Efficiency tells you how much of the energy put into a device is transferred usefully. You calculate it with: efficiency = useful output energy transfer ÷ total input energy transfer. Both energies must be in joules.
The answer is a decimal between 0 and 1. To give it as a percentage, multiply by 100. For example, a motor has an input of 500 J and a useful output of 400 J. Its efficiency is 400 ÷ 500 = 0.8, which is 80%. The other 100 J is wasted.
A device can never be more than 100% efficient, because the useful output cannot be bigger than the input. If your answer is more than 1, you have probably divided the wrong way round. A lamp with an input of 200 J and 50 J of useful light has an efficiency of 50 ÷ 200 = 0.25.