Short answer.
I think 12 VDC would be fine, without the AC option.
Especially, as you say, boondocking.
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Long answer.
For me the thing to know is... from being off (at a nominal temp) to getting to the initial desired temp (34-35 degrees for a fridge, or just below 30 degrees for a freezer), how many amps are required and how long does it take to get there?
Then how many amps does it take to keep it at that temp for a total time of 24 hours?
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My case study...
I've got 2, built-in, top loading, 1.5 cu ft, 12 VDC only units by Indel-Isotherm (marine industry).
These use the very common Danfoss BD35F, air cooled compressor, very quiet operation.
One as a fridge, the other as freezer.
Starting at an "off temp" of around 75 degrees cooling down to 33 degrees, measured with a VWR digital thermometer using a liquid filled bottle probe, takes about 4 amps and 40 minutes or so (OK, let's say 1 hour), loaded with already cold food.
A worst case scenario is to load it with food that is not already cooled and it should draw more amps and take a longer time to reach the desired temp. This could be a way to need AC power.
Then to hold that temp it's under 1 amp per hour.
So, in the first 24 hours it takes 4 amps (hour #1) + 23 amps (hours #2 thru #24) = 27 total amps (for me, for each unit).
That's what the sales info basically said for my units and that's what I've experienced in the couple of months of operation, so far.
As I've got two of the above units, they will use approximately 54 amps per 24 hour day.
I've got a solar panel that both of my cooling units will operate off of during daylight hours, with no drain on my battery. So, if I figure a 12 hour sunlight period (it's often more), then I can cut the amps per day back down to half... 27 amps for both units for overnight only operation.
I personally don't like to take a battery much below 50% of it's rated amperage, and I've got an AGM, 100 amp/hour battery, so that is plenty for overnight operation. I could probably be OK with a 60 amp/hour battery. And, the solar panel is large enough to recharge the battery and run the cooling units at the same time when the sun is up.
I personally prefer to not have any electrical connection to my truck. Just my choice.
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So, see if you can find out the details of the initial amp draw to get to the desired temp, and then what is the average amps per hour to hold that temp.
Sometimes the power consumption will not be listed in amps, but in "watts per 24 hours". That's OK as Amps = Watts/Volts. So, watts per day of 270 / 12 VDC = 22.5 amps per day. That's usually an average, so add in another 20% or so, for the startup amps to get from nominal temp to 33-35 degree for a fridge, a little more for a freezer. So, for a fridge... 22.5 amps average + 4.5 amps (20%) = 27 amps.
Incidental usage of adding in "new, not cold" food items will add some amp draw to this equation.
If you have a front opening door that will let some cold air out every time the door is open, instead of a top loader that keeps the cold air mostly in the box, then add in some more amps to take care of opening the front door every now and then.
Once you calculate your 24 hour amperage draw, ask yourself... is your battery up to that?
And, ask... how will the battery get recharged so it can keep up the next 24 hours at the continuation amp draw rate?
Calculate your output, and then calculate your input, to get to a balance or surplus.
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I believe that all of the units mentioned above by others are roughly like mine (amp/hour draw rate), however, you should confirm it.
My units are going to live permanently in my ATC Bobcat (built-ins), but I know that the portable units mentioned above are very good and very popular.
