For my 3' by 3' tent I was using a 100 watt closet case. It wasn't enough so I finished the grow with a SunSystem 315 CMH. But in the Summer it was a battle to keep the temperatures down. So I upgraded my closet case to 140 watts just for hot spells.
Eventually I said "F" it. And am now using a Migro 400 which is the most efficient and best of all. In the winter I have to switch to the CMH to keep the temperatures up. With the Migro you might be just using 100 watts when the plants are young. Not enough to keep it warm enough for proper growth..