Damn. Not unexpected but heat loss on my garage is close to 6kW for a 72 degree F differential. That is just to maintain, more heat would be needed to increase temperature. With a 40 F differential, 3.2 kW is required to maintain. Oh well, I'm happy just heating while I am out there. Most of the time, the warmish breeze from the heater will be enough to make it bearable.Most people don't heat the garage unless they are working inside it. That means you want a heater that can raise the temperature quickly, radiator's don't work like that.
If you turned the heater off altogether I'll bet your garage would stay above zero this time of year. In January when the ground under and around your garage has lost it's summer heat, a 1500watt rad won't raise the temp more than a few degrees.
Electric construction heaters can deliver up to 4x the heat when called upon, this not only raises the room temperature faster, it will raise it higher, circulate it better, and hold it steadier when doors are opened and closed.
A well insulated 20x20x8 garage need about 4KW of heating available. If you want to properly size you heater, try this link. Use the sq' of your garage door, and slab on grade. Heat Loss Calculator