Well it actually depends on your ballast efficiency....a 250w is the bulb consumption, ballasts can consume up to 50w(cheap chinese ballasts @ 70% efficiency).
My (250w)ballast has a 400w ignitor on it so it draws close to that at startup, a spike if you will, and come down to about 285w when fully lit...
Good sunpark ballasts will run at 85-90% efficiency drawing less than 10w(on a 250w) during their operation...
Just to be clear i'm talking about magnetic ballasts.......Digital/e-ballasts also consume power, and their efficiencies/lumen output are still being debated vs magnetic..