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Old 04-20-2006, 06:20 PM   #1
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re: Fridges

So I have been thinking and talking to the technicians here, and I am at a bit of a crossroads about what to do about a fridge (and yes I know I don't need one ). I was thinking going with an rv fridge, but even at my prices It's alot of money. Also they are very finicky about how level the vehicle is, and can have some expensive problems. The power consumption stinks on power. So why not use an energy efficiant domestic machine like all the fancy high end coaches?

I was looking at the whirlpool site and they have a 6.9 cu/ft model that uses 386 kw/yr, which works out to 0.4 amps an hour with, although it depends on how efficient your inverter is and at what temp you keep it, ambient temp, and how often it is opened.

But those numbers look resonable. I want solar anyways and this fridge is about $400 new, so that leaves it open to used and I could get solar for the price of a Norcold. Right?

I know they aren't designed to handle road vibration but it should be able to be mounted on rubber grommets, and you could even have a little solar powered fan to keep things moving behind the unit. Also no holes in the roof or the side of the bus.

What do you all think?

-Richard
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Old 04-20-2006, 08:12 PM   #2
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I think that if you can afford the solar, go for it . The only downside might be those cloudy days that turn your milk into cottage cheese. As for vibrations....the rubber grommets seem like a good idea and I certainly would do that, but I wouldn't worry TOO much about vibrations. If that new fridge is half as tough as my $25 garage sale dorm fridge that has been kicked, flipped, dropped, and just generally thrashed you'll be fine.

As best as I can tell.... 120 volts*.4 amps*1.1 (assuming 90% inverter efficiency)=52.8 watts per hour. That's a lot of solar energy, isn't it? Of course you can run off the engine when you're moving, a genset, etc if need be. What solar set up are you looking at running? I can't afford one right now, but they do intrigue me a bit. I've also looked at various wind setups as an alternative. I'm not particularly "green" but I am cheap
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Old 04-20-2006, 11:25 PM   #3
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Somthing I do need to look into is how much power one would consume when it is kicked on. They don't run that often (every 3-4 hours the pump kicks on).

For solar I am thinking about an 80 or a 110 watt pannel with a regulator. I have a sine wave 750 watt tripp inverter that has a 3 stage 75 am variable charger. This should keep things charged up of a geni quickly on overcast days. I plan on a minimum of six 6 volt batteries (220 ah ea.). They should run aprox $80 ea, the pannel and regulator about $650 to $880. The system can handle up to 4 pannels before needing another regulator. The cool thing about the inverter is it negates the need for a converter because it can handle AC and DC.

I just don't know what to do about a freaking fridge . A norcold is going to run upwards of $850. I don't have levelers and I am not sure how much of an issue that is going to be.

Please help .

-Richard
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Old 04-21-2006, 12:14 AM   #4
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That solar set up sounds kinda costly.. Have you considered getting one of those Honda EU geni's? There are also cheap-o versions of the same genirators around too.

6.9cu.ft. is about the size of the fridge I have in my bus (I think) its one of those 3' tall or so "dorm" fridges, which runs fine off my 750watt inverter or a Honda EU 2000

Just a thought....
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Old 04-21-2006, 01:38 AM   #5
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Quote:
Originally Posted by captainkf

I just don't know what to do about a freaking fridge . A norcold is going to run upwards of $850. I don't have levelers and I am not sure how much of an issue that is going to be.

Please help .

-Richard
A lot depends on how you plan to use your bus. We'll be living in it full-time, and depending on solar for 100% of our electricity. (We may add a wind turbine...). And like you, we didn't want to deal with leveling our bus to within 3% of the horizontal plane to accommodate the propane function. So we decided to go with an all electric model. Looked at Waeco, SunDanzer, Norcold and NovaKool. Even made a spreadsheet of volume vs. cost vs. efficiency (sorry, I looked and can't find it.... )

NovaKool models are 12vdc (so no need to worry about level operation). Our 4.2 cubic foot model (R4500) uses (on paper, anyway) 2 amps @ 12vdc. and cost about $800 (plus $100 freight). Yes, that's some dough....but, as we found out, when it comes to this kind of thing, there is no free lunch (again, wish I could find that spreadsheet). And 4.2 cubic feet is really all we need (2 adults). Before purchasing the fridge we made a cardboard box 4.2 cubic feet in volume...and everything currently in our big ol' household fridge fits.

Anyway, hope this helps. Good luck with your decision.

Sean

PS: Here is how we installed the fridge. Just food for thought.
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Old 04-21-2006, 08:42 AM   #6
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As best as I can tell.... 120 volts*.4 amps*1.1 (assuming 90% inverter efficiency)=52.8 watts per hour.
Forgive me if I'm wrong, but that would give you 52.8 Watts per hour @ 120VAC, no? In order to convert that to 12VDC and find the drain on the 12VDC side that was supplying the inverter, you'd have to multiply it by 10, wouldn't you? That would be a 12VDC demand of 528 Watts per hour, which would consume the production 3 or 4 solar panels of 150 to 200 Watts. There wouldn't be any left over for battery charging.

Of course, that assumes a constant demand. In fact, the reefer would only operate for discrete blocks of time every few hours, depending on insulation.
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Old 04-21-2006, 10:21 AM   #7
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Re: Math

If it uses 386 kw/yr then divide by 364 (days in the year) to get 1.06 kw/day. Then divide that by 24 (hours in a day) and get 0.0441 kw/ hr or 44 watts per hour. Then divide watts per hour by the voltage being used (120) and get 0.366 amps per hour ... ? Right? Now this is a general average under the best possible circumstances. However assuming the fridge only ran 25% of the day multiply the 0.36 ah by 4 to get an actual (ish) power consumption running to get 1.44 ah. Now add 15% onto this for power loss in the inverter and along the lines to get 1.66 amps per hour.

What do you all think of my fancy math.

-Richard
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Old 04-21-2006, 11:03 AM   #8
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[quote=Eric von Kleist]
Quote:
Originally Posted by "the_experience03":c0c42
As best as I can tell.... 120 volts*.4 amps*1.1 (assuming 90% inverter efficiency)=52.8 watts per hour.
Forgive me if I'm wrong, but that would give you 52.8 Watts per hour @ 120VAC, no? In order to convert that to 12VDC and find the drain on the 12VDC side that was supplying the inverter, you'd have to multiply it by 10, wouldn't you? .[/quote:c0c42]

NO!, that's not correct.. watts are watts no matter what the voltage. An appliance that requires 120 watts @ 12 volts, still requires 120 watts at 120 volts. It will however take 10 times as many amps to run a device at 12 volts (rather than 120.)

watts= volts x amps

amp = watts/volts

volts= watts/amps

120 watts @ 120 volts = 1 amp

120 watts @ 12 volts = 10 amps

back to the math in the queston at hand....52.8 watts per hour to run the fridge

52.8watts/12volts = 4.4 amps of 12 volt current. So the when the fridge is running it will draw approximately 4.4 amps from your batteries. If you were to run the same fridge off of shore power, you would be using approximately 0.44 amps of 120 volt current, but either method is still using 52.8 watts.
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Old 04