Efficiency

Some devices are designed to transfer energy, e.g.

A light bulb is designed to transfer electrical energy into light energy.	The petrol engine in a car is designed to transfer the chemical energy in fuel into kinetic energy

Not all of the energy is transferred into what it is supposed to be. In the case of a bulb a lot of energy is transferred into heat energy. We can show the whole picture in an energy flow diagram.

The heat energy in this case is wasted. It will warm up the room a little bit but we may not want that. From the diagram you can see that MOST of the energy is wasted and that very little is transferred into useful light energy.

The efficiency of a device tells us the fraction of the energy which is transferred usefully.

For example,
if we supply a bulb with 100J of energy every second and it only transfers 7J of this into light energy then its efficiency is 0.07 or 7%.