The efficiency of any contraption that uses energy to
do a job is defined as
(amount of useful energy output) / (amount of energy input) .
In describing or talking about the efficiency f the machine,
it's most often expressed as a percentage.
It's important to notice that the output is taken to be the useful output.
Why is that important ?
Here's a simple example:
If you use an incandescent light bulb as a source of light, then
the useful output is the light, and its efficiency is quite low.
If you use an incandescent light bulb as a source of heat, then
the useful output is the heat, and its efficiency is quite high.
