How big of an inverter

Hi everyone,
We bought a 9 cu. ft. ac Magic Chef refrigerator when our Dometic ac/dc frig broke. We want to run the new refrigerator off of an inverter.
The question is what size should we buy? I could not find the electric stats only a paper telling me how much money a year we would save.
Any ideas.

Thanks
Gordo

Need some specs for the target fridge. Watts/year, amps/year (or hour), start-up amps, etc. Maybe if you can find the manual it'll have full specs.

One question inverter/fridge specific is about whether you'll need pure sine wave inverter (more expensive) or not. Maybe other folks can speak to that...

The "save per year" factoids are likely not taking into account the cost of an inverter, the cost of installation parts and labor (even if you do it yourself), the potentially-increased cost of batteries over time, etc. You might want to factor that stuff in, too...

-Chris

Didn't see the amp draw specs on the online manual. The is usually a sticker in side the fridge with those specs.

Ted

I run an under counter domestic fridge on a 1000W inverter. On my AC panel I can see the amps drawn, it is about 1.6A running and about 6.8A on startup. Rig runs just fine on inverter. A bigger fridge will probably draw more. Get an AC clamp meter and measure what the startup amps are. Probably 1500W would give you plenty of margin. I think I am a little tight on margin with my 1000, but works.

And keep in mind that home style fridges that have auto defrosting have a 'heating cycle' to accomplish this, and draw a lot of power in the process.

You could try to work backwards.....

Looks like those in that size range are about 300- kWh per year. Let's use 365 to make it easy.

That's 1kWh per day. At 24hrs in a day, that is 0.0416 kW average operating power, or ~42 Watts.

At 110V, 42 Watts is .38A average. At 12V, this would be approximately 3.5A average, assuming a 100% efficient inverter. At an inverter efficiency of 85%, this would rise to about 4.2A continuous draw from your battery banks, or 100 amp-hours per day.

If 4.2A is average, and we assume a 50% duty cycle of on/off, then I would guess that the current when running would be 8-10A and would therefore require a 200-300W or so inverter in running mode in order to have some margin.

What I cannot find is the startup current of a typical fridge compressor that your inverter would have to support intermittently. Maybe some googling of RV forums or similar "off-grid" groups would produce some general information.

I also think motors/compressors are more efficient and have a longer life on a pure sine-wave input.

Just a swag....

O C Diver said:

Didn't see the amp draw specs on the online manual. The is usually a sticker in side the fridge with those specs.

Ted

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There is always a label with the power requirements and consumption. Often, it is on the rear near where the power cord enters.

Rule of thumb, starting current is approximately 6X running current for household AC motors. Inverters can handle this short term surge.

If I were getting a new inverter I would spend the few dollars more and get full sine wave

If there are no specs on the current draw and you don't have a clamp-on ammeter, buy a $20 Kill-A-Watt meter to measure its start-up and running power requirements.

Good math boathealer. However the simple solution to startup power is to purchase a digital inverter refrigerator. Then your startup multiplier is 1.0

If you want to determine startup amps, use a clamp meter or other meter and let fridge run for about ten minutes and turn it off. Then turn it back on again and take reading. Compressor is stalled from high head pressure and will just sit there and hum. Get your reading and turn it off.

I use a 3000 watt MSW inverter, and it runs everything except the AC.
If you ever want to run something else and the fridge at the same time, it makes sense to go bigger. I have a 6500 watt Onan gen so the inverter is more of a backup. And also no noise, which is nice.
I seem to recall trying a 750 watt 1500 watt surge inverter, and it could not start the fridge.


I also use a UPS in inverter mode to power my nav PC, was a freebie. I disabled the buzzer by cutting its trace on the board. I wired it direct to the house batteries, in parallel off the lugs of the 3000 watt inverter. They sit next to each other. I have backup of the backup for the nav pc and it will run some LED AC lights too that little UPS inverter.

Buy big! Cost difference is small between wattage ranges and also consider Ebay!

Most in fact all the apartment size fridges that I am aware of have self defrost, my Summit 8.2cuft included. My 1KW true sinewave Xantrex used to burp when the self defrost cycle began with a displayed load of 800-850 watts. That inverter now rests comfortably in inverter heaven...%$#@!

Next I replaced it with a 2KW sine wave inverter with a surge rating to 2.5KW. That inverter stayed with me until I really put it to the test this summer when our genny had a problem....since repaired. Wifey attempted to microwave something that required 10 minutes. She checked the food after 5 and replaced it in the microwave for another 5. Just as the microwave's buzzer buzzed alerting her to the cooking was complete, I noticed smoke coming from the cabinet where the inverter was mounted.

Yeah.... real smoke! And where there is smoke there is...well, you know. I quickly shut down its power source and opened the windows. Fortunately I had another spare inverter to carry us over for the rest of the trip. I replaced my spare with a 3KW with a 3.5KW surge and that now sits where the original Xantrex sat. Time will tell if this beast holds up.l

Good points, with my 3000 watt inverter, I can run the fridge and the MW oven and no burning out the inverter..

sdowney717 said:

Good points, with my 3000 watt inverter, I can run the fridge and the MW oven and no burning out the inverter..

