Fridge 12v vs AC?

[Xavierp]

Hi all.

My new fridge can be powered with AC or DC
I am off the grid
I to save any power I can

which is best AC or DC connection for the fridge

Thx all!!

[CaptainRivet]

Originally Posted by Xavierp

Hi all.

My new fridge can be powered with AC or DC
I am off the grid
I to save any power I can

which is best AC or DC connection for the fridge

Thx all!!

What kind battery and how many AH you have, what brand and wattages is your inverter and how many watts the fridge uses in 12 and in 110 or 230V?
Inverter running 24/7 anyhow eg because starlink? If that answer is no go for DC as alone the draw of inverter itself is significant.

[rslifkin]

Efficiency wise, there's no inherent reason why a DC vs AC fridge matters if you're already running an inverter 24/7 for other reasons. It's down to the efficiency of the specific fridge you install.

[wholybee]

I could go either way. There will be efficiency loss with the inverter, which would favor a DC unit. Less an issue if the inverter is on anyway. However, the DC units efficiency is affected by battery voltage. So if you have too much voltage drop in wires to the unit, AC might gain an advantage. LFP would be much preferred over LA for this reason.

I have met a few sailors that get more battery life running the inverter and their fridge in A/C mode, than running the same fridge in DC mode. But in general, I think the opposite is true, that DC would be more efficient.

But really, if you want a fridge that is efficient, it is ALL about the insulation. Having had a couple brands of already built fridges, they don't hold a candle to a custom built and well insulated unit.

[CaptainRivet]

Originally Posted by wholybee

I could go either way. There will be efficiency loss with the inverter, which would favor a DC unit. Less an issue if the inverter is on anyway. However, the DC units efficiency is affected by battery voltage. So if you have too much voltage drop in wires to the unit, AC might gain an advantage. LFP would be much preferred over LA for this reason.

I have met a few sailors that get more battery life running the inverter and their fridge in A/C mode, than running the same fridge in DC mode. But in general, I think the opposite is true, that DC would be more efficient.

But really, if you want a fridge that is efficient, it is ALL about the insulation. Having had a couple brands of already built fridges, they don't hold a candle to a custom built and well insulated unit.

all true...in case of voltage drop, especially in 12V i highly recomment to use 12/12/30A Victron isolated Tr-orion which creates from 9-17V a stabilzed adjustable output. that i installed right at the main power input of the switchboard and output voltage at 14,8V compensates voltage drop&stabilze the power so the compressor of fridges/freezer as well as sweet water pump have minimum stable 13.6V at the input terminal. like this they run much better with much less losses...yes the stabilzing&upscaling costs energy but the losses are much smaller and eg waterpump runs shorter and keeps pressure much better. also good for livespan of pumps/compressor and if you have electric toilets..short all with a motor.

[Xavierp]

I spoke with the Whynter, the company that makes the freezer/fridge.
They gave me very detailed info which results in showing:

AC - 1.07 Kwh / 24h = 10 Ah
DC = 65.6 Ah

Is it even possible?

Attached is their specs sheet.

[wholybee]

Originally Posted by Xavierp

I spoke with the Whynter, the company that makes the freezer/fridge.
They gave me very detailed info which results in showing:

AC - 1.07 Kwh / 24h = 10 Ah
DC = 65.6 Ah

Is it even possible?

Attached is their specs sheet.

No, that does not make sense at all, and it does not match the spec sheet attached. The spec sheet is a bit odd because it gives consumption as kWh for AC power, and Ah for DC power. Converting the units and using 90F ambient at -0.4F freezer:

AC = 1.01kWh
DC = 0.747kWh

And converting to Ah run by a 12V battery:

AC = 84Ah
DC = 62.3Ah

The error in your number might be incorrectly converting the 1.07kWh to Ah. If you are counting Ah in a 12V battery (running and inverter) you need to divide by 12) it appears that 1.07 was divided by 110.

[UFO]

Originally Posted by rslifkin

Efficiency wise, there's no inherent reason why a DC vs AC fridge matters if you're already running an inverter 24/7 for other reasons. It's down to the efficiency of the specific fridge you install.

Yep - Found that with my freezer - A lot of new 240V Freezers are now highly efficient - I got the most efficient one that I could find that fitted and the Amp draw, even with inverter was less than my previous 12V Freezer and for a bonus a 1/4 of the price!

[Wandering1]

Originally Posted by UFO

Yep - Found that with my freezer - A lot of new 240V Freezers are now highly efficient - I got the most efficient one that I could find that fitted and the Amp draw, even with inverter was less than my previous 12V Freezer and for a bonus a 1/4 of the price!

They all use evaporator plates, compressors and condensers. It is possible to mismatch them and lose some efficiency but you can't argue with the Carnot cycle. Most likely the isulation was poor in your old fridge or the compressors was worn, the gas weight was wrong. The inverter will have an inefficiency that a dc compressor fridge simply doesn't have

[Statistical]

Originally Posted by Wandering1

They all use evaporator plates, compressors and condensers. It is possible to mismatch them and lose some efficiency but you can't argue with the Carnot cycle. Most likely the isulation was poor in your old fridge or the compressors was worn, the gas weight was wrong. The inverter will have an inefficiency that a dc compressor fridge simply doesn't have

DC compressor is a bit of a simplification. Compressors are AC motors usually 3 phase AC motor. A "DC compressor" is simply one that takes DC and uses an onboard inverter to turn it into 3 phase AC. It is running on AC it has DC to AC conversion losses.

