Refrigerator power consumption question

[CharlieO.]

If looking at the specs of a 12v refer and it claims rated current to be 9 amps
how does that convert over to amp hr. usage per day?

If it is 9 amps then it would be safe to say that my alternator would have no problem keeping it running while underway?

I only have one battery bank of two batteries for start/house loads with the basic Motorola alternator on my Lehman.

I would am also probably going to add a small house bank in the future and would like to size it correctly to run the fridge for at least 24 hrs. at anchor or docked without shorepower, but would at least like to run the unit while out cruising to start the season.
*
Thanks for any info.

 

[GoneFarrell]

9 x 24 = 216 amp hours. Assume 50% duty cycle (swag) 108 a-h over 24 hours. Likely running current is less.

Usually want to discharge no more than half rating, that says a battery bank about 220 a-h. Feel free to correct me on this.
9 amps seems like a lot, my never-cool pulls under 5 amps.

You may want to consider a couple solar panels for summer too when duty cycle is higher.

 

[O C Diver]

9 amps refers to the start up or largest draw rate and usually not the running amp draw. Further, it depends on how much the compressor runs which is based on the ambient temperature of the inside of your boat, how often you put warm groceries in, how often and how long the door is open, and the insulation R value.

In short, amp draw has no bearing on power consumption unless you leave the door open all the time.

Ted

 

[CharlieO.]

I'm thinking about adding a 200 amp hr. Lifepo4 battery with BMS to start out with. I just don't want to undersize the battery from the get-go.
I only have room for a small under counter fridge around 33 inches tall.

 

[Jeff F]

My Norcold uses about 100ah per day on average. More in hot weather.

 

[Marco Flamingo]

I have a new undercounter 12V GE fridge. 9 amp startup, 3 amp running, but there are some weird things about it. I put a Bluetooth recording thermometer in it and was surprised at the temp swing. It only comes on a couple times a day according to the thermometer (when we aren't at the boat). On for three or four hours, then off for eight. Temp in the crisper drawer drops to 29 degrees, then compressor turns off until the crisper temp is mid-40s. It wasn't below freezing long enough for anything to freeze in the crisper, but didn't seem right. Amp usage overall is low, but temp swing is excessive.

Part of that problem was that the temp sensor was right under the freezer plate. I moved it further down into the fridge and that helped. I've tried a few other things to see if I can get the temp more stable (like Blue Ice in the freezer since we don't use it). Overall, I'm happy, but I was hoping for perfection with a new fridge. Not the case (yet). Maybe it's just too much information from the recording thermometer.

 

[Comodave]

To help conserve power make sure that the refer is properly vented, low intake vent and high discharge vent to let the hot air out of the cabinet. I also put a low amp draw fan in the cabinet to help evacuate the hot air. The one I use draws 50 mAmps.

 

[Mambo42]

Most important with a fridge or freezer is that the insulation is up to speed. If that is done well and you don't open the fridge too often the fridge just needs to maintain temperature which should only draw 1/2 or 1/3 of the listed amps. So if it says 9 amps you should calculate with 3 amps per hour.
However, if the fridge is of poor quality and the compressor needs to run all the time to keep the fridge at temperature you will be using 9 amps each hour, so times 24 hours will turn that into 216 Ah.
So best tip is to look for a very energy efficient fridge, which may cost a bit more when you buy it, but it will save you tons of money afterwards.
I have 1 large fridge/freezer, 1 small freezer and 1 small fridge (220 V) plus 1 x 12 V fridge onboard. I keep them full so I have a cold bank in the fridge and for all that cooling equipment I use about 150 W at 220 V or 0.75 Amps at 220 V or 7.5 Amps at 24 V (which would be 15 amps at 12 V). So 9 Amps for a 12 V fridge is a bit much in my opinion, but I guess that is rated power, it is not what it will do if you keep it in a cool place, don't open the door too often and only put cold stuff in that fridge.

 

[CharlieO.]

I haven't zeroed in on any particular fridge yet, the 9 amp one was one I came across is one that would fit my existing open very well. You have all provided very good information.

 

[O C Diver]

I haven't zeroed in on any particular fridge yet, the 9 amp one was one I came across is one that would fit my existing open very well. You have all provided very good information.

Don't know if you have considered an apartment or under counter home unit and running it off an inverter.

You've probably seen this thread on my apartment refrigerator installation:

https://www.trawlerforum.com/forums/s31/my-new-apartment-refrigerator-41392.html

I found the Energy Guide (yellow label) to be quite accurate. If you look at apartment units which are often more efficient (better insulation), you can take their yearly KW consultation, divide it by 365, and divide that by 12 (volts DC) to get an approximation of amp hours per day. You'll need to multiple that number by 1.2 to cover inverter conversion and stand-by consumption.

Ted

 

[CharlieO.]

Don't know if you have considered an apartment or under counter home unit and running it off an inverter.

You've probably seen this thread on my apartment refrigerator installation:

https://www.trawlerforum.com/forums/s31/my-new-apartment-refrigerator-41392.html

I found the Energy Guide (yellow label) to be quite accurate. If you look at apartment units which are often more efficient (better insulation), you can take their yearly KW consultation, divide it by 365, and divide that by 12 (volts DC) to get an approximation of amp hours per day. You'll need to multiple that number by 1.2 to cover inverter conversion and stand-by consumption.

Ted

Thanks Ted, I have considered and am still considering using a 120v fridge and inverter, I just haven't compared the numbers yet.

 

[CharlieO.]

Thanks Ted, I have considered and am still considering using a 120v fridge and inverter, I just haven't compared the numbers yet.

