Energy Efficiency Calculator
Calculate the efficiency of a machine or process from its useful energy output and total energy input.
How it works
efficiency = (useful energy output / total energy input) × 100%
Efficiency is the fraction of the energy you put in that comes out doing the job you wanted, expressed as a percentage. The rest is not destroyed — energy is always conserved — but is lost to surroundings as heat, sound or friction. No real machine reaches 100%, and the same formula works with power (watts) in place of energy (joules).
Worked example
A device delivering 60 J of useful output from 100 J of input is 60 ÷ 100 × 100 = 60% efficient, wasting 40 J (40%) as heat and other losses.
Frequently asked questions
Can efficiency be over 100%?
Not for energy. A result above 100% means the output figure is larger than the input, which breaks conservation of energy — so recheck the numbers. (Heat pumps quote a coefficient of performance above 1, but that compares heat moved to electricity used, not the same energy in and out.)
Can I use power instead of energy?
Yes. Efficiency is a ratio, so as long as output and input use the same units — both joules, or both watts — the percentage comes out the same.
Where does the wasted energy go?
Mostly to heat from friction and electrical resistance, plus some sound and vibration. It spreads into the surroundings and is hard to recover, which is why waste heat is the hallmark of inefficiency.
Related calculators
Educational estimate. Check the formula and units for your exact case.