4.4 Know and use the relationship between useful energy output, total energy input and efficiency

Efficiency = useful energy out ÷ total energy in (x 100)

Efficiency - %
Energy - J (joules)

Eg: If you have a lightbulb and it is "fed" 100J of energy, of which 10J are transferred into light energy and the rest is wasted as heat energy, what is the efficiency of the bulb?

Eff = E. useful out ÷ E. in
= 10 ÷ 100
= 0.1 x 100
= 10% efficient 

Conclusion: The poor person that bought this light bulb is going to have a very hot house/office/building and is going to have an enormous electricity bill. Also, it's a waste of money. And bad for the environment.
(don't write that [this "conclusion"] in your exam - the examiner probably won't find it funny.)