Just buy the size you want/need. I thinnnnk I'm right, perhaps some one that truly understands will pipe up and inform us, but, I have it in my mind that in the old days a speed controller used electricity. As you say, it used the excess. Now, with solid state digital controllers, they just dribble out the power needed & don't use the excess so to speak. If the fan was set to half speed, it would use about half the electricity. With either the old style resister type, or the new solid state type, at full speed, the controller uses zero power, it just lets full voltage through.