Energy Efficiency

Energy can be transferred from one form of energy to another. However, it is not always transferred to the form of energy that we want. A device is said to be efficient if it can convert a large amount of input energy into useful energy.

  • An efficient device wastes only a small amount of its input energy
  • An inefficient device wastes most of its input energy

Efficiency is calculated in terms of energy. So, to calculate the efficiency of a device, we use the formula below:

Efficiency= useful energy/total input energy

This means that the efficiency of a device is just the proportion of input energy that is transferred to useful energy output.

To calculate the percentage efficiency, the equation is:

efficiency % = useful energy/total input energy x 100

Let’s look at an example:


If a microwave has an efficiency of 0.7 and has a total power input of 600 W, work out the useful power output.

We can rearrange the equation:

Efficiency = Useful energy / Total input

To produce:

Useful energy = Efficiency × Total input

So, useful energy output = 0.7 × 600 = 420 W

Increasing Efficiency

No device is 100% efficient, which would mean that the total input energy is transferred into useful energy. Also, no device has an efficiency greater than 100%. As this would mean that energy is being created, which breaks the law of conservation of energy.

However, certain devices, especially those with moving parts, can be made more efficient using lubrication.

Lubrication is a substance (often a liquid) that reduces friction and wear between moving objects or surfaces. We can use lubrication to increase efficiency.