Sankey diagrams show what happens to the input energy and in what proportion is transferred into various forms. This is a Sankey of a filament lamp
Note: Downwards arrow is the wasted energy.
So if we take into account that energy efficiency = total useful output / total useful input,
- Light is the useful energy (75 J)
- Efficiency = ( 75 / 100 ) x 100
- Efficiency = 75%