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Old 24-11-2015, 08:23   #1
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120V Reefer off Inverter?

I think I've gone as far as I can go with my 12v refrig. My re-insulation of the old converted ice box didn't go well; it still comes on too often for my taste and is noisy, and creates lots of condensation in small, dark spaces.

There are 12v/120 small reefers available for under $200, Avanti to name one. I was set to buy one til I remembered that my hero Sailorchic runs her 120v off an inverter. For the same $200, I can buy a 120v unit and a decent inverter that I need for my ShopVac, chargers and the like.

These apt frigs consume about 1.5 amps which is way better than my AB unit. Curious about the loss to the inverter but assume I'll still be far ahead of the AB and have some good noise abatement as well.

Am I overlooking something?
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Old 24-11-2015, 08:37   #2
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Re: 120V Reefer off Inverter?

I don't think so, I'd get me a decent 300W pure sine wave and go for it.
Check though that a 300W will run your shop vac, some I think are pretty high power.

On edit, you will lose some efficiency, I'd guess about 25%, but gain the ability or running anything else you want, depends on how tight your energy budget is I'd guess.

Another edit, if your saying the apt fridge consumes 1.5 amps at 120 V , then of course that is 15 amps at 12 V, which is a lot, but duty cycle means everything, meaning if it only runs 1/3 of the time, then it will consume the same as one that runs continuously, but pulls 1/3 the amps
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Old 24-11-2015, 09:00   #3
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Re: 120V Reefer off Inverter?

1.5 amps at 120 volts is 15 amps at 12 volts out of your batteries, plus the loss due to inverter inefficiencies.

The best way to go is a Waeco/Dometic fridge that runs on 12/110 volts. Costs more, maybe $600 for a 40 - quart unit, but it will chill quickly and even freeze stuff, as opposed to the $200 thermo units. Will use 35 or so amps a day.

Another issue with RV fridges is that they are vulnerable to corrosion problems and sometimes are wired improperly for boats.
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Old 24-11-2015, 09:02   #4
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pirate Re: 120V Reefer off Inverter?

I knew I wasn't considering the whole picture. A secondary goal to operating the "lower" amperage 120v unit was the hope that my wind gen could keep up with it, which it cannot with the AB. But if wishes were horses ... Thx
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Old 24-11-2015, 09:31   #5
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Re: 120V Reefer off Inverter?

Quote:
Originally Posted by Blue Crab View Post
I think I've gone as far as I can go with my 12v refrig. My re-insulation of the old converted ice box didn't go well; it still comes on too often for my taste and is noisy, and creates lots of condensation in small, dark spaces.

There are 12v/120 small reefers available for under $200, Avanti to name one. I was set to buy one til I remembered that my hero Sailorchic runs her 120v off an inverter. For the same $200, I can buy a 120v unit and a decent inverter that I need for my ShopVac, chargers and the like.

These apt frigs consume about 1.5 amps which is way better than my AB unit. Curious about the loss to the inverter but assume I'll still be far ahead of the AB and have some good noise abatement as well.

Am I overlooking something?
We consume less than 6A from our keel cooled 12V frigoboat fridge and freezer.

Insulation done well will reduce your duty cycle but not your running consumption. Insulation slows down heat transfer.

Anything you run through an inverter will have losses. Both ac to dc and dc to dc. Any cheap domestic unit will be a power hog.

Condensation and frosting can be reduced by keeping the cold, damp air mass moving.

You might like to record the temp deltas in your system. This will help you identify what to tackle first.

You dont indicate whether your refrigerant circuit is functioning properly. Is your compressor working properly?

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Old 24-11-2015, 09:46   #6
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Re: 120V Reefer off Inverter?

I believe SC's major objective was money consumption, that is if it cost thousands of dollars to get a "real' boat fridge to save a few amps, that she was better off spending a couple of hundred in Solar that more than made up for the electrical inefficiency of the Dorm fridge, or at least that was my understanding.
We often spend a whole lot more for just a little increase in efficiency
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Old 24-11-2015, 10:16   #7
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Re: 120V Reefer off Inverter?

As someone who has done this for, gee 8 years now, here's my take. A small 120V fridge will pull 1.4 to 1.6 amps at 120V when it's running. I've found that on a hot day, 90 degree California delta day, my fridge would run 2-1/2 minutes in 14 minutes, or roughly 10 minutes an hour.

Yes when it runs it's using 15 amps, which is heart stopping lots, but it only runs roughly 10 minutes per hour. So that brings the Amp/hr usage in at 2.56 Amp/hr, or about what a average 12v system would use.

