I have had a 4000 watt 240 volt electric heater (13648 BTU rating) in my shop for maybe 4 months now. I use it every day. My garage is 3 stalls. The doors are wood with noticeable lite outside. The ceilings are 9 feet with insualtion, walls as well are insualted.
So id say the shop is 18x35. Temps here for the past 4 days have been 20 in morning 35 in day. My shop which does share a common wall with house is about 40-45 in morning on all days during winter. If I leave it running for 1-2 hours its up to about 60 and then as day goes on I can get it up to 70. Its quiet, it has a thermostat.
It cost $99 from Northern Tool item # 173004-1812. I got free shipping when I bought it.
Im happy with it, I didnt have 600-1000 to get anything real big or even 400 at the time. My electric bill went up about 30-40 a month. Its very light, I leave it on top of my jointer and it shares the plug with the TS.
Just thought Id let you know how well a $100 heater works for me. I also from time to time leave door open into house from garage, its laundry room so some heat does come from there.