um, I say start with 50, you can always go up later, spot check temp to make sure it doesn't go above 90. Buy the bigger dome that takes more watts, just in case. You will find the snap from 30 to -30 you'll be pulling out human heating pads. Having one of these on hand for crazy weather helps allot. I have my viv in a chilly location, and the back of the viv is all wrapped up in plastic and blankets. Also I have hand warmers for emergency, if the power goes out. You can stock up on these now, they're real cheep right now.