How To Calculate Efficiency

Efficiency is the fraction of the energy you put into a device that comes back out as the thing you actually wanted. A motor taking a thousand watts and delivering seven hundred and fifty watts of turning force is seventy five percent efficient, and the other quarter leaves as heat and noise.

The formula is a simple ratio: useful output divided by total input. Multiply by a hundred to express it as a percentage, or leave it as a decimal if you are going to use it in a further calculation. Both forms describe the same thing.

The key word is useful. A car engine rejects most of its energy as waste heat through the radiator and exhaust, and that energy is genuinely lost even though it still exists. Efficiency counts only the output that does the job you wanted done.

Efficiency is always below one hundred percent for a real device. The second law of thermodynamics guarantees that some energy ends up as low grade heat that cannot be recovered. A claim of more than a hundred percent means the measurement is wrong, not that someone has beaten physics.

Because it is a ratio of two similar quantities, efficiency has no units. Watts divided by watts, or joules divided by joules, cancels out. That is why the same number applies whether you measured instantaneous power or total energy over a period, provided you are consistent.

The complementary quantity is loss, which is one minus the efficiency. An eighty percent efficient device wastes twenty percent. Keeping both in view is useful because losses are what you can actually attack: better bearings, insulation, or lubrication all reduce the loss term.

Efficiency can be calculated at any point in a chain. A power station might be forty percent efficient, the transmission grid ninety five, and a motor eighty five, and the overall figure is the product of all three. Chain efficiency falls fast because the fractions multiply.

It is not always the goal. A heater is deliberately close to one hundred percent efficient because the waste heat is the product. An incandescent bulb was about two percent efficient at making light, which is terrible for lighting but perfect for keeping a henhouse warm.

Comparisons only make sense between devices doing the same job. A petrol engine at thirty percent and an electric motor at ninety percent are not really competing, because they store and deliver energy in fundamentally different ways. Always check the boundary of the measurement.

Efficiency varies with load. A motor has a sweet spot and runs inefficiently when barely loaded or overloaded. That is why quoting a single figure without the operating point is incomplete, and why variable speed drives save so much energy in practice.

Finally, energy efficiency and exergy efficiency are different measures. The first counts energy, the second counts how much of it is still available to do useful work. A heat pump can appear to exceed one hundred percent energy efficiency because it moves heat rather than creating it.