Sankey diagrams summarise all the energy transfers taking place in a process.
The thicker the line or arrow, the greater the amount of energy involved.
This Sankey diagram for an electric filament lamp shows that most of the electrical energy is transferred as heat energy rather than light energy.
The Principle of Conservation of Energy
In the above Sankey diagram, note that 100 J of electrical energy is supplied to the lamp.
Of this, 10 J is transferred to the surroundings as useful light energy.
The remainder, 90 J (100 J – 10 J) is transferred to the surroundings as wasted heat energy.
The energy transfer to light energy is the useful transfer.
The rest is ‘wasted’.
It is eventually transferred to the surroundings, making them warmer.
This ‘wasted’ energy eventually becomes so spread out that it becomes very difficult to do anything useful with it.