Of what, the UVB bulb? Each size comes with its own specific wattage, so that will depend on the length you get. I like my UVB to cover the span of my cage, so a 24" light is perfect for the 24" wide cages, for example, and the 24" light has a certain wattage that it comes in. I know they come in T8 and T5 style tubes, the T5s are more efficient. I get whichever bulb fits in the fixtures I have, typically a T8 right now.