Last updated: 2026-06-15

Efficiency

Efficiency answers a simple question: of all the energy you feed into a machine, how much actually does the job you wanted? The rest is lost — to friction, resistance, drag and waste heat. This calculator turns the useful output and the total input into an efficiency percentage and the amount of energy thrown away.

The formula

Efficiency is the ratio of useful output to total input, written as a percentage:

Because it is a ratio of two energies, $\eta$ has no units of its own. The energy that does not come out useful is wasted,

and the useful output can never be larger than the input — that would mean creating energy from nothing.

Worked example

A petrol engine burns fuel releasing $E_\text{in} = 1000\ \text{kJ}$ of chemical energy and delivers $E_\text{out} = 600\ \text{kJ}$ of useful work at the wheels:

with the remaining

leaving as heat through the exhaust and radiator.

Energy or power — the ratio is the same

You can divide two energies or two powers and get the same number, because dividing both by the same time interval changes nothing. A motor that draws $1000\ \text{W}$ of electrical power and produces $850\ \text{W}$ of mechanical power is $85\%$ efficient, exactly as if you compared the energies over an hour.

Why nothing reaches 100%

Friction, electrical resistance, air drag and sound all bleed energy away, so a real machine always returns less than it takes in. Heat engines face a stricter limit still: the second law of thermodynamics caps their efficiency well below 100%, set by the temperatures they work between. Improving efficiency is therefore about cutting losses — better lubrication, insulation, aerodynamics — never about beating conservation of energy.