Hope this helps!
Which size do I need?
We have tested the 40 watt phereanel in several 2'x4' cage designs including our own PVC and various thicknesses of plywood. Participants in this test have also included ARBREPTILES (Clay Davenport), Boamaster (Mark Venhorst) and other private breeders. We have all reached pretty much the same guidelines.
In a 2x4 cage that is 12" tall or less in a room temp of 75° F+ you will see good results with the 40 watt RHP. The taller the cage the more likely you will want to set up basking sites that are elevated and closer to the RHP to give your animals a gradient. Generally taller cages 18" or taller are set up for arboreal use so this is not a problem. If your room temp is lower than 74 F or your cage is 12"+ you may want to move up to the 80 watt.
Cages less than 2'x4' will generally work well with the 40 watt panel. Once again, height and room temp is a determining factor. We have had very good success with several Ball Python keepers in the Northern parts of the US using the 80 watt panel in our 2x3 cages. While normally a bit much for this size cage we have found this combination to work very well in the cooler room temps often found in these areas with the higher temp requirements of these snakes. Make sure you use a thermostat!
If your cage falls into the 2'x4' or 2'x6' range you will want to upgrade to the 80 watt panel. The 120 watt panel is for generally for larger cages 2'x6' or larger, open air enclosures or cool room temperatures. The 160 watt panel is intended for 6' and 8' cages. When in doubt you can move to the next higher wattage, just make sure you use that thermostat!
Where I got this from! Radiant Heat Panel FAQ: Reptile Products You Can Trust | Reptile Basics Inc