Assume 12 foot high average walls
Assume no doors or windows and excellent air sealing
Assume all surfaces insulated to R19, giving a wall and ceiling "system" performance of around R10Assume same climate as Wichita, KS You have 30*12*4 or 1440 square feet of wall
You have 900 square feet of ceiling.
Total 2340 square feet At an average of R10, Your shop loses 234 BTU per hour per degree of temperature difference between inside and outside If the inside is 65 degrees, and the outside is 0, you need 15000 BTU per hour to maintain the temp.
So you don't need a very large unit to heat the shop. Wichita has an average of 4800 base 65 heating degree days per year. This equates to 4800 * 24 or 115200 degree hours. At 234 BTU per degree hour, this means it would take 27 million BTU per year to heat your shop to 65 degrees, 24x7. At 3413 BTU per kwH, this is 7900 kwH of electricity per year of resistance heat.
At $0.112 per kwh this is $885 per year. A heat pump would do better, a good unit achieving around 1/3 of this or 2600 kwh per year. Since this qualified as heating and cooling equipment, you might be eligible for the "low rate" of .056 per kwh or $146 per year for the heat load electricity only. And you can put it into cooling mode in the summer if you want to, of course paying more for the electricity to do that. If a gallon of propane costs $2.10 and contains 92000 BTU and you have an 80 percent efficiency furnace, it takes 370 gallons of propane to provide 24x7 heat, or $777 per year. One benefit of propane is that you can significantly oversize the unit and not lose that much efficiency, making for a faster warm-up in really cold weather compared to heat pumps. But other than that, propane isn't all that great at these prices. If you only use the shop 5 days a week for 6 hours a day on the average, and you have minimal thermal mass (you just warm up the shop when you need it, no heated slab) then you can expect to use only about 20 percent of the above numbers -- a bit more if your useage is primarily at night, less if it is during the day. The heated slab makes for a pleasant surface to work on, but it is hard to argue for that to be the sole source of heat for a building used only 20 percent of the time. There is a secondary benefit in the sense that radiant heat "feels warmer" than ambient heat, allowing you to run the air at a lower temp and still be comfortable, but I doubt this is enough of an effect to make up for the 20 percent useage issue.
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