I use a cheap 700 watt modified sine wave inverter that has a 10% loss. I figure it's costing me about 12 amps/hr's per day for the inverter. True sine wave inverters will double that loss and really are not needed for a cheap compressor motor. I've not had any problems running things (cell phone changer, laptop or fridge off a modified sine wave inverter.

You will want a 700 watt inverter to handle the starting inrush current for the fridge compressor, which is quite high. A 450 watt inverter may or may not start the fridge.

So for $200 ish you can make it work.

Now, after doing this for 8 years, what I want to do is replace my old magic chef dorm fridge with a edgestar 3.1 cf fridge with a separate freezer. EdgeStar 3.1 Cu. Ft. Energy Star Fridge/Freezer - CRF321SS

Why Edgestar. Because it's rated at 80 watts or 1/2 what most fridges in the same size are rated at. Plus the freezer section works better that is it will keep ice cream frozen, where a regular dorm fridge will not.

One thing I notice is that the fridge like I have with a un-insulated freezer section (really just the evap coil) has too high a temperature swing to keep the frozen foods frozen.

Note that there are lots of small fridges out there but most have really high amp requirements, as does my magic chef I suspect the compressor in the edge star has a smaller displacement and it may run somewhat longer then the cheaper models. Its one thing I want to look at.

One thought I had to reduce daily energy usage, was to mount a cheap inverter to the fridge and wire the fridge thermostat to the inverter power switch. That would save the parasitic inverter losses when the fridge is off. I've not done that yet.

I'm hoping to get the new fridge after Christmas this year and replace my 8 year old fridge that is using now using 10 more watts per hour then when it was new. I will of course report my findings to the board here.
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Old 24-11-2015, 10:33   #8
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Re: 120V Reefer off Inverter?

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Originally Posted by a64pilot View Post
I believe SC's major objective was money consumption, that is if it cost thousands of dollars to get a "real' boat fridge to save a few amps, that she was better off spending a couple of hundred in Solar that more than made up for the electrical inefficiency of the Dorm fridge, or at least that was my understanding.
We often spend a whole lot more for just a little increase in efficiency
That was part of it. My previous boat had a 12V fridge in it and it used about 3.5 amps/hr an hour for a 6-7 CF box. It was not insulated very well and it was Florida, so was a pig.

Myself I know that energy use would generally be close to the same for a given box size. Yes having a well insulated box will greatly reduce heat transfer.

It was partly to save money and partly to see if it could be done. Everyone said you cound not do it. But being an engineer and blonde I figured for $200, lets find out.

Really the daily energy load of the 120V fridge is not much higher then a average 12V fridge. Yes having a well insulated box will greatly reduce heat transfer.

But I do not think I use appreciatively more energy at 120V then a 12V compressor would on average. Yes some folks only use 30 amps/hr a day, but that's more a factor of box insulation.

I should note that I have a total of 320 watts in solar panels and a PWM solar controller, Just a basic system. I use roughly 50 amp/hr a day for the fridge more at 100 degrees less in 50 degree weather. The rest for my cad laptop, wifi thingy and lights.

Put it another way, a water heater can have a 1000 watt or 4000 watt heating element. One uses more power then the other. But the daily energy load or cost will be very close to the same for the same size tank. One just runs a bit longer. It's similar with fridge compressors.

Ok that's a simplification as some compressor systems dol have better efficiencies, but it's ballpark anyway.
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Old 24-11-2015, 11:33   #9
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Re: 120V Reefer off Inverter?

you will use less power running it of 12v then invertering. if you can buy a dual one.


also you need an inverter that can handle the high start up loads. the last small fridge I tried to invert, I could not start it with a 1000w inverter. even though fridge was only a couple hundred watts. the draw at start up is many times the running draw
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Old 25-11-2015, 09:06   #10
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Re: 120V Reefer off Inverter?

I always find it interesting when this subject comes up. The inefficiency of front loading refrigerators is seldom discussed.
When opening a front loader all the cold air is immediately lost as cold air descends. This is why reach in top loaders have been the standard for so long.
Yes if we're single handing and are limiting our access to the front loader it can be a good answer, however if you have more than one person accessing the fridge then your cost to re-cool after loosing all the cold air equates to the box size, thermal mass of the contents. An empty fridge will take longer to cool than a full unit. Time is energy.
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Old 25-11-2015, 09:32   #11
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Re: 120V Reefer off Inverter?

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I always find it interesting when this subject comes up. The inefficiency of front loading refrigerators is seldom discussed.
.
True, But cold air has little mass so in reality not a lot of cold (actual BUTH) is loss. All the cold food stored in the fridge and freezer will easily recover the air temperature inside the fridge.

