and click on 6lb - 40lb Specifications.
You can figure out how many gallons of water the tank holds from it's water weight capacity (stamped on the tank), and multiply that by a percentage to find usable gallons, and multiply that by 91.500 btu/gal or...
you can just look at how many gallons of LP Worthington says the tank holds and multiply that by 91,500 btu/gal.
or more simply, if you have a 40 lb tank, it holds 40 lbs of propane and multiply that by 21,560 btu/lb for 862,400 btu.
Usage is a little harder. If you have a 34,000 btu input furnace and it's running 1/3 of the time, on average throughout a 24 hour period, it's consuming 11,333 btu/hr or 272,000 btu/day.
The refrigerator is similar. You have to figure out how many btu/hour it consumes if running 100% then figure out what percentage of the time it's running.
Same with the water heater left on constantly. If you only run it long enough to heat one tank of water, then it takes 1 btu to raise the temperature of 1 lb of water 1 degree F. A 6 gallon tank is about 50 lbs, so to raise it 80 degrees (from 60 to 140) or 4000 btu. How much energy it consumes to keep it there is a different story.