A workshop winch motor takes in 800 J of electrical energy and delivers 240 J of useful work.
What is the motor's efficiency?
Write the two values down: useful energy out = 240 J, total energy in = 800 J.
efficiency = useful energy out ÷ total energy in
efficiency = 240 ÷ 800 = 0.30
As a percentage: 0.30 × 100 = 30. The motor is 30% efficient.
Sometimes a question gives the wasted output instead. The input splits into useful plus wasted, so useful energy out = total energy in - wasted energy out.
A check on every answer: a real device's efficiency lands below 100%. An answer at 100% or above means the two values swapped places.
Q: A toaster takes in 600 J, and 450 J of that ends up in the bread. Calculate the toaster's efficiency, as a percentage.
Write down the values:
useful energy out = 450 J, total energy in = 600 J, efficiency = ?
Write down the equation:
efficiency = useful energy out ÷ total energy in
Substitute in the values, and calculate:
efficiency = 450 ÷ 600 = 0.75
As a percentage: 0.75 × 100 = 75
The toaster is 75% efficient.
Q: A shop vacuum takes in 1500 J and sends 900 J out as wasted heat and sound. Calculate its efficiency, as a percentage.
Write down the values:
total energy in = 1500 J, wasted energy out = 900 J, efficiency = ?
Find the useful output first: 1500 - 900 = 600. useful energy out = 600 J.
Write down the equation:
efficiency = useful energy out ÷ total energy in
Substitute in the values, and calculate:
efficiency = 600 ÷ 1500 = 0.40
As a percentage: 0.40 × 100 = 40
The shop vacuum is 40% efficient.
A ceiling fan takes in 360 J, and 90 J of that becomes the kinetic energy of the moving air. Enter the fan's efficiency, as a percentage.
A phone charger takes in 50 J and puts 42 J into the battery. Enter the charger's efficiency, as a percentage.
A hair dryer takes in 2400 J. 720 J of that is wasted. Enter the dryer's efficiency, as a percentage.
A pottery kiln takes in 5000 J through its heating element, and 3600 J of that ends up in the clay. Enter the kiln's efficiency, as a percentage.
A gas lawn mower's engine takes in 1800 J from its fuel. 1314 J leaves through the exhaust and the hot engine parts as wasted output. Enter the mower's efficiency, as a percentage.
A 3D printer's motor takes in 150 J while moving the print head and delivers 51 J of useful work. Enter the motor's efficiency, as a percentage.