What does efficiency mean?
Efficiency measures how much of the energy put into a device ends up where you want it. At GCSE it is efficiency = useful output energy transfer ÷ total input energy transfer, or, using power, efficiency = useful power output ÷ total power input. The result is a decimal between 0 and 1; multiply by 100 to give a percentage. The Greek letter η (eta) is often used as the symbol.
No real device is 100% efficient. The energy that is not transferred usefully is wasted: it is dissipated, usually by heating the device and its surroundings, sometimes as sound. Energy is still conserved, since useful energy plus wasted energy always equals the total input, so an efficiency above 100% means there is a mistake in the values.
A real-life picture: a filament lamp transfers only a small fraction of its input energy as light and the rest heats the room, while an LED lamp gives the same light with far less wasted energy. Lubricating moving parts to reduce friction and adding thermal insulation are common ways to improve efficiency. A Sankey diagram shows the split, with the widths of the arrows drawn to scale.