Hey onlyname.

For the efficiency, does this mean that 100% effeciency uses only the needed power, but 50% efficiency requires twice as much power to actually do what's meant to do?

If this is the case, then it would make sense to take into account efficiency is to take the per watt rating and divide by the efficiency where this is a number from 0 to 1 inclusive. If 100%, then nothing changes: if its 50% you double the figure, 25% you quadruple and 0% then you can't supply enough to get the job done.

To convert a percentage to a fraction simply divide by 100.