How big is your shop. how high are your ceilings. what are the lowest temps you've had for testing? Mine is 26 x 42 with 12 ft ceilings in Georgia.Virtually uninsulated. Low to mid twenties is about the typical lows for nights with 30's -40's for days. I think I will need 3. Opinions?
I think they put out about !7,000 to 18,000 BTUs each, which is not a lot for a big shop. My building is 72x40 with insulated open ceiling and walls but only about R 7 or so in the ceilings; more like R-15 in the walls of the shop part. The walls are 15 feet high with the peak of the ceilng about 23 feet. I plan to put in a drop curtain to divide the storage part from the shop part, which is 32 x 40. I have a 100,000 BTU salamander heater. I am hoping that with the salamander heater and the electric heaters I can bring up the temperature on the cold days in the shop part to something workable (50s) and then maintain it with the electric heaters. I also am planning on putting in 4 ceiling fans to being the heat down from the open ceiling.
This is Nebraska so we get some pretty cold temperatures and this is not going to make the shop part into t-shirt tempertures, but I am hoping workable temperatures. At 9-10 cents per kilowatt rates, I am thinking about 50 cents per hour per heater, but someone can probably do more accurate math than my back of the envelope math. I only plan to run them when I am in the shop.
In contrast, I think the salamander heater uses about a gallon of diesel an hour. I got some more diesel yesterday, and I think it is about $4 in this area. I do not notice the smell (but it does smell, according to my wife) but I don't like the noise of the salamander. The electric heaters are very quiet.
If my calculations are correct, I would think 3 might work for your shop in Georgia. I don't know your electric rates in your area. I would also think a couple of ceiling fans would be helpful in bringing the heat down to floor level. People I talk to tell me the ceiling fans make a big difference. Plus, insulation and stoppage of any air leaks makes a big difference too.
Without a drop curtain and ceiling fans, my use so far probably is not real helpful. The day we installed them, it was in the 20s. Without the ceiling fan and the drop curtain, and with using the salamander heater, we had the building to about 50. With the salamander heater shut off, it appears the electric heaters were maintaining the heat.
Ask me again in a year and I can give you a better answer, but probably a bit hard to generalize from my situation to yours.
Wish you the best with your plans.
Good question. 
