That would depend on what the ambient temps of the room the tank is going in, distance from lights to basking zones, etc. It is easiest just to setup the tank and then adjust accordingly depending on the lighting you have by either dimming the lights or adjusting the distance of the basking zone to get the right temps.
I find you don't need a lot of wattage unless you have a very large enclosure. In my 24"x18"x24" I only use the flourescent canopy and one 20 watt halogen puck light. That is enough to provide the right temp basking zones plus raise the overall amibient temp to the low 80s. But again each setup is different and you need to setup and test to make sure you are getting the right temps.