Energy is never made or destroyed, only moved between stores. Pick a device and follow its energy through a Sankey diagram, where the width of each arrow shows how much energy flows.
Energy in
Useful out
Wasted
Sankey diagrams and efficiency
The total energy going in always equals the energy coming out: that is conservation of energy. In a Sankey diagram the arrow widths add up: the useful transfer goes straight on, the wasted energy (usually heat) branches off. Efficiency = useful energy ÷ total energy in, so a more efficient device wastes less as heat.
Check yourself
A device takes in 100 J and usefully transfers 25 J. What is its efficiency?
25%. Efficiency = useful ÷ total = 25 ÷ 100 = 0.25, which is 25 per cent. The other 75 J is wasted, usually as heat.
Energy is never used up, so what does it mean to "waste" energy?
Wasted energy has been transferred to a store that is not useful, usually spread out as heat to the surroundings. It still exists, but it is now too spread out to do anything useful.
Name the energy store that fills up when you lift a book onto a shelf.
The gravitational potential store. Lifting the book higher transfers energy from your muscles into its gravitational store.