No device transfers all of its input energy usefully, so we measure how good it is using its efficiency. Efficiency compares the useful energy you get out with the total energy you put in:
efficiency = useful output energy transfer ÷ total input energy transfer
Efficiency can also be found from power:
efficiency = useful power output ÷ total power input
Efficiency has no unit. It is given as a decimal or as a percentage. To change a decimal to a percentage, multiply by 100. For example, a lamp takes in 100 J of electrical energy and transfers 20 J as light. The efficiency is 20 ÷ 100 = 0.20, which is 20%. The other 80 J is wasted, mainly by heating the surroundings. A motor that takes in 500 J and transfers 400 J usefully has an efficiency of 400 ÷ 500 = 0.8, which is 80%.
Because some energy is always dissipated, the useful output is always less than the input, so efficiency is always less than 1 (or 100%). You can increase the efficiency of an energy transfer by reducing the waste, for example by lubricating moving parts to reduce friction, or by using thermal insulation to reduce energy lost by heating.