Quote Originally Posted by stinkyattic
Don't do it! You could at the very least shorten the life of your bulb and/or ballast, and at worst, end up with the bulb actually breaking.
Please don't give this sort of advice: "in theory it would work" when you don't actually have a good concept of the 'theory'. In theory, you should NEVER mismatch electrical components!
first of all, this info came from an impeccable source, Overgrow, second if you read it you will see that it says to NOT mix and match.
"Yes Ballasts and Bulbs SHOULD BE MATCHED."

"The bulb wattage MUST BE matched to the ballast. A smaller bulb will usually be fed a wattage close to what the proper bulb takes, and will generally overheat and may catastrophically fail. Any catastrophic failures may not necessarily happen quickly. A LARGER bulb WILL be UNDERpowered, and will operate at REDUCED efficiency and may have a shortened lifetime. The ballast may also overheat from prolonged operation with an oversized bulb that fails to warm."

i quoted overgrow rather than speak from experience because i don't have any with HID's (actually, i used 2 175watt MV's for my first grow ever). i just tried to help the guy.