Efficiency tells us how much of the input energy a device transfers usefully. The equation is efficiency = useful output energy transfer ÷ input energy transfer. Both energies are in joules (J), so the units cancel and efficiency has no unit.
For example, a motor takes in 500 J of electrical energy and transfers 400 J usefully. Its efficiency is 400 ÷ 500 = 0.8. To give this as a percentage, multiply by 100, so 0.8 becomes 80%. In the same way, an efficiency of 0.6 is 60%.
If you are told the wasted energy instead, subtract it from the input first. A motor that takes in 500 J and wastes 100 J has a useful output of 400 J.
No real device is 100% efficient. Some energy is always dissipated to the surroundings, so the useful output energy is always less than the input energy and the efficiency is always less than 1 (100%).
