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Old 09-27-2018, 11:21 AM   #1
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Power usage for A+++ Fridge

I never had a chance to measure the true power consumption of my previous digital inverter fridge/freezer, but as I am shopping now I got in touch with Samsung tech support and an engineer, and received the following information that I though I would share:

Model: Samsung RB31 Fridge (top) / Freezer (bottom)
Size: 11.5 cubic feet (7.3cf and 3.5cf net)
Motor: Digital inverter technology
Energy consumption:
* 172 kW annually
* 480 watts daily
* 725 watts running with defrost mode

Therefore it will consume 530 watts daily (at 90% inverter efficiency) = 67 amps at 12VDC, or about 63 watts per cubic foot per day. Since it has a digital inverter motor it should run just fine on a 800 watt PSW inverter.

I wonder how that compares to the expensive marine fridges and the Danfoss homemade fridges that so many sailboaters brag about?
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Old 09-27-2018, 11:47 AM   #2
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Quote:
Originally Posted by makobuilders View Post
I never had a chance to measure the true power consumption of my previous digital inverter fridge/freezer, but as I am shopping now I got in touch with Samsung tech support and an engineer, and received the following information that I though I would share:

Model: Samsung RB31 Fridge (top) / Freezer (bottom)
Size: 11.5 cubic feet (7.3cf and 3.5cf net)
Motor: Digital inverter technology
Energy consumption:
* 172 kW annually
* 480 watts daily
* 725 watts running with defrost mode

Therefore it will consume 530 watts daily (at 90% inverter efficiency) = 67 amps at 12VDC, or about 63 watts per cubic foot per day. Since it has a digital inverter motor it should run just fine on a 800 watt PSW inverter.

I wonder how that compares to the expensive marine fridges and the Danfoss homemade fridges that so many sailboaters brag about?

That is quite good. What he's quoting are the Energy Star test results, and I have found them to be very accurate predictors of actual performance.
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Old 09-27-2018, 11:51 AM   #3
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First the fact that it has an "inverter motor" really doesn't say much about its compatibility with a 110V PSW inverter output. Some electronics does not act well on a modified square wave source.



I have a few points of comparison with sailboat custom fridges cooled with Danfoss compressors. Both were approximately 8 cu feet- 6 fridge and 2 freezer both cooled by the same evaporator. One was very well insulated but used a simple air cooled Danfoss compressor/condenser. The other had the standard manufacturer's insulation, but the cooling was with a keel cooled condenser with a Danfoss compressor. So each had their own beneficial features and their power consumption was similar.


Each required 50-75 amp hours at 12v to operate. This is 600-900 or 75-110 watt hours per cu foot. So your 63 watt hours (I presume this is what you mean) per cu foot sounds very good.


Your system uses an "inverter motor" but so does a Danfoss system. The other important thing for a marine system is not to size the compressor too big. Small is usually better for efficiency.



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Old 09-27-2018, 03:04 PM   #4
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The fridge on my boat uses an average of 1700 watts/day, but then you must factor in that my fridge is bigger, new fridges are about 40% more efficient than 11 year old versions like mine, and the energy star data doesn't take into consideration the extra energy required to cool all the warm stuff placed in the fridge on provisioning day. So the data that you received from Samsung sounds about right and is entirely plausible.

After you install your new fridge, test it for a week with a Kill-a-Watt, or similar, and let us know the results.

I can't answer your question as to how this compares with an expensive "marine" fridge. Marine fridge makers have chosen not to participate in the Energy Star program, and I know of no other comprehensive objective testing of these units.
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Old 09-27-2018, 03:22 PM   #5
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On provisioning day if you buy already cooled stuff or throw a bag of ice in with the groceries it can help a lot. If that isn't possible, try to shop at night so stuff doesn't heat up on the way home.
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Old 09-27-2018, 08:10 PM   #6
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Something to consider and I don't know how those ratings are established, but in the house with air conditioning in the summer the house still stays in the mid-70s wear that same unit on your boat is working in temperatures that can exceed 95 degrees. Makes a big difference I think
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Old 10-15-2018, 07:02 PM   #7
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I have a custom built fridge and freezer on the sailboat. It is insulated with vacuum panels, R50. What I learned was, one of the biggest contributors of heat to a box is warm things put into it. You can do the calculations to convince yourself, or do measurements. A six pack of beer stuck in the fridge costs you about 60 watts. On a boat with 3 or 4 people, between sticking stuff in and opening the door, it isn't hard to get a couple hundred watts even if the refer is 100% efficient and lossless. They guys helping me design the box said figure about 1/3 heat gain through the walls, about 1/3 in edge effects (doors, seals, corners), and about 1/3 putting warm stuff in. Measurements confirm this.

I've got a Nova Kool on it's last legs in the tug, NK says 650W/day. I'm replacing it with a Isotherm, they say 840W/day. That makes 480W sound pretty good, just don't ever open the door or put anything warm in it.
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Old 10-15-2018, 07:16 PM   #8
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Well that makes sense. That is the purpose of the refrigerator to remove heat from food. But that is the whole purpose. You could just leave it unplugged and it would not use any energy
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Old 10-16-2018, 06:23 AM   #9
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"I wonder how that compares to the expensive marine fridges and the Danfoss homemade fridges that so many sail boaters brag about?"

Most mfg. will have similar mechanical power use , if of the same era.

The difference is the thickness and quality of the insulation.

The sailors top loading style helps a bit , but the ability to site the reefer in a location with 4-6 inches of highest quality insulation .

Exterior insulation can be added (glued on ) to a std house box.
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Old 10-16-2018, 10:07 AM   #10
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FF, adding insulation becomes more complicated with residential units because most use a sidewall for cooling the condenser coils. I've contacted the technical support department at Samsung and they insist on a certain air gap (clearance) on all sides and top. I suppose if one can identify exactly where the coils are, then the rest of the unit can be further insulated.
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Old 10-16-2018, 02:56 PM   #11
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"because most use a sidewall for cooling the condenser coils


I guess that is the price of a few more cubic inches of interior volume.

The Sun Frost uses nice thick insulation , but is pricy, and of course ,big outside compared to inside, the price of efficiency.

Sun Frost: Energy Efficient Refrigerators, Freezers, and Sustainable ...


www.sunfrost.com/
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