120V AC Fridge Energy Usage

[bdosborn]

Just for fun I threw a watt-hr meter on my 120V kegerator fridge to see how efficient it is compared to my 12v fridge. It used 295 w-hrs over a 24 hour period. That's 22 a-hrs at 12V, which is right around what my Tundra 12V compressor fridge would use at 78F ambient. But WTF, the kegerator is about twice as big as the Tundra! I expected it to use at least twice as much energy given the larger size. I have the kegerator set at around 34F and it's down in the basement. It' pretty cool down there which helps a lot with energy use. I think I'll meter it some more and see if that number is correct.

Anyone else meter a dorm size fridge to see how much energy they use? I think I'd rather buy a cheap 120V fridge and a nice inverter than an expensive 12V fridge.

Bruce

[rjgimp]

I have heard similar things from others but have not seen much for good solid real numbers.

[MickinOz]

I think I'd rather buy a cheap 120V fridge and a nice inverter than an expensive 12V fridge.

Bruce

I considered it. Well, this being Oz, I considered a 240V fridge.
Happened to have a Samsung 49L bar fridge sitting in the shed doing not much.
But, in the end I didn't use it for the following reasons:
Rated at 85W = around 7 amps at 12 volt, however have to factor in the inefficiencies of a small inverter, add about 10%
Light-ish insulation compared to proper car fridge. A bit conjectural I know, but seems that way and if I'm right it'll run more than a proper car fridge.
Front opening door very attractive compared to the lift up lid that is pretty standard on car fridges, but every time you open the door all the cold air falls out and the fridge must chill a fresh batch of ambient air.
Not meant for being used while moving, and you certainly will struggle to find any sort of recommendation regarding what angle the fridge can operate at.
I'd guess vibration will probably do some damage earlier than it would damage a car fridge.
Then I inherited Dad's old car fridge, so question resolved.

On the other hand, inverters and fridges are very cheap.

[rjgimp]

Then I inherited Dad's old car fridge, so question resolved.

That dynamic often negates any other consideration.

[bdosborn]

Front opening door very attractive compared to the lift up lid that is pretty standard on car fridges, but every time you open the door all the cold air falls out and the fridge must chill a fresh batch of ambient air.

Air doesn't hold much heat, its a great insulator so opening the door doesn't really affect battery usage. I tested mine by pulling a beer out every 1/2 hour or so and it didn't affect the energy usage at all.

Not meant for being used while moving, and you certainly will struggle to find any sort of recommendation regarding what angle the fridge can operate at.

The big RV crowd is replacing their failing absorption fridges with full size AC units. Dometic had a bad batch and ammonia leaks were causing fires. I haven't seen any posts bout premature fridge failures for the AC type but maybe I ought to do some more checking.

I want a bigger fridge, and the DC units are in the $800-$1,000 dollar range. I can get the same size AC unit for $180. So I could break a few and still be ahead.

Bruce

[Tom&Shelly]

Air doesn't hold much heat, its a great insulator so opening the door doesn't really affect battery usage. I tested mine by pulling a beer out every 1/2 hour or so and it didn't affect the energy usage at all.

As a some-time research engineer, I salute your adherence to scientific integrity!

Tom

[bdosborn]

I just don't remember how many beers I took out.
Bruce

[OP827]

Do 12VDC compressors use brushless motors that suppose to be more efficient in comparison to 120VAC compressors that use induction motors or it is not the case?

[MickinOz]

Front opening door very attractive compared to the lift up lid that is pretty standard on car fridges, but every time you open the door all the cold air falls out and the fridge must chill a fresh batch of ambient air.

Air doesn't hold much heat, its a great insulator so opening the door doesn't really affect battery usage. I tested mine by pulling a beer out every 1/2 hour or so and it didn't affect the energy usage at all.

Not meant for being used while moving, and you certainly will struggle to find any sort of recommendation regarding what angle the fridge can operate at.

The big RV crowd is replacing their failing absorption fridges with full size AC units. Dometic had a bad batch and ammonia leaks were causing fires. I haven't seen any posts bout premature fridge failures for the AC type but maybe I ought to do some more checking.

I want a bigger fridge, and the DC units are in the $800-$1,000 dollar range. I can get the same size AC unit for $180. So I could break a few and still be ahead.

