How to calculate how long a car battery will last as backup

[Chin_Lee]

Wow thanks Lyle. So if A+B=C what's D? JKing.

Thats helps a lot. At least now I know maybe a heater is not such a good idea but I would imagine that two Tunzes at 13 and 23 watts will last:

13 + 23 = 36 watts which equals 0.3 amps

So if the battery is 60 amp hour and factor in the efficiency factors, that battery really has about 46 amp hours.
60 amp hour * 0.9 * 0.85 ~ 46

So to calculate how long the two tunzes will run, the hours will be the variable

36 watts / 12 volts * h = 46

So breaking it down, it becomes:
36h/12 = 46
3h=46
h=46/3

Looks like that battery will run both my tunzes for about 15 hours. Wow my grade 8 math is coming back to me.

I think to keep my tank warm in the event of a prolonged black out, I will consider boiling some tank water with my gas stove attached to my BBQ and pouring it back into the tank (slowly of course)..... won't be easy but it will work.

Thanks again Lyle