Now after all this running around with sensors and propane heaters - there's really no possible way we could NOT install a natural gas heater. Browsing the Menards ad for the week yielded a 75,000 BTU Beacon Morris unit on sale for about $450. Figuring that's a pretty good deal, we headed over there and gathered the supplies we needed. Oh the supplies... B-vent to get out the roof, flashing, crazy angle pipe things, a cap, storm collar, black iron pipe for a drip leg on the gas line, 60' of 1/2" copper flex, some flare nuts, some 14/2 wire, a thermostat, some 18/5 thermostat wire, hangars, and various other random things - probably some beef jerky since you always buy beef jerky at Menards. All in - a whopping $750 bill at the checkout. Seems like a lot, but it's much less than the $2,200 quote I received from a local HVAC shop for a 45,000 btu heater. Fast forward a few hours and we've got ourselves cheap(er) heat!
Scenario 1: 20 degree day, maintain 50 degrees
On for about 8 minutes, off for about 56 - that's a total cycle of 64 minutes and it's on for 12.5% of the time. It's a 75K BTU heater, meaning it will consume 75K * 12.5% or 9,375 btu per hour. 24 hours * 9,375 btu / 100,000 * $0.67 = $1.50. That's $1.50 per day for a 30 degree temp differential.
Scenario 2: 20 degree day, maintain 70 degrees
On for about 7 minutes, off for 11. Cycle of 18 minutes, on for 39% of the time. 75K * 39% = 29,250 btu. 24 hours * 29,250 / 100,000 * $0.67 = $4.70. $4.70 per day for a 50 degree temp differential.
Scenario 3: Back to 50 degrees
I had some crazy idea like i could figure out the heat loss of the garage with data like this - but there are really just a ton of variables in addition to the heat loss. The biggest issue I see is the heat that moves from the objects in the room (that have been heated up to 70) in to the air around them as the air cools. Look at the difference in how long it takes to drop from 59 to 49 the first time, and then again the 2nd time after the air is heated again.
And my car in the middle of being waxed - since I had to keep the garage at 70 for a while to gather data, I figured I'd make use of the warm weather. That laptop in the background is gathering the data: