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Old 10-08-2012, 10:33 AM   #1
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Efficient fridges?

I'm looking to get a compact fridge (~4 cu ft), not too expensive (~$350 maybe), and am curious about their efficiency. I've seen projects online to convert an old AC fridge into a DC one, but the compressor/etc costs around $1000 alone. I'm also concerned about the current drop over a 15 ft distance on DC wiring (I'd hope it wouldn't need 0 or 2 gauge, since those wires are expensive).

I did find this fridge online:

http://www.amazon.com/Danby-DAR440BL...t+refrigerator

It says the energy consumption is 329 kWh. I tried doing the math to see what wattage it consumes in an hour (on average), and I must be doing something wrong because I got 38W/hr. Which is less than my laptop, rated at 75W (I'm assuming that's per hour). Here's the math I did:

329 kWh-per-year * 1000 = 329,000 Wh / 365 days-per-year = 901 Watts per day

901 Watts per day / 24 hours = 37.5 Watts per hour

If I'm not mistaken, my laptop is rated for 19V * 4 amps = ~75 Watts , which I'm assuming is 75 Watts every hour?

Is this right? a 37 Watts per hour fridge that runs off AC?

I've seen some DC fridges online that run off about 5 Watts per hour (114 Watts per day), so maybe 37W/hr is about right.

Am I getting pretty much a good bang for my buck? 37W/hr 4 cu ft fridge (no freezer) for $325? My understanding is that a full size fridge and freezer can use about 500 kWh/yr or (if my math is right) 1300W/day or ~60W/hour.

Are more efficient options within a reasonable price range? Or is it just cheaper to spend the $85 and get another 109 Amp-hours (59 if you count the 50% discharge limit) on a battery (which would give me about another 15 hours per battery)? I guess what I'm wondering is if I really have nothing to worry about assuming my battery bank is properly sized.

Also, could I surround (some/most) of the fridge with R-6.5 foam panel insulation (that I already have) to add to it's efficiency? And if so, how much might that help?
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Old 10-08-2012, 11:32 AM   #2
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Re: Efficient fridges?

Read a bit more carefully. That's 329 kwh PER YEAR & $35 PER YEAR. To give you a measure, we were on metered electric Nov 2009 - May 2010. We were paying 14 per kwh. From what I gather, metered electric is still running 14 to 15 per kwh. At 14 per kwh you are looking at $46.06 per year/ $3.84 per month.


We have a 4cf GE from WalMart. I have a old smaller 2nd ref that will be replaced with another of the GE (we will have two refrigerators giving us a combined 8.8 cf refrigerator). We put a small 3.2 cf ref in the Class C and lived with it (fulltime) from 2006 until we moved into the bus (Dec 2011). I like the undercounter ref. We also have a 12 cf upright freezer. We are set up as 30 amp.
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Old 10-08-2012, 11:44 AM   #3
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Re: Efficient fridges?

This one will save you some cash.....



http://www.hhgregg.com/danby-4-4-cu-ft- ... DAR125SLDD
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Old 10-08-2012, 11:47 AM   #4
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Re: Efficient fridges?

OOOH. Thanks!

Yeah I took into consideration it was 329 kWh per year. That's why I divided by 365 and then 24 (to get per hour).
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Old 10-09-2012, 12:24 AM   #5
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Re: Efficient fridges?

Actually, I know some people who do that very thing. They are off grid, though they use a veg oil generator.

I'm curious how to get it to not freeze the contents and stay at the 34-36 deg F (or whatever).

My only problem with the chest freezer is that I don't have vertical room to flip up a chest door. That's a lot of vertical space that can't be used for anything else. I only really have space for a side-door unit. At least with a door that swings out, it can just open out into the walking isle. I would need to figure out a way to utilize the space above, without making getting into the fridge totally impractical.

The theory sounds obvious and the premise sounds really appealing. Thicker insulation (by design, as a freezer), cold air sinks and doesn't escape so easily when opened from top, etc.

Actually, now that I think about it, the front of the passenger side (side window #1) is a perfect spot since I don't need the upper space since I need the window visible for driving.
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Old 10-09-2012, 12:51 AM   #6
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Re: Efficient fridges?

