The best answer is not to dim, but to get a ballast that is switchable...say a 400-600-1000 or just a 600-1000= much better than dimming...just don't use a 400watt bulb on 600 or 1000
As for where the power went? it never got used...say you have a garden hose and it produces 5 gl a min unrestricted, then you put a nozzle on it to reduce flow to 2.5 gl per min...you still have the possibility of 5 but by restricting it you only get 2.5...so you reduced the amount by 1/2 and can still unristrict any time you please
As for where the power went? it never got used...say you have a garden hose and it produces 5 gl a min unrestricted, then you put a nozzle on it to reduce flow to 2.5 gl per min...you still have the possibility of 5 but by restricting it you only get 2.5...so you reduced the amount by 1/2 and can still unristrict any time you please