Efficiency of Energy Transfer

"Wasted" energy

Energy cannot be created or destroyed. It can only be transferred from one form to another, or moved. Energy that is "wasted", like the heat energy from an electric lamp, does not disappear. Instead, it is transferred to its surroundings and spreads out so much that it becomes difficult to do anything useful with it.

Electric lamps

Ordinary electric lamps contain a thin metal filament that glows when electricity passes through it. However, most of the electrical energy is transferred as heat rather than light energy. This is the Sankey diagram for a typical filament lamp.

Sankey diagram for a filament lamp

Modern energy-saving lamps work in a different way. They transfer a greater proportion of electrical energy as light energy. This is the Sankey diagram for a typical energy-saving lamp.

Sankey diagram for a typical energy-saving lamp

From the diagram, you can see that much less electrical energy is transferred, or "wasted", as heat energy.

Calculating efficiency

The efficiency of a device such as a lamp can be calculated using this equation:

efficiency = (useful energy transferred ÷ energy supplied) × 100

-The efficiency of the filament lamp is 10 ÷ 100 × 100 = 10%.

This means that 10% of the electrical energy supplied is transferred as light energy. 90% is transferred as heat energy.


-The efficiency of the energy-saving lamp is 75 ÷ 100 × 100 = 75%.

 This means that 75% of the electrical energy supplied is transferred as light energy. 25% is transferred as heat energy.

(Note that the efficiency of a device will always be less than 100%.)