I see this 5.2 cu ft chest freezer at best buy for $160 at my local Best Buy, and it only consumes 172 kWh/year, which comes out to 470W/day or 19W/hr. I must be dreaming. My computer uses triple that power. Haha. It seems to be about 1/3rd of the energy usage of that other fridge I found. I'm tempted to go buy it tomorrow. =)

Only downside is that it doesn't have an interior light. Kinda makes it hard to use at night, without a flashlight or lighting directly from above. But I could easily add a lightbulb from above!

http://www.bestbuy.com/site/Igloo+-+...=1218603010333
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Old 10-09-2012, 07:47 PM   #7
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Re: Efficient fridges?

[quote=Iceni John]
Quote:
Originally Posted by "tomas_maly":1010xeoy
I'm curious how to get it to not freeze the contents and stay at the 34-36 deg F (or whatever).
The generally-accepted way of doing it is to use an external thermostat, similar to what home-brewers use. A friend of mine has a cheapo chest freezer with such a thermostat, and it works great. I plan on running mine off its own dedicated inverter, so when the thermostat turns it off (when it's cooled down enough) it will turn off the inverter itself. This way there won't be any power used at all by the inverter while it's not actually powering the fridger. For a light inside, it's easy to put a LED light in there that has a microswitch operated by the door.

John[/quote:1010xeoy]

I think it's worth testing to see how long the freezer stays cold when the power is turned off. I have the impression that it could be up to a day, if not opened, and so I would just make the habit of turning off the inverter itself at night.

I would probably implement 12V lights if needed at night - maybe somehow rewire the built-in ceiling lights to run off the batteries or have their own switch next to the light instead of the single switch being all the way at the driver's seat. I'd imagine it wouldn't be that tough.

I actually bought the freezer I mentioned from Best Buy for $160. It's only 170-ish kWh/year, which is by far the lowest I've seen. That rating is also for it functioning as a freezer, so being used as a fridge, I'd imagine it'd be even more efficient.

I think it's also worth checking if a freezer has a 'refrigerate' mode on the thermostat. I'd imagine most (manual defrost) would. You might not need the external thermostat. Probably worth experimenting with the highest setting and an accurate thermometer to see for sure, before buying the external thermostat. I got a fridge/freezer thermometer and put it inside and it was able to go between 20 and 40 deg F based on the dial setting. I've put a bottle of orange juice as well as a water bottle and will see how they are in the morning.
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Old 10-09-2012, 08:34 PM   #8
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Re: Efficient fridges?

In our first motorhome...... we replaced the RV fridge with an apt size residential fridge. It would stay cool all day during travel,
sometimes it was up to 8 hours but we never opened it during the day while we where traveling. I think anything much more than 8 hours would require a generator or another source of power.
It was during the summer and the temps inside the fridge where still around 40 degrees. give or take a few. It was cool enough that the food didn't spoil and the freezer keep the ice frozen.


Just putting in my 2 cents.....

Roy B.
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Old 10-10-2012, 09:48 PM   #9
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Re: Efficient fridges?



This is what our current setup looks like. We've been using this system for a little over a year and plan to move these two units right into our bus. (They double as a great counter top, hence the grated cheese)



Fridge and freezer go on the left.

Here is our real world evaluation:
Per the Kill-A-Watt meter, the fridge uses 200 watts a day.
The freezer is more of a power hog, using about 700 watts a day.
We have two thermometers per unit.
Current temps are:
Freezer bottom: -2
Top: 7
Fridge bottom: 30 (It's a warm 30. Milk takes a few days before the milkbergs start forming)
Top: 35
We keep the fridge on the cooler side because there can be a pretty decent gap between the top and bottom of the fridge. For the last two months I have been experimenting with a small pc fan sitting on the bottom of the fridge circulating air upwards. So far it has helped tremendously.
We make sure the fridge never gets above 40 degrees.


Figuring out how to store the food items has been an interesting journey. Placing items in various sized locking/stackable containers seems to work best for us.
The freezer is easy enough but the fridge does cause some issues sometimes. Keeping the often used items handy can be a little tricky but in all reality it's not to bad.
It's funny to see the reaction of friends and family... ya they flip.
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Old 10-11-2012, 12:21 AM   #10
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Re: Efficient fridges?

Ryan,

Is your fridge a real fridge, or a freezer used as a fridge? I'm impressed with the 200W per day. What's the capacity (cu ft)? What is the Energy Star rating (kWh/year)? Mine is 172, which (if it's right) comes out to be around 471 Watts per day. But that's rated for freezer use, so I will have to get one of those kill-a-watts to find out for sure (I'm sure it's less when at 34ish deg F).
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