Note that you need to determine the btu consumption RATE. A calculation may show that you need 200K btus, but over what period of time? If it's a 24 hour period, that translates to 8333 btus/hour. Whatever the rate, you need to translate that to the same rate the the heater is specified in so that you have a real comparison. A large candle can generate 200K btus, but it will take a lot of burning over a very long period of time to reach that total.
The "delta T" (the amount of temperature differential) is based on the difference between the "static" temp and the desired temp, and is normally based on outdoor temp vs. indoor temp. This is the case because heat rises and represents the actual temp differential across the insulation, and therefore the actual heat loss RATE across the insulation. Your heat sources must be able to generate heat at the same RATE in order to maintain a stable temperature in the room.
I say heat "sources" because there is generally some natural earth heat that comes up through the floor, however, this is a relatively small amount and should be discounted to offset other unconsidered sources of heat loss. For example, we generally assign an R value to a wall as though the entire wall is insulated, but in most cases it isn't. A stud wall with 16" stud spacing is only about 90% insulated, the other 10% is the studs and plates, and wood doesn't have near the R value of insulation.
I went through all the heat load calcs on my building and determined a 10 btu per sq. ft. per hour requirement to maintain a 70 degree temp differential between inside and outside under worst case temp extremes. The building is about 1500 sq. ft, so I need a heat source capable of generating 15K btus per hour, minimum. Theoretically, a 35K btu per hour propane wall heater would have to operate approximately 26 minutes of every hour to maintain a steady 70 degree temp differential between inside temp and outside temp. If the outside temp is -20 degrees F, at that rate, the building would stay at +50 degrees F inside.
The propane consumption rate of this heater can also be determined under these conditions. 1 Gallon of propane = 92K btu, and the furnace's efficiency rate is 80%, so we really get only 73.6K usuable btus from a gallon of propane, or 4.9 hours of heat (at 15K btus per hour) per gallon of propane. If we had to maintain this temp differential all day long, it would require about 5 gallons of propane per day to heat the building.
Fortunately, I don't have to maintain this heat differential all day long. This calculation is based on a "worst case" scenario. Most of the day the outdoor temperature is not 70 degrees below the desired indoor temp, so the 10 btus per sq. ft. per hour is not required all day long.
