Efficiency Calculator
Find energy efficiency as a percentage.
Results update as you type.
How it works
Efficiency tells you how good a machine or process is at turning the energy it consumes into useful output, and this efficiency calculator expresses that as a clean percentage from the output and input energies you enter. A perfectly efficient device would convert every joule of input into useful work, scoring 100 percent, but in the real world some energy always escapes as heat, friction, sound, or vibration, so practical efficiencies are always lower. This figure matters because it drives cost, performance, and sustainability. A more efficient engine burns less fuel for the same output, a better power supply wastes less electricity, and an efficient appliance saves money and reduces emissions over its lifetime. Comparing efficiencies lets engineers, buyers, and students judge competing designs objectively rather than by guesswork, and an efficiency calculator makes that comparison instant. The formula works because energy is conserved, so useful output divided by total input reveals exactly the share of energy that did its intended job. The remainder is the loss. Whether you are analysing a motor, a solar panel, a light bulb, or a simple machine like a pulley, an efficiency calculator turns raw energy figures into an intuitive percentage you can act on and compare across systems.
efficiency = (useful output ÷ total input) × 100%
Efficiency measures what fraction of the energy you put into a device comes back out as useful work, so dividing useful output by total input and multiplying by 100 expresses that fraction as a percentage. The formula works because energy is conserved: every joule supplied either does useful work or is lost, usually as heat, sound, or friction. The ratio therefore shows how much of the input actually served its purpose, and it can never exceed 100 percent because a machine cannot produce more useful energy than it consumes.
Source:Standard physics (conservation of energy)
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter useful output energy and total input energy.
- 2
Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
Efficiency updates instantly as you type — no button to press.
Frequently asked questions
Can efficiency ever be more than 100 percent?
No. Because energy is conserved, a device can never deliver more useful output energy than the total energy supplied to it, so efficiency is always at or below 100 percent. A result above 100 percent would mean energy was created from nothing, which is impossible. If your calculation exceeds 100 percent, recheck your figures — you have likely mislabelled the input and output or missed a hidden energy source.
What counts as useful output versus input?
Input is the total energy you supply to the device, such as the electricity, fuel, or mechanical energy fed in. Useful output is only the portion that performs the intended task, like the light from a bulb or the motion from a motor. Energy lost as waste heat, sound, or friction is not counted as useful output, which is exactly why real efficiencies fall below one hundred percent.
Why does the calculator return NaN for zero input?
Efficiency divides useful output by total input, and dividing by zero is mathematically undefined, so the calculator returns a non-numeric result to signal that the calculation cannot proceed. A device with zero input energy has no meaningful efficiency because it is not consuming anything to convert. Enter a positive input value representing the actual energy supplied to obtain a valid efficiency percentage for your system.
What is a typical efficiency for common machines?
Efficiency varies widely by technology. Electric motors often reach 85 to 95 percent, while petrol car engines convert only around 20 to 35 percent of fuel energy into motion, wasting the rest as heat. Incandescent bulbs are famously inefficient at roughly 5 percent, whereas LED lights exceed 40 percent. Comparing these figures shows why some technologies replace others on energy-saving grounds alone.
Does this work for power as well as energy?
Yes. Efficiency is a ratio, so you can divide useful output power by total input power and multiply by 100 to get the same percentage, as long as both quantities use the same unit and time basis. Whether you measure in joules of energy or watts of power, the ratio is dimensionless and yields an identical efficiency figure for a steady, continuously operating device.