How is 50% efficiency five times worse than 90%? Isn't that a factor of roughly two? 90% efficiency: spend 100 joules to get 90 useful joules, wasting 10. 50%

How is 50% efficiency five times worse than 90%? Isn't that a factor of roughly two? 90% efficiency: spend 100 joules to get 90 useful joules, wasting 10. 50% efficiency: spend 100 joules to get 50 useful joules, wasting 50. You could say that 50% efficiency is five times more wasteful than 90%, but that doesn't strike me as a useful metric. More useful is to observe that spending 100 units of energy at 90% efficiency gets you roughly twice as much usable energy compared to 50% efficiency (9/5 = 1.8 times, to be precise).