I've run heaters in tents before, if you are venting the air out its a lot of electricity in the end. This run my biggest electrical impact is the dehumidifiers.
The thing about my garage is that even though it is partially insulated, the temps fluctuate a LOT depending on the outside temperature. So I have been adjusting things almost daily. A previous owner divided the garage, and the back 2/3's were made into a room that has insulation in the walls... then they started to insulate the roof but ran into problems and just stopped. So currently there is a large opening in the ceiling (of the "room" space) which I have covered with plastic... but no insulation there. Among the long list of projects, I'm going to cover that hole and insulate the ceiling and vent the roof for passive summer cooling. That should seal up the room area much better.
I have a cool tube for my 600w HID. Kind of ironically, to varying degrees I'm just pumping the warm air back in. There are two passive intakes and one is taking cool air from the room and the other is getting warmed air. I think that's why some of my plants reacted to the cold and not others -- depending on which vent they were near there could be as much as a 10º difference in air temps.
This looks pretty odd, but it actually works. The ducting on the left is from the output on the cool tube exhaust fan, the ducting on the right is the intake vent at the bottom of the tent (on one side). The closer I put them, the less ambient air mixes in and the warmer it gets which gives me some control. It's a PITA to monitor but it has kept me going for this grow.
View attachment 3887990