on propane cylinders: 1lb of propane contains about 21.5k btus.
My math says 21.8 (91,600/4.2), but what's .3 BTU between hearth heads!
So a 100 lb tank contains about 2.15Mbtu.
But they will only put about 80 lbs in a 100 lb tank around here...or about 19 gallons...so 91,600x19 =1.74mbtu
If you know the consumption of the heater, it is easy enough to work out how many hours it will run. I think heating any reasonable space with propane will be a drag and you tend to be stung real bad on the refilling cost with regular cylinders. There are smaller propane tanks (they look like fat squat cylinders) that are probably 250lb in capacity that I have seen in RV parks where people stay year round. But they are owned by one of the propane companies and rented on a contract basis. I think the rule is that the bigger the tank, the higher the tank rental and the higher the "minimum" annual consumption before you start paying penalties
We actually have a maintenance fee calculated into the price per gallon around here...
Not sure what the rate is, because it varies...
If you heat with LP, you pay a lower price than if you cook with it or use it solely for hotwater...
You can get the fuel cheaply, if you own your tank(s), but there are no guarantees...
One of my buds bought a tank & found it had a leak in a welded seam...
He blamed the vendor & the vendor blamed the shipper & he lost his $$$, because it couldn't be resolved...
All after he buried the tank...
He was able to get the leak repaired & re-pressure tested, but that cost him even MORE $$$...