Originally Posted by tuffr2
We will have to disagree
Not quite sure what we are disagreeing about. It's simple math.
Standard group 24 batteries provide 70 to 85-ish amp hours. You generally don't want to run them completely flat so 80% is a good number for the actual usable capacity:
70 Ah x .80 = 56 amp/hours of usable capacity.
The amp draw numbers I mentioned were quoted from various manufacturers' spec sheets. Using the worst number I found, 11 amps results in
56 Ah / 11 A = 5.09 hours of continuous running
Using a more reasonable number for run time of, say 25% -
5.09 hours x 1/.25 = 20.36 hours of runtime
So, the math says 20-ish hours of run time with no charging.
Using the ubiquitous F150 as an example tow vehicle, the alternator output is in the 120 amp range. After replacing the starting draw there is more than enough left to replace the 11 amp draw of the furnace.
Ignoring the math, boondockers would be really upset if they couldn't get a single full night out of their furnace. My son, in his popup with a group 24 battery, gets 2 to 3 nights.