well most ballasts pull roughly 30-50 watts... so your total power consumption should be roughly 640 watts.
a 1000w ballast is good for lamps up to 1000w.... any lamp larger than 1000w will fry the ballast because it is drawing (using) more current than the ballast can regulate....... an HID lamp has little or no electrical resistance.. which means that if u didnt have a ballast on the lamp, when you turned it on and the lamp ignited, unlimited power would flow through the lamp, making it explode... think of it like having the gas pedal stuck on a car. the ballast limits the current going through the lamp.. thats it. nothing super complicated. an hid lamp has another part besides the ballast that is called the starter... this gives a high voltage pulse to the lamp to get it hot enough to ignite... pretty simple- starter gets lamp hot, lamp starts, ballast keeps too much power from running through the lamp once its ignited.