Bruce

While you are certainly correct in saying air doesn't hold much heat it does hold some.
Which is why, when you use air as an insulator, you have to make sure it can't move.

More than one fridge operating manual I have read lately recommends closing the lid gently so as to avoid displacing the cold air with hot thereby causing the compressor to run unnecessarily.

here in Oz, you could taking that cold beer out in 45C heat, and trust me the compressor will kick in.

And yes, you can certainly be ahead money-wise buying an AC unit, but bear in mind a big RV probably won't knock a fridge around as much as a teardrop would.
But anyway, it is up to you. Get one, set it up and report back. I'm keen to know how much inverter losses you see, and how big the inverter has to be for staring current.

Do 12VDC compressors use brushless motors that suppose to be more efficient in comparison to 120VAC compressors that use induction motors or it is not the case?

They use all sorts.
The Engel fridge doesn't use a rotating motor at all - it has a piston that is pulled back and forth by magnetic fields.
Run by a built in inverter it seems, so likely to be not as efficient as suggested.

Danfoss/Secop compressors seem to be rotating motors with a crank driven piston, again operated by a "control module" that is called an inverter when ordering spares.

[OP827]

Check this video that talks about sizing of inverter. https://youtu.be/oVy9TlGvLIE

[Socal Tom]

Check this video that talks about sizing of inverter. https://youtu.be/oVy9TlGvLIE

And you need to factor in the invertor efficiency. Most are in the range of 80 to 90% efficiency and a good one will cost almost as much as the fridge. FWIW, we used to haul a dorm fridge with us when we camped at RV parks. We would set it outside and it would hold all the drinks.

[bdosborn]

here in Oz, you could taking that cold beer out in 45C heat, and trust me the compressor will kick in.

I hear you, I did my tests when it was 78F, a lot cooler. I only wanted to make sure that there wasn't a huge draw from opening the door every once it awhile.

Energy Star refrigerators have supposedly gotten a lot more efficient in the last 10 years, all though I haven't been able to find any numbers on the gains. So I wonder if the DC fridges have kept up? We've got a bar fridge up a the cabin, I'll throw the meter on it and see what I get. I'm going to throw it on the trailer and see what the AC draw is when the refrigerator is running. I was planning on getting a 2000w Victron multiplus inverter/charger anyways, I might as well work through my options.

Bruce

[bdosborn]

The Victron 2000 watt is pretty efficient in its lower power ranges. The AC fridge draws around 150 watts, so I'll be in the 85% efficiency range:



That's right on the knee of the efficiency curve.



The 2000w should be a bit more efficient at that load than the 3000 but I couldn't find any data for the 2000. I can live with that loss, assuming the data they publish is issued by engineering and not the marketing department.

Victron Efficiency paper

Bruce

[bdosborn]

Found a web site graph on refrigerator efficiency. Its a little old but still relevant. Apparently the energy star requirements ratcheted down 25% in 2014.


Refrigerator Improvements

I've got a DC amp-h meter on the fridge in the camper so I ran it for 24 hours yesterday; 19 amp-hrs. Not surprising when the garage is at 45F or so. I put the AC watt meter on the shore power to the camper, just to see if the meter is in the same ballpark as the DC meter. The Iota charger probably isn't as efficient as an inverter but it should give some indication of the losses to be expected. 10 years ago there would have been personal web sites that showed these kind of tests. Too bad Google killed the net.

Bruce

[iconicflux]

Have a look at the Norcold Polar line. Their documentation says the 12v 8cu/ft fridge pulls .93A.

Probably too big for a teardrop but it's what I got for my camper.

Just for fun I threw a watt-hr meter on my 120V kegerator fridge to see how efficient it is compared to my 12v fridge. It used 295 w-hrs over a 24 hour period. That's 22 a-hrs at 12V, which is right around what my Tundra 12V compressor fridge would use at 78F ambient. But WTF, the kegerator is about twice as big as the Tundra! I expected it to use at least twice as much energy given the larger size. I have the
kegerator set at around 34F and it's down in the basement. It' pretty cool down there which helps a lot with energy use. I think I'll meter it some more and see if that number is correct.

Anyone else meter a dorm size fridge to see how much energy they use? I think I'd rather buy a cheap 120V fridge and a nice inverter than an expensive 12V fridge.

Bruce