I did come across a 120v that would fit nicely claiming 1.5 amps usage.

 

[mvweebles]

I have a pretty decent energy monitor at a condo in Yucatan so pretty warm climate. 16cf fridge uses about 500wh/day which works out to around 45-Ah at 12V. A single 100W panel might power that much in mid-summer but not in other seasons.

As an aside, my full size 3kw Magnum inverter has a meaningful standby current, probably approaching 20-25 Ah/day (12V). Suggestion to use a smaller inverter to power a fridge makes sense.

Peter

 

[twistedtree]

I did come across a 120v that would fit nicely claiming 1.5 amps usage.

See if you can get the Energy Guide info for it, as Ted says. I too have found that info to be very accurate for actual, daily power consumption.

 

[CharlieO.]

See if you can get the Energy Guide info for it, as Ted says. I too have found that info to be very accurate for actual, daily power consumption.

I'll have to stop in there today, it says they have one in stock locally. I'll see if they have the Energy Guide info on it.

 

[CharlieO.]

This is the one I think I will go with. It is on sale for $87 and in stock locally. Does this seem like reasonable on energy consumption? And would 200 amp hr of lifepo4 be enough for a 24 hr. run? Plus an inverter.

Thanks.

 

[twistedtree]

If you go to the web site show on the bottom of the sticker, you should be able to find a list of all fridges and compare to others of like size.

 

[SteveK]

I did some math and stand to be corrected.
That fridge will draw ~2.5A average per hour or 60 amps in 24hr leaving ~140A in the battery.

If the fridge is the only draw IMO yes it will work.
A 200Ah battery will give you about 8.3A per hour over 24hr before completely depleted.

 

[O C Diver]

This is the one I think I will go with. It is on sale for $87 and in stock locally. Does this seem like reasonable on energy consumption? And would 200 amp hr of lifepo4 be enough for a 24 hr. run? Plus an inverter.

Thanks.

My math works as follows:

228 KW per year ÷ 365 = .624 KW per day or 624 watts per day.
624 watts ÷ 24 = 26 watts per hour
26 watts ÷ 12 volts = 2.17 Amp Hours
2.17 × 1.2 (inverter conversion and stand by loss) = 2.6 AH per hour.
2.6 times 24 = 62.4 AH per day.

This is basically the same as Steve posted, but shows the math should anyone else be interested.

Ted

 

[CharlieO.]

With the given information what size inverter would be recommended? Thanks

 

[SteveK]

With the given information what size inverter would be recommended? Thanks

If the fridge is all you will be running then a 100-300 watt inverter should be enough. Just inverter. You need to allow for a spike on startup.

But if you also want an inverter charger then go with a 2000 W unit that can fast charge at about 100A too. This will give you option to plug in other appliances like coffee maker, toaster etc.

 

[O C Diver]

With the given information what size inverter would be recommended? Thanks

Victron Phoenix inverter model 12 / 500 (12 VDC 500 watts). Pay very little more for the extra capacity and these Phoenix models have the same low stand-by draw. For the critical nature of a refrigerator, I would like to be operating in the bottom third of inverter capacity. It's around $150.

You could also choose the 375 watt unit, but nice to have extra capacity.

Ted

 

[Max1]

It depends on the duty cycle, and how many cubic feet your fridge is.
The temperature you set also affects the duty cycle.
If you check my thread on my MT 36 project In the taiwanese trawler thread, I posted a couple of graphs of the various duty cycles based on temperature setting. In the real world, I don't subscribe to shore power at my marina. I run my fridge 24-7 all summer from solar. In a twenty four hour period, I will draw about fifty amps from my battery. However, The impact is zero during the day because the sun is shining, So the overnight draw of my fridge is about twenty five amps, Easily recovered the next morning when the sun comes up.
I think my fridge is six cubic feet.

 

[Pete Meisinger]

Most 12 volt refrigerators will kill a single battery in less than a day. Of course a multiple battery system will be a big help but..

The best thing you can do is turn the refrigerator off at night when you go to bed. It should hold just fine until you fire up the genny for morning coffee and hot water.

pete

 

[rslifkin]

The best thing you can do is turn the refrigerator off at night when you go to bed. It should hold just fine until you fire up the genny for morning coffee and hot water.

pete

Without a whole lot more insulation and being set much colder, most refrigerators won't stay anywhere near a safe temperature for food (under 40 degrees) all night without running. If you're keeping any kid of meat or other sensitive items in the fridge, turning it off at night is a terrible idea without some kind of temperature logging in the fridge. At a minimum, food will go bad much sooner.

If you can't keep the fridge running overnight, then you don't have an adequate electrical system to support having a fridge at all, it's that simple.

Overnight is usually less power use than during the day for the fridge for us, as it's cooler at night (fridge doesn't work as hard) and the fridge isn't opened all night. Typical overnight power use for our fridge is in the 20 - 30 ah ballpark at 12v based on the data I have. Even if it ran non-stop all night it would only use somewhere around 50 - 60 ah.

 

[CharlieO.]

Most 12 volt refrigerators will kill a single battery in less than a day. Of course a multiple battery system will be a big help but..

The best thing you can do is turn the refrigerator off at night when you go to bed. It should hold just fine until you fire up the genny for morning coffee and hot water.

pete

Sorry Pete, no genny on this boat. I think I've finally settled on the 120v fridge, DC2DC charger, inverter and a 200 ah Lifepo4 battery, and possibly add a bit of solar later on.

I think this will complement and enhance our use profile. We are mostly weekend warriors and generally like to dock at night but plan on working on anchoring overnights.