I think it depends on what kind of ballasts. As Ziggy said, MH
ballasts get seriously hot and are pretty big. I have two PC
ballasts attached to the back of my canopy so they are pretty much out of the line of fire regarding getting wet and the small amount of heat they do put out doesn't stay in the canopy. They are a little smaller than Ziggy's VHO
ballasts and also don't weigh much.