One thing to factor in is how long it will take to effectivly filter all your water. We have two great articles about UV
units in our articles area. The effectivness of the 8W unit will be partially calculated by the speed of the water flowing thru it as well as the bulbs age.
As far as running it when you first setup the tank there is no need until the fish have been introduced. Once fish are introduced then you should run it on a regular schedule if your going to use one at all.