22K BTU/pound of propane. 400K/22K=18.18pound per hour/4.23pound/gallon = 4.3 gallons. Throw in some efficiency losses and 5 gallons/day, then add a bit more for routine hot water use, and you may have another gallon (probably less). Now, that's at full output, and that is likely to make the water hotter than you need. So...

If you figure roughly 8#/gallon of water and you have 15gpm*60min*8=7200# of water per hour. So, it will take 7200BTU for each degree you need to raise the water temp. WIth these numbers, you can do the rest of the calculations.

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