What a Sankey diagram shows. The arrow on the left is the total energy put in. It splits into the useful energy, which carries straight on, and the wasted energy, which bends away. The width of each arrow is proportional to the amount of energy, so the diagram must be drawn to scale.

Conservation of energy. Energy cannot be created or destroyed, only transferred from one store to another, by heating, electrical work, radiation or mechanical work. So the widths of the arrows coming out always add up to the width of the arrow going in: total input = useful output + wasted output.

Wasted energy. Most wasted energy ends up in the thermal store of the surroundings, which warm up very slightly. It is spread out so thinly that it is very hard to use again; we say it has been dissipated.

Reducing waste. Lubrication reduces friction between moving parts. Thermal insulation reduces energy transfer by heating. Streamlining reduces air resistance. Using a device designed for the job, such as an LED instead of a filament bulb, wastes less in the first place.

Efficiency. Efficiency is the useful output energy transfer divided by the total input energy transfer. It can be written as a decimal or a percentage. No real device is 100% efficient.

The numbers. The values for each device are typical, rounded figures chosen to make the diagrams clear. Real devices vary.