Although we can't destroy eneryg we can **waste** it, by transferring it into non-useful forms. The less energy wasted, the more efficient that machine is.

We use Sankey diagrams (or Energy Transfer diagrams) to show the useful and wasted energy (example below).

** Efficiency**** (energy) **

Efficiency = useful output energy x 100

total input energy

Efficiency is either expressed as a percentage (less than 100%) or left as a number (less than 1). Usefull energy input isn't usually equal to total eneergy output, as no device is 100% efficient. Wasted energy is usually spread out as heat. Electric heaters are the one exception to this as all the energy is converted to useful heat.

To use the formula you need to find how much energy is supplied to the machine and how much useful energy the machine delivers. Then divide the smaller one by the bigger one.

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