I have a 600watt ballast but am only using a 250 watt bulb until my flowering stage when then i will move up to a 400 watt and finally a 600watt, my qustion is when im using my 250watt or 400watt bulb is my ballast only using 250watts of power or the full 600watts??
No, that is not right. The ballast is a large inductance, which will limit the current going to the lamp, but not consume much power itself. (If you know any electronics, this is why we use an inductive reactance and not a simple, and much cheaper, plain resistor.) HID lamps are weird things, they can display a NEGATIVE resistance, which is why we can´t just plug them directly into the mains supply, a ballast is needed or they will self destruct instantly. A 600W ballast certainly does not consume 600W itself, the 600W refers only to the correct power lamp that the ballast will drive.
I never knew how they worked. So it's like a power supply on a computer then? it only uses the wattage needed?