There are lots of different ways of describing efficiency, but I think that what you are looking for is percent efficiency. What this measures is how much of the energy that you put into a machine you can get back out. For example, if you put 100 Joules of energy into a machine, and got 50 Joules back out (and the other 50 Joules was wasted by the machine), you would have 50% efficiency.
You can calculate percent efficiency by taking how much energy you got out, dividing it by how much you started with, and multiplying by 100%. So, if you put 10 Joules in and got 4 Joules out, you would have:
% Efficiency = (4 J) / (10 J) * 100% = 40%
So, if you put in 50 Joules and got 45 Joules back, you would have:
% Efficiency = (45 J) / (50 J) * 100% = ?
(published on 10/22/2007)