The wattage of the UVB bulb varies only by length of the bulb. So when you buy a bulb you just want to make sure the bulb covers at least 2/3'rds of your tank. The UVB bulb should overlap with your basking bulb. I put my UVB behind my basking bulb. The UVB makes D3 and the heat from the basking bulb helps the body turn that into calcium. Both are very beneficial for healthy bones. Hdrydr31 linked you to some real helpful information in getting him set up properly.