Jerry:
What keeps your garage at 40 deg F. when the outside temperature is -10 F. ? Is it buried in a hill?
The amount of power, rate of expenditure of energy, to maintain a given internal temperature is a function of the outside temperature and the thermal resistance to the outside. Thermal resistance will not be a constant because of wind variations. There also may be some thermal gain from solar radiation on the outside surface.
All the above is difficult to calculate.
Note: if you building is mostly in a hill then you might be mostly heat sinking to ground temperature.
You can experimentally determine energy required with some electric heaters. On your coldest day with high winds run the electric heaters to maintain 60 deg F. As a guess maybe 5 to 20 KW worth of heaters. Use a KWH meter to measure energy over a days time. This will give you an average energy requirement for a worst case day.
Another way is to use a torpedo type kerosene heater and measure the fuel used over the 1 day period.
Electrical heat has 100% efficiency, combustion heaters have less efficiency. There is data available on the energy content of various fuels, and there are conversion factors between different units of energy measurement. Thus, you can make measurements with one means and compare with a different one.
In my garage which is about the size you are describing and with fiber glass insulation filling 3.5" in the walls and mostly heating a 22 x 15 area 5 kw on continuously wiil heat to about 60 deg F. with outside about +0 to 10 deg F.
1 KWH = 3413 BTU. So my requirement is about 17,000 BTU/HR. 134000 BTU are in 1 gallon of kerosene. 21548 BTU are in 1 pound of propane.
At 100 % efficiency I would require about 0.12 gallons of kerosene, or 0.79 # of propane per hour.
Our electrical rate is about $0.11/KWH and our natural gas is about $0.04/KWH. If fuel oil is $3/gallon, then its cost per KWH is about $0.08 . Note these are 100% efficiency.
The difference between your heat sink temperature (building outside) and the internal temperature in combination with the thermal resistance from inside to outside determines the amount of energy require to achieve the internal temperature. If you double this difference then you double the energy required.
If I required 0.79 # of propane per hour and we use 24 x 30 for the hours per month, then I would require 568 #/month for the 60 deg F. internal at 0 to 10 deg F. outside.
Would our temperature be 0 to 10 F. for a continuous month? No. Averaged over a winter it is more in the range of 30 F.
Your area is larger than mine, but your average required temperature difference is much lower.
When you look at costs you may find that added insulation would be cost effective.
For improved efficiency, but at a high capital cost, you might look at a ground based heat pump.
Is your estimate of 60 #/month realistic? I have no idea. You need more information than what you have provided so far. But it won't be 600 #/month.
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