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[hobbyheater]

Ran a small experiment this morning.
Our heat storage tank holds 1,260 us gallons the hot water tap was run at a flow rate of 1.3 us gallons per hour for 1 hour and in that hour the storage temperature of the tank dropped from 168 degrees to 161 degrees,roughly a consumption of 80 BTU'S per hour for 80 gallons of hot water or 1,000 BTU,S for one  gallon of hot water!

You need 8.3 BTU for every degree you raise the water temperature. You should be able to heat about 150 gallons of water from 60 to 120 degrees while dropping the storage tank temp 7 degrees.

Actual heat rise of the water  was 45 to 161/168 degrees so using your numbers 80 gallons was a good result!
In our system 1 LBS of wood gives roughly 6 gallons of hot water or 166lbs for 1,000 gallons.
The wheel barrow load in the picture is roughly 166lbs.