Energy cannot be created or destroyed, therefore the total energy before and after a change or transfer remains the same. This is known as the conservation of energy.

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Therefore: Total energy before = Total energy after

However, energy can also be wasted.

Wasted energy

Energy can be wasted during transfers between different energy stores.

For example, when a bulb is lit, it releases heat energy into the surroundings. This heat energy is not necessary for the bulb’s primary function, so it is considered wasted energy.

Most wasted energy ends up increasing the thermal energy of the surroundings, causing them to heat up slightly.

Sankey diagrams

Sankey diagrams are also used to show energy transfers. In these diagrams, the size of the arrows indicates the amount of energy being transferred.

The Sankey diagram above is for an electric lamp. As you can see, the total amount of energy given out by the lamp is 100 J. Out of the total 100 J, 10 J is transferred as useful light energy, while 90 J is transferred as non-useful thermal energy.

The arrow representing the transfer to thermal energy is larger because more energy is transferred in that form. However, this is considered wasted energy because lamps are primarily used to produce light, not heat.