What is the Efficiency of a machine?

Efficiency is a measure of how good a machine is. Formula: Efficiency = (Output / Input) × 100%.

If you put 100 Joules of fuel energy into a generator and it only gives you 80 Joules of electrical energy (with 20 Joules lost as noise and heat), the generator is 80% efficient. No machine is 100% efficient—friction always takes a tax!