There are many threads on this topic-but most will suggest 5.0. I recommend usind 10.0's on dark meshed Reptariums/Flexariums or on very large cages. You DO NOT want small baby chams hanging on top of the cage directly under a 10.0-so even on Reptariums if there are small baby chams or juvies that hang from the roof I raise the bulb about 3-5" off the top.