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I just checked my purchase history on Ebay where I purchased my inverter. It's a 3.5KW true sine wave unit rather than 3kW in my above post that I paid $338 bucks with free shipping. In any case, buy big and consider ebay.

Mine was listed as an EDECOA Power Inverter at Ebay if that helps.

foggysail said:

I just checked my purchase history on Ebay where I purchased my inverter. It's a 3.5KW true sine wave unit rather than 3kW in my above post that I paid $338 bucks with free shipping. In any case, buy big and consider ebay.

Mine was listed as an EDECOA Power Inverter at Ebay if that helps.

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You got a good inverter for that price. How long have you been using it?

Some numbers - my boat came with a old 2200 watt inverter that ran a house type double door reefer and small box freezer. Only info on the refer is 6.5 full load amps.

Interesting side issues but for a dedicated inverter (not my choice by any means) most will be big enough. Typical refrigerator will draw 300 watts during the defrost cycle and less than 200 for operations. A 1000 watt inverter should take care of the start up draw.

But then I am just guessing.

Thanks to everyone for all the info.

Did you find the power label yet?

FlyWright said:

If there are no specs on the current draw and you don't have a clamp-on ammeter, buy a $20 Kill-A-Watt meter to measure its start-up and running power requirements.

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Ditto on the”Kill-A-Watt meter
Run for several days and average the usage.
Have done this and it pretty acurate.
Back into the wattage to determine your batt bank size and inverter size.
Go with pure sign wave, and go bigger, you will find many more uses for it.

Do not buy a much bigger inverter than you need. Their efficiency is usually highest near peak.

Bay Pelican said:

Interesting side issues but for a dedicated inverter (not my choice by any means) most will be big enough. Typical refrigerator will draw 300 watts during the defrost cycle and less than 200 for operations. A 1000 watt inverter should take care of the start up draw.

But then I am just guessing.

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I don't know where you get your 300 watts but my inverter's load meter indicated 800 watts were used for my Summit 8.2cuft fridge in the defrost mode.

And those who plan to purchase a 1KW inverter for a fridge with auto defrost should consider going with a higher wattage unit.....but it's your money

kev_rm said:

Do not buy a much bigger inverter than you need. Their efficiency is usually highest near peak.

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If you're that concerned about energy, I suggest you use ice instead of refrigeration

sdowney717 said:

You got a good inverter for that price. How long have you been using it?

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EDECOA Power Inverter 3500W 7000W Pure Sine Wave 12V dc to ac 120V LCD Display

The seller asked a higher price than I paid because he advertised "or best offer." I just recently (August) purchased and installed it.

I suggested something much lower and he reduced his sale price to that I quoted earlier. Life expectancy! Who knows! This I do know, the inverter that went smokey worked better, lasted much longer than my old Xantrex. And yes indeed, I did kill it with our microwave. Maybe the manufacturer overrated the inverter for sales purposes. And that thought crossed my mind when I could have purchased an inverter with a lower rating than I did.

My inverters have always operated 24/7 and see loads only when there is no dock or genny power available. Efficiency??? Who cares! I have 860 AHs of golf carts providing 12vdc plus 1180 watts of solar. But most modern inverters operate with efficiency greater than 90% at full load. At lower loads the efficiency will drop but the total power consumed will of course be less. If in doubt, check the heat of an idle inverter yourself vs one operating under load.

I went the cheap route with a Cheap Chinese Crap (CCC) Xantrex 1000W nine years ago for about $120. Five years later, I fried it by my own fault and replaced it for the same cost, then that one died under the 1-year warranty and Xantrex replaced it no questions. Then I bought another one for $110 that sits in shrink wrap in my onboard spares. So all in, I've got $240 in 9 years of inverter use plus $110 for a spare.

It's used to run a 3 cu ft apt fridge, 900W microwave and an electric skillet. It can run any 2 of the 3 simultaneously.

Al-- the inverter I posted the URL to is Chinese. Cost was just over $100/KW

foggysail said:

Al-- the inverter I posted the URL to is Chinese. Cost was just over $100/KW

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I saw that. Good prices for a pure sine wave inverter.

Sine wave units are usually required to get the best out of heavy loads, air cond etc.

The cheap square wave units are great at small 120V RV or house fridges.

These reefers do not have TX valves to modulate the refrigerant flow , they simply have a pin hole the refrigerant sprays thru.

The head pressure is bled off very rapidly when the unit stops, so large amperage is not required to start it a few min later.

The house fridge danger is the hot door seals and heaters to dry out the thin insulation is not measured in the annual consumption claim.

These heaters can use as much energy as operating the fridge.

Look up the fridge on line from the mfg repair lit before purchase and see how much energy is required.

Although more expensive RV fridges usually have better insulation and no heaters.