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Originally Posted by dkoldman
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By Chance, can we play out the information provided below...  Math Alert: Some may wish to stop reading NOW 
You State: "Your 2.5 Amps is at 120 VAC, so roughly 25 Amps coming out of batteries at +/- 12 VDC to power inverter. If it ran for 2 hours, you may have pulled up to 50 Amp-hours, which is a lot from two small house batteries even if in great shape."
If I follow this right, you are saying that whenever I have amperage based on 120VAC, I should multiply by 10 if I wish to run the same load from a battery source? I assume you mean 120VAC/12VDC = 10?
So lets say the exact time the Norcold ran off the battery was 2 hrs. That would mean it pulled 50 DC AHs off the battery before the AGS started the genset to recharge. The same batteries new should have 130 DC AHs available capacity, but I have heard that you really on have half of that? So it means I may have drawn 50 AHs off 65 available AHs before the AGS thought it was too low? (or maybe it really is 130AH available?)
If the above is all true, what perplexes me is the 1.5 AH Insignia Fridge that is in the outside kitchen. It is plugged to the Inverter; so by math just learned; it is pulling 15 amps DC off battery? We have gone overnight (Quiet time 10pm - 7:00AM) with that outdoor fridge on batteries plus TV lights etc. That is 8 hrs and 90AHs. So if I only had 65ah available, it would not be possible, but if the 130ahs were available maybe if batteries were in good condition?
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If we were to look at the “numbers” for guidance, or at least evidence of what you observed during test and also regarding outside-kitchen fridge, there are a couple of very important points to be added to the energy-consumption estimates that you would need to include for accuracy. Otherwise your estimates will be way off.
With propane fridge running on inverter, “IF” it uses 300 Watts (and that’s in right ballpark), electrical current from battery will be even greater because of inverter not being 100% efficient. In total, batteries would need to supply about 30 Amps or more in order to make 300 Watts of heat at refrigerator.
More importantly, your batteries may be rated at 65 Amp-hours each, for total of 130 Ah, but that applies only to a very slow discharge rate over 20 hours. That works out to 6.5 Amps for 20 hours. When you discharge lead-acid batteries much faster than that, like at 30 Amps instead of 6.5 Amps, you will NOT get 130 Ah. It will be much less.
As I stated previously, running absorption fridge from batteries is not very efficient because of all these factors combined. I would only do it if driving with alternator powering the inverter, and stopped for gas or bathroom break for a few minutes, in which case I would let batteries power the fridge for a short period. If stopping for more than a few minutes, I’d go back to propane.
As to your outside kitchen fridge running much longer on batteries, it’s not only due to the lower power requirement, but also because it cools with a compressor which means that once temperature is reached, it cycles on and off as needed. Through the night on battery power, the fridge compressor may have only run a couple of hours depending on outside temperature. By comparison, propane fridge barely stay cold enough when running on electricity, so it’s likely it will draw closer to maximum current all night long.
Basically, total energy consumption of those small outside kitchen refrigerators is a small fraction of what your propane house fridge requires when on electricity. It requires a lot lot less. Comparing how long the two refrigerators will run off batteries is like night and day.
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