Watts, it really depends, I use 50 watt (Red) in my vision, I was told not to go any higher, because like the post above they will burn. Then someone told me that as far as saying what watt to use it depends on alot of things. I have that 50 watt (red) going in my vision and for a 20 gallon long tank, and then in my big glass tank I use a 100 watt (Reds) on both sides and then a 50 watt with the 100 watt. I never turn them off, and in the summer I just dont use as much. Whatever size you use just put a thermometer under there and see how hot it gets. And you should be good to go. You don't want to fry them and reptiles will fry and not move just like they will freeze and not move, at least some.
I have a question about those ceramic lights that just heat up and dont really produce light, has anyone had problems with those. When I put mine in it smelled like it was burning, maybe it was just the start of the light when it was heating up.