio of useful energy (or power) out of a system to the total energy (or power) put in, expressed as a percentage. Energy is conserved, so the difference between input and useful output does not disappear — it is converted into forms you did not want, usually heat through friction, electrical resistance or exhaust.
The calculator converts both quantities to SI units first (watts for power, joules for energy) so you can mix units: 1 hp = 745.70 W, 1 BTU/h = 0.29307 W, 1 kWh = 3.6 MJ, 1 BTU = 1,055.06 J and 1 kcal = 4,184 J. Rearranging the same formula gives the output from a known efficiency, or the input required to deliver a target output.
- η
- Efficiency (percent)
- E_in
- Total energy or power supplied
- E_out
- Useful energy or power delivered
Worked examples
1,000 W in, 850 W of useful output
η = 850 / 1,000 × 100 = 85%. The remaining 150 W (15%) is lost, usually as heat in the windings, bearings and air.
Electric motor: 10 kW electrical in, 12 hp shaft out
12 hp × 745.70 W/hp = 8,948.4 W. Dividing by the 10,000 W input gives an efficiency of 89.48%, with about 1.05 kW lost as heat.
Input needed for 5 kW of output at 80% efficiency
E_in = 5 / 0.80 = 6.25 kW. The extra 1.25 kW supplied is the energy that will be lost to inefficiency.
Frequently asked questions
What is the formula for efficiency?+
Efficiency = useful output ÷ total input × 100%. For example, a light that uses 10 W and emits 4 W of visible light energy is 40% efficient. Use the same units, or let the calculator convert them.
Can efficiency be more than 100%?+
No. Energy cannot be created, so a device cannot give out more useful energy than it takes in. Apparent values above 100% come from unit mistakes or, for heat pumps, from moving heat rather than creating it (described by COP).
Where does the wasted energy go?+
Almost all wasted energy ends up as heat, from friction, electrical resistance, turbulence or exhaust gases. Some is lost as sound, vibration or light. It is still conserved, just not useful.
What is the difference between power efficiency and energy efficiency?+
They give the same number when the device runs steadily, because energy is power × time. Use power when you know ratings (W, hp) and energy when you have totals over a period (kWh, BTU).
What are typical efficiencies of common machines?+
Large electric motors and transformers are typically above 90% efficient, while petrol car engines convert roughly a quarter to a third of fuel energy into work. Any heat engine is capped by the Carnot limit.