Energy Efficiency
All the energy transfers (useful and wasted) that are
associated with a device can be represented by a Sankey
diagram.
A Sankey diagram uses arrows to represent all the output
energies.
The thickness of each arrow shows the amount of energy
The formula for energy efficiency is:
Useful energy is measured in joules (J).
Total energy is measured in joules (J).
Energy efficiency does not have any units.
It is a number between 0 and 1 which can be converted into a percentage by
multiplying by 100.
Efficiency of a lightbulb
This filament bulb is supplied with 100 J of electrical energy, which it converts to 45 J of light
energy.
1. How much energy is wasted?
2. In what form is the energy wasted?
3. In what form is the energy wasted?
Answers
1. Wasted energy= Total – Useful
= 100 J – 45 J
= 55 J
2. Heat
3. Efficiency = Useful/ Total = 45 J/ 100 J =0.45 or 45%
All the energy transfers (useful and wasted) that are
associated with a device can be represented by a Sankey
diagram.
A Sankey diagram uses arrows to represent all the output
energies.
The thickness of each arrow shows the amount of energy
The formula for energy efficiency is:
Useful energy is measured in joules (J).
Total energy is measured in joules (J).
Energy efficiency does not have any units.
It is a number between 0 and 1 which can be converted into a percentage by
multiplying by 100.
Efficiency of a lightbulb
This filament bulb is supplied with 100 J of electrical energy, which it converts to 45 J of light
energy.
1. How much energy is wasted?
2. In what form is the energy wasted?
3. In what form is the energy wasted?
Answers
1. Wasted energy= Total – Useful
= 100 J – 45 J
= 55 J
2. Heat
3. Efficiency = Useful/ Total = 45 J/ 100 J =0.45 or 45%
