I sure hope UofODuck isn't isinuating there is such a thing as too much light for a tank.
If anyone within 100 yards is not at least temporarily blinded for an hour or more when you crack the canopy open with the halides powered up.... you need more power :D
Serously, take into consideration the ambient air temp and how much heat you can dissipate from the lighting... no sense setting up a reef if you are going to slow cook it.