At most, I'll loose 2 CF of air from the fridge. Air weights roughly 0.0855 pounds per CF so I would loose ~0.17 pounds of air, worst case. Assuming 90 degree air temperature (90-35 degree fridge=55 delta T) that would need (.17*55)= 9.45 btu's to recover. Allowing for 10 openings a day, you need to recover 94 btu's over a days time. That's roughly an extra 2.1 amp/hr's per day, worst case. It's just not that bad.

So while a front loading fridge IS less efficient, It in reality is not that bad and has the advantage of not having a top load open more looking for that packet of food at the bottom. It's all trade offs.
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Old 25-11-2015, 10:36   #12
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Re: 120V Reefer off Inverter?

I run an Isotherm Cruise 65 (about 2.3 cu ft) fridge with "freezer" being the evaporator coil box (with a flimsy door) It draws 2.75 amps (@12Volts) normally but shifts up/down as temps demand. It runs happily off an 80Watt solar panel (puts out about 4-5amps in normal 'sun'). It has a small Danfoss compressor, a BD35 I think. These are variable speed controller driven (hence the change in amperage drawn). For good info find the Sea Frost pages and read up on the tech'l sections. No endorsement here, I know nothing about them, nor have purchased from them. They spec BD35/50/80 compressors and controllers for some of their systems.

A fridge with a simple evaporator coil will cycle more often than the same fridge with a holding plate. Fridges with plates are noticeably more expensive. To get around this I pack the freezer box with gelpacks and let them freeze solid. Meaning I have a fridge, with no freezer space available. Poor man's holding plate, works for me. Best add-on is a very tiny 12V box fan inside, permanently 'on' to keep the air moving past the freezer section.

The BD35 runs off 12-24/120V using their power supply, or 12-24 without. Newest ones have a solar panel option which handles 9-32Volts as input. It appears these can be direct wired to a dedicated solar panel without any controller. I stand to be corrected here, anyone running one of these systems? I prefer having a controller and excess panel amps keeping the batteries topped up. Have to wonder how the fridge runs at night/clouds otherwise!

Bottom line, with the power loss inherent with inverters I'd never consider a household grade 120V only fridge. Leaving build quality /insulation /weight /installation issues aside. Apologies to SailerChic, but a big thanks for the thermal analysis on front loading fridge door opening.
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Old 25-11-2015, 17:22   #13
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Re: 120V Reefer off Inverter?

I have a 7.5 c.f. Avanti fridge running off a Xantrex 2000 PSW inverter, 5 years running.

My Tri Metric battery monitor shows 9.8 amps draw, 12 volt, when only the fridge is running. That includes the inverter losses. It runs about 20 minutes per hour.

The Xantrex draws .5 amps ideling. Many others draw more. That you need to check.

I had a cheap inverter that drew 4 amps idle. That was more then the fridge, over a 24 hour period.

Check the yellow energy tag that the government requires in all appliances. Mine said 365 KWH per year. 1 KWH per day.
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Old 25-11-2015, 17:32   #14
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Re: 120V Reefer off Inverter?

I use an apt. sized 120v refrigerator with separate freezer door. 4+ years and still happy. One does have to be aware of the amount of time the door is open but I can see better when looking for something so I can get the door closed fairly quickly. If it goes belly up it can be replaced at any big box store for around $400. Very cheap and I have frozen meat and ICE CREAM! Worth a using a little more power every day.
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Old 25-11-2015, 20:58   #15
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Re: 120V Reefer off Inverter?

But cold air has little mass so in reality not a lot of cold (actual BUTH) is loss. All the cold food stored in the fridge and freezer will easily recover the air temperature inside the fridge.
Incorrect as the specific heat capacity of air is low

sailorchic34 is similarly mistaken in describing energy usage as Amps/hour
Correct is Amp-hours
Her calculation of energy loss from opening door is suspect.

"At most, I'll loose 2 CF of air from the fridge. Air weights roughly 0.0855 pounds per CF so I would loose ~0.17 pounds of air, worst case. Assuming 90 degree air temperature (90-35 degree fridge=55 delta T) that would need (.17*55)= 9.45 btu's to recover. Allowing for 10 openings a day, you need to recover 94 btu's over a days time. That's roughly an extra 2.1 amp/hr's per day, worst case. It's just not that bad.
Allowing for 10 openings a day, you need to recover 94 btu's over a days time. That's roughly an extra 2.1 amp/hr's per day, worst case. It's just not that bad. "

"Not that bad" but very wrong in heat energy and electrical energy calculation,

Assuming accuracy of 94 BTU (which it is suspect) and converting it to Amp-hours at 12 volt we have 94 BTU = 28 Watt-hour = 28 X (Volts x Amps)-hour = 28 x(12 x Amps)-hours
which is 336 Amp-hours at 12 volts

Something wrong in Sailorchick34 calculations
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