Calculate useful-output percentages and compare devices on equivalent energy inputs.
Method and worked examples
Efficiency describes the fraction of an energy input transferred usefully for a specified purpose. Efficiency = useful energy output / total energy input ×100%. Use matching units, such as joules for both quantities. If total input is 500J and useful output is 200J, efficiency is 40%. To find useful output from a percentage, convert the percentage to a decimal and multiply by the input.
A more efficient device transfers a larger fraction usefully; this does not automatically mean its total energy use is lower if its size or operating time differs. Compare equivalent tasks or stated inputs. In an account containing all energy sources, useful output cannot exceed total input. Improving insulation or reducing friction can increase the useful fraction without creating energy.
Example 1
A motor receives 800J and transfers 600J usefully. Find its efficiency.
Efficiency =800600×100=75%.
Example 2
A device is 30% efficient and receives 200J. Find useful and other output energy with no storage change.
Useful =0.30×200=60J. Other outputs =200−60=140J.
Watch out:Efficiency is a fraction. Total consumption also depends on input power, task size and operating time.
Calculating and comparing efficiency | Year 9 Science | Ascent Arcade