OK, listen up.
HID lamps don't work the same as most electrical appliances. While they may RUN at 600 watts or so, it's the huge initial start-up current surge to strike the arc of the bulb that causes problems.
Basically, this means you can ignore everyone trying to add up amps and watts on here. Yes, it works that way for most electrical apparatus, and it works fo trying to estimate electricity consumption No, it doesn't work that way with ballasts when trying to work out fuse ratings.
If this is a problem, there is one easy way around it. Simply have the a/c on a different timer set to come on a couple of minutes after the light.