I'll write more from my desktop later on because I'm on my cell phone, now, but The holding plates Will keep the boxes cold for a long period of time
One of the arguments people try to enforce about current draw of 12 volt Refrigeration I don't agree with. For weekend/a week
The draw is so low that you almost don't have to make modifications in your electrical system since many boats already have around 200 amp hrs and 2 banks.
Weeks away at sea on the hook for weeks will Evoke a different argument.
I've not looked But the engine driven compressor He's usually big enough to cool the whole boat Horsepower wise I would say 3 to 5 horsepower
Edit (from my desktop)
The factors to use as a guide are easy to find.
watts to btu W X 3.415 (3.4 is ok)
1 horse power = about 745 watts.
horse power of refrigeration effect 12,000 btu = ton (old schoolers call it horsepower.
boxes should be at around R-25 per sq ft. some more.. many much less.
CAPTA 8 cu ft is not a big box. are they side by side?
On engine driven systems, you can have more then one box (or device like evaporator or holding plate.) the capacity of the device is determined by the refrigerant metering device (cap tube/s or TX valves.)
many if not all 12 volt condensing units draw around 40-60 watts.
Most people with boats know how to figure out amp hours and "wattnot"
You are all of course welcome to have a field day with this information.
It's not written in stone and I don't pretend to be a total expert, it's just stuff I know from being in HVAC all my life.