I use a 220V-5000 watt electric heater in my 18X20 garage and it keeps it shirtsleeve warm even at -30C, and the roof isn't insulated yet. I also have a cheap ceiling fan to move the air around, keeps the heat near the floor where you need it.
“She may not look like much, but she's got it where it counts, kid. I've made a lot of special modifications myself.”
— Han Solo