Solar Refrigerator advice please

[Maggie]

Let me say first that I am in the process of downsizing and selling my property in preparation for using my equity to purchase small rural acreage for an off grid home base and then I plan to build out a rig to travel in part of the year but also live in when at my home base. I thought I wanted a chest type refrigerator such as the Dometic CFX-100W (about 8 CF)  it is a 12 Volt system that uses 1.5AH/h. More recently I been considering something larger and upright.

The refrigerator I'm considering investing in is an upright for off-grid use that is 16.6 CF and is a 24 Volt system that has a [font=Cabin, sans-serif]DC cooling system (Danfoss/Secop compressor), the UGP-470L1 and states that it only draws 865 Wh/24hrs (in a +25°C/77°F ambient, set at -14°C/+7°F in the freezer and +4°C/39°F in the refrigerator)  I was going to post a link to the refrigerator but since I'm not sure if that is allowed in this forum I haven't done that. [/font]

[font=Cabin, sans-serif]I have several questions if anyone is knowledgeable about the 12 volt and 24 volt systems. I want to how do I compare the Wh usage from the 24 volt system refrigerator with the Ah usage of the 12 volt refrigerator. Also, I assume I'd have to wire the 12 volt AGM batteries, that I plan to buy, in series to achieve the 24 volt system, but I'm not sure how much solar and batteries I would have to dedicate to this refrigerator. I'd sure appreciate some advice from some of you with experience in these systems. Thanks![/font]

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[Tom_M]

Here's some specs I found for the fridge:

• [font=arial, helvetica, sans-serif]Total Amp Hr. Draw (24V DC): 28 Ah/ 24 hrs (set to -14°C freezer/ +4°C fridge performance in a +25°C ambient)[/font]
• [font=arial, helvetica, sans-serif]Power Consumption: 148W on 24V[/font]
• [font=arial, helvetica, sans-serif]Avg. Run Current: 6.2A on 24V[/font]

Rather than convert your coach to a 24 volt system it would be simpler to use a DC-to-DC converter to boost 12 volt to 24 volt for the fridge. The run current would then be 12.4 amps. The power consumption would remain the same at 148 watts.

 

[Seminole Wind]

dont forget that the dc to dc converter is not 100% efficient and will add to the power consumption. the quality of the dc-dc converter will dictate how much loss but you can likely count on a minimum of 10% extra in general. it also looks like she is building out a rig and may not have a built "coach" that needs converting?

to the original poster
the basics to compare the 24volt watt hours to the 12 volt amp hours you would use the standard formula of "volts X amps = watts" watts will stay the same regardless of volts, so i recommend converting everything to watts for basic comparison.

as a general rule of thumb. when volts go up amps go down to do the same work. so if you had a fridge like mine that can run on 12 or 24 volts it will use half the amps when running on 24 volt vs 12.  so in your case for a quick estimate you could divide the amphour measure for the 12 volt unit to be comparable to the 24 volt unit.

or to be more clear do the full math. take the 12 volts times it by the 1.5ah/h that would be 18watt hours per hour and then times 24 hours to get 432 watt hours per day. these number seam reasonable for a simple comparison, the big fridge is 2x the capacity and looks to use about 2 times the power.

be advised in real life use it may be different than the test numbers the manufacturer provided. there are no standard test conditions for comparing fridges. some variable they often dont list in their numbers is what was the ambient temp of the room they tested in. was the unit full or empty. did it run 24 hours to stabilize temps before recording numbers. did they do some sort of open and close pattern during testing to emulate going in and out of the unit like a person would through out the day. so you can see if one unit was tested empty as soon as it was plugged in, in a hot garage and they opened and closed the doors through out the test it might not be an accurate comparison to a unit full of food that was run overnight to stabilize the temps in and air conditioned room and the doors left closed.

as the previous response mentioned it is possible to use dc to dc converter to run the 24 volt fridge off the usual 12 volt battery bank. but since you are talking about building out your own trailer you may not already be saddled or tied to a 12 volt system. there are a lot of advantages to a 24 volt system with a 24 volt battery bank, especially if you watt any larger draws like a good sized inverter and if you plan to have a good sized solar panel array. if it was me in your situation based on the info provided so far. i would go with a 24 volt battery bank and build out my trailer around that.

i would suggest you get a good idea "on paper" of what other electrical stuff you want to have, both while on the road and when back on the property. then pencil out a 12 volt system and a 24 volt system and compare the beniefits of each before deciding on whether to go 12 or 24 volt.

 

[tx2sturgis]

Dometic CFX-100W (about 8 CF)  it is a 12 Volt system that uses 1.5AH/h. 

{snip}

the UGP-470L1 and states that it only draws 865 Wh/24hrs 

{snip}

...how do I compare the Wh usage from the 24 volt system refrigerator with the Ah usage of the 12 volt refrigerator. 

Good information has been posted, but here is the answer to the question you asked:

The Dometic, according to the numbers you provided, will consume 432 watt-hours per 24 hours.

( 1.5 ah times 12 volts times 24 hours)

Compared to the other unit, at 865 watt-hours per 24 hours.

Amazingly, about twice the power consumed in a day compared to the smaller unit.

As stated in other posts above, test conditions vary, and so will your results.

 

[Trebor English]

UGP-470L1 is 24 volt. Virtually all the other fridges are 12 or 24 volt.

 

[wayne49]

A 16 cu ft will use twice the power of a 8 cu ft, all else being equal.
Go back to the original post for the capacities.

 

[jonyjoe303]

fridge specs
Largest upright solar DC fridge in North America
Outstanding performance/very low DC power drain
Super efficient construction features 10 cm (4") of insulation in the freezer and 6 cm (2.5") in the fridge
Danfoss/Secop compressor - the world's leading DC compressor (2 year warranty)
Has an in-line fuse for protection
Thermostat
Adjustable heavy-duty wire shelves
Storage basket
Vertical can holder
Interior light
Total Watt Hr. Draw (24V DC): 865 Wh/24 hrs (set to -14 deg. C freezer/ +4 deg. C fridge performance in a 
+25 deg. C ambient)
Total Amp Hr. Draw (24V DC): 28 Ah/ 24 hrs (set to -14 deg. C freezer/ +4 deg. C fridge performance in a 
+25 deg. C ambient)

a 12 volt 102 ah agm battery has about 1224 watts (102ah x12 volts = 1224 watts) , but of that you can only use 50 percent (usable) or 612 watts. Since you will connect 2 in series to make 24 volts (the watts won't double), you will have a 24 volt 1224 watts battery (612 usable), you will need at least 4x 12 volt 102ah batteries (24 volt 1224 watts usable - 865 watts = 359 watts left over )to run your fridge for 24 hours and stay within 50 percent of the batteries limit. You will only get the usable watts, if you fully charge the battery every day. The agm lead acid are very unforgiving if you undercharge them or drain them too low.

 

[Seminole Wind]

fridge specs
Largest upright solar DC fridge in North America
Outstanding performance/very low DC power drain

content snipped

Since you will connect 2 in series to make 24 volts (the watts won't double),

sorry, you mixed that up. it is the amps that dont double. the watts most certainly will.