There are two small advantages to "DC" compressors.
1) They can use 3 phase power without additional steps this tends to lead to slightly more efficient compressors than single phase power. Not sure any marine refrigeration system uses 3 phase compressors.

2) The onboard inverter can be sized to high efficiency. A general purpose house inverter is likely rather efficient at 50% to 90% load but at night if you turn everything else off and your 4000W house inverter is now only running the fridge drawing 30W (so <1% load) it is a lot less efficient.

Both have an impact but the impact is small. Same thing with the touted benefits of DC generators. A "DC generator" is a 3 phase AC alternator which gets rectified to DC inside the generator there is still an AC to DC conversion happening.

[CaptainRivet]

Originally Posted by Statistical

DC compressor is a bit of a simplification. Compressors are AC motors usually 3 phase AC motor. A "DC compressor" is simply one that takes DC and uses an onboard inverter to turn it into 3 phase AC. It is running on AC it has DC to AC conversion losses.

There are two small advantages to "DC" compressors.
1) They can use 3 phase power without additional steps this tends to lead to slightly more efficient compressors than single phase power. Not sure any marine refrigeration system uses 3 phase compressors.

2) The onboard inverter can be sized to high efficiency. A general purpose house inverter is likely rather efficient at 50% to 90% load but at night if you turn everything else off and your 4000W house inverter is now only running the fridge drawing 30W (so <1% load) it is a lot less efficient.

Both have an impact but the impact is small. Same thing with the touted benefits of DC generators. A "DC generator" is a 3 phase AC alternator which gets rectified to DC inside the generator there is still an AC to DC conversion happening.

Simple solution get the 500 or 800W victron Phönix inverter that runs 7/24/365 your fridge/freezer and other small AC loads and this uses <8W on standby when your fidge/freezer compressor is off. Small 350W once have more...the Phönix 500 and 800W is the absolute king in standby power. You can often get them used for a steal as people upgrading to bigger ones, paid 130Euro for my 800W one.
And your big inverter runs galley and other high power 230V stuff during day on demand.

[BMD21]

Well, have a look at the refridg electrical circuit. If It's like my Norcold it runs on DC. When it gets AC, it uses an inverter to generate DC. So, if one uses an inverter to generate AC, the refridg uses an inverter to convert the voltage back to DC. Why pay for the efficiency of two serial inverters?


And there are DC motors; I use one every time I start my truck.


Boris

[CaptainRivet]

Originally Posted by BMD21

Well, have a look at the refridg electrical circuit. If It's like my Norcold it runs on DC. When it gets AC, it uses an inverter to generate DC. So, if one uses an inverter to generate AC, the refridg uses an inverter to convert the voltage back to DC. Why pay for the efficiency of two serial inverters?


And there are DC motors; I use one every time I start my truck.


Boris

You are sure about DC? Compressors normally run on AC...made from DC...

Because the newer generation of fridges and especially freezers have ac compressors and they are super efficient, they still run on DC though. But they are much better in AC then them running on DC ones that convert then to AC. They also show this on there spec sheets.
And to run these best you use the victron phönix which has a inverter efficency of over 90% and only <8W in standby that uses much less energy then your DC one or run this freezer on 12 or 24V.

[Statistical]

Originally Posted by BMD21

Well, have a look at the refridg electrical circuit. If It's like my Norcold it runs on DC. When it gets AC, it uses an inverter to generate DC. So, if one uses an inverter to generate AC, the refridg uses an inverter to convert the voltage back to DC. Why pay for the efficiency of two serial inverters?


And there are DC motors; I use one every time I start my truck.


Boris

Exception to every rule. 99.9%of compressors run on AC maybe you have the 1 in 1000 that doesn't. In general DC motors are either terrible efficiency/reliability (brushes) or very complex and expensive (BLDC). Neither is ideal for a refrigerator compressor.

BLDC are starting to get cheap enough that they are going into high end marine AC unit but a marine AC is going to have 20x the power requirement of the average refrigerator so higher cost for higher efficiency has different economics there.

[Bowdrie]

Generally speaking three phase motors are cheaper/more efficient and eliminate the need for start capacitors and a centrifugal or electronic switch to connect/disconnect a start winding.
Those "Danfoss" compressors do run on AC.
We can't use a DC motor inside a hermetic compressor where brushes are used, the brush-fire breaks down the refrigerant.
Back 35>40 years ago EBM-Pabst made hermetic compressors that did run on DC.
They used a stationary external commutator that was fed by a bunch of wires, each wire using a hermetic thru-shell connector, together with a magnetically coupled rotating brush holder/slip ring assembly.
They didn't last long on the market, short brush life, overly complex, and frankly, almost no one in the field actually understood how they worked.
I use what has already been mentioned, a small/dedicated invertor that runs an AC unit with a changeover switch to use shore power.