I can't tel you the number right off hand, but use an Advance electronic ballast.
Once you get into this size ballast, most any of them will easily run a 30watt tube.
For kicks, you can get a ballast designed to run four bulbs, run two red and blue wires to each bulb, doubling them up essentially, and you are doing what is called overdriving. I have done this as have many others. It is quite safe and will give you around 45-50 watts output from each bulb!
Expect to pay around 17 bucks for the ballast.
GL