The formula for efficiency is: efficiency = (useful energy transferred / energy supplied) x 100. It is important to note that the efficiency of any machine or device will always be less than 100 percent.
When calculating efficiency, the simple rules of energy should be kept in mind. For example, energy cannot be created or destroyed. Energy can only be transferred or moved. Energy, however, can be wasted, which is why calculating efficiency is important. Examples of wasted energy include running an air conditioner with the windows open, or a non-energy saving lamp that transfers more heat energy than light energy.