T8 UVB bulbs are the standard for many reptile set ups, and come in a choice of UV levels to ensure you have the right bulb for your reptile. In order to get the right size bulb for your set up, firstly choose the UV level you want, then work out the correct length. For many set ups, such as Bearded Dragons, the UV bulb should be around two-thirds of the length of the vivarium. Once you have the right length, you will know the wattage, so you just choose the correct wattage in the controller to power this. A reflector is often recommended as well, as this will boost the UV level massively, improving the output.