A chest style freezer is efficient, due to the thick insulation, and the lid and seals being on top. There is nothing with the mechanicals which makes them more efficient than any other compressor fridge design other than that. Yes they do have a larger more capable cooling unit to get to those sub zero temps, and when used as a slightly sub 40f fridge they will use much less energy, compared to when they are holding sub zero temps..
But less than a 12v compressor fridge holding the same interior temps? I doubt it, highly, but have no data to confirm my suspicion, which merely makes it an opinion.
One thing to keep in mind is one has to run a 120v compressor on an inverter. An inverter is at best, 90% efficient, and it has to be left on all the time, even though the freezer might only run for 5 or 10 minutes an hour.
The Inverter, turned on, powering nothing, consumes some electricity. this Idling current is different for each and every inverter, but the range is typically 0.25 amps to 1 amp per hour. Some inverters have standby modes that have much lower idle current consumption when turned on powering nothing, but one pays significantly more for this feature.
I have a 400 watt PSW inverter that is 0.24a idle consumption, and a 800 watt MSW inverter that is almost 0.8a per hour.
My MSW inverter, turned on, powering nothing, consumes more battery power than my front loading Vitrifrigo 12v compressor fridge uses in an hour, 95% of the time. In warm ambients or when placing warm items within, or just opening the door a lot it will consume more electricity.
It should be noted that my fridge and fridge cabinet has extra insulation, and the cooling unit (compressor/condenser) is very well ventilated, making it more efficient that it was out of the box.
So while the chest style freezer conversion no doubt works, the question of whether it will ultimately use less battery power than a 12v compressor fridge, is in my opinion, highly unlikely.
If one already has a 120vac chest freezer, and the ability to modify the thermostat, and an inverter which can handle the rather large start up surge, then it makes sense to go this route, in my opinion.
Get the idea of a thermo electric cooler out of the equation, they are supremely inefficient and are really only good for use when actually driving and the alternator can make extra energy to run it. They average 4amps per hour consumption where a 12v compressor fridge of similar internal volume is about 1amp per hour, depending on its size and other factors. They are also only good to about 40F under ambient temps. 90F ambients and your food is at best is at 50F, and the accepted cutoff for food safety is 40F
Of course the chest style freezer conversion can and does work, but one needs to run the large inverter 24/7/365, and likely needs more battery capacity, and more charging capacity to run it, largely negating any cost savings. This applies to the cheap 120vAC dorm fridges run on an inverter too.
There is also the possibility of the compressor motor not liking the modified square wave of a MSW inverter. It can run hotter, wear out more quickly, and there is the possibility that road vibrations/stresses can overwhelm a cooling unit designed to remain stationary in a stick and brick dwelling, where as the portable/ 12v units were designed to go into boats or RVs and take those stresses in mind when designed.
A PSW inverter is much more expensive than a MSW inverter.
There is no real way to beat the system with refrigeration on battery power. The initial price of a 12v unit always causes people to search for different options, and the inverter powering a household unit is always the idea brought up as if nobody has ever thought of it before.
If one already has a large solar system and large amount of battery capacity, and a large inverter, then going the residential 120vac fridge/freezer makes sense, as long as the unit does not succumb to road vibrations/stresses and fails prematurely.
If one is designing a system from scratch, one can usually get away with half the battery capacity, and recharging capacity, by getting a 12v compressor fridge in the first place.
It is always easier and cheaper to use less electricity, than it is to is store and return more energy into a larger battery bank so one can power more less efficiently.
So it is mostly getting over the sticker shock of a 12v compressor unit like. Norcold/Dometic/NovaKool/Vitrifrigo/TruckFridge/Edgestar/Whytner/Waeco/ARB, and some other brand names which escape me at the moment.
My preference is for the 12v fridges which use the Danfoss/Secop compressor rather than those units using a Sawafuji compressor for the noise factor. The BD35f compressor has an adjustable speed compressor from 2000 to 3500 rpm and can be run at any speed in between. It is about 2.2 amps at 2000 rpm and 6.4 amps at 3500rpm. It 3500 rpm it can be used on something like a 6 cubic foot fridge freezer, so on a 1.8cubic foot at 2000 rpm unit it is barely working hard at all, and has a huge overhead available.
Some units are one set speed always, mostly the frontloader fridges, and it seems like the chest style units can automatically vary the compressor speed depending on their load.
I am very pleased with my Vitrifrigo c51is 1.8 cubic foot fridge. Much more so than I was with my Norcold de-0040( lasted 5 years) which was loud and liked to vibrate the whole van until I took many steps at vibration and noise suppression. My VF is more efficient than the Norcold was as well, holding cooler temps for the same battery power consumed.
My particular VF fridge was 100$ more expensive for the AC/DC model. I bought the 12vDC only model and put that extra 100$ toward a 12v powersupply/charger, capable of not only powering the fridge whenever I have grid power, but also recharging the battery(s) at upto 40 amps, less the 2.2 to 2.8 amps the VF consumes when the compressor is running, and any other DC loads I am running at the time.
The TruckFridge is among the least expensive 12v models available, and utilize the Danfoss/Secop compressor. They sell front loading and chest style fridges.
http://truckfridge.com/
I bought my Vitrifrigo through this guy. Best price and free shipping 38 months ago, he can get any VF model even if there is no direct link on his site:
http://westyventures.com/parts.html
The VF is Italian made vs the TF which is Chinese made. They appear very similar but the VF's condenser/fan and plumbing is a bit different. The VF's setup allowed me to modify the fan and the airflow for better efficiency, the TF would not have allowed this.
If one plans on removing the fridge and running it in a stick and brick, then it makes sense to choose a AC/DC unit, but if one is only ever going to run it on DC power, then paying extra for an AC/DC model makes less sense. When one has grid power one should be running a charger/converter/power supply to charge the batteries, and this can then power the fridge too.
Most chest style 12v compressor fridges are dual voltage though. It seems that only the front loaders have the DC only option.
The chest style fridges are said to be more efficient that front loaders, as the cold air spills out with the door opened. My opinion is that they are more efficient due to the fact that there are no thin door seals at the bottom of the unit. Whenever the top lid or front door is opened, most of the cool air within the unit is displaced anyway, and the actual thermal mass of the cool air inside is quite small. So while one who leaves their lid or door open for a minute or two for convenience will benefit from a top loader, the person who was not raised to have the door open for the entire food preparation session sees much less benefit .
So while the chest style is inherently more efficient as there are no door seals on the bottom, and a percentage of cooler air remains inside when the door is opened, one needs to factor in the convenience of a front loader, vs a chest style, as they will use it.
I have an older, good friend, who is a stick and bricker, and he was given a chest style freezer, and always complains about the size of his electric bills. He decided to run this chest style fridge just one hour a day to freeze everything and then approach 40f before turning it back on. I think he lasted about 2.5 months before giving up and going back to a regular fridge. He saw no appreciable difference in his electric bills and was tired of worrying about whether his food was sub40f or not, but for that first month I got an unwanted update everyday about how well it was working and how happy he was thinking he was 'giving it to the man' and beating the system, and flouting my recommendations against expending the effort to do so.
Whenever I hear his latest idea for this that or the other thing, I remind him of his chest freezer fiasco, and he grumbles away.
Right now he has determined that his water heater is responsible for about 400$ a month of his 600$ a month bill. I had to listen to his idea to set of a solar thermal collector and run a series of pumps and switches. He only plans on keeping the home for 5 more years. the 'return on investment' portion of the equation is always lost on him. When some math is done, the Idea is usually scrapped. as he will not live the ~30 more years needed before he starts saving money.
A couple years ago he was going to buy 10K worth of batteries and a 20K diesel generator and build a soundproof enclosure for it, as that 500$ a month electric bill was so infuriating to him. I keep recommending going back to a gas water heater which he scrapped 6 or 8 years ago in favor of a electric heat pump style water heater, which he could never get to work right
Kind of OT but whenever I see something about beating the system via some great idea that apparently nobody has ever thought of before, I think of My friend and his latest ideas to spend thousands to save hundreds, a decade or more in the future which he will not be alive for to ever see.
Unless you already own the freezer, large inverter, adequate battery capacity and solar recharging capacity to power an AC fridge/freezer, then you will not save any money by going this route, over a 12v compressor fridge/freezer, that was built with minimal electrical consumption in mind in the first place.
