It will depend on the temperature of the outside room, and on whether you add any insulation around the tank. A 30-gal isn't really that much bigger than what you have, so it shouldn't need THAT much more wattage to heat it.
But that said... a 30 gal isn't all that much more room for your snake, either. We're you planning on this being another intermediate size, or a final adult home? If it's an intermediate size, I'd say don't bother and just get whatever you're thinking of for a forever adult home, and worry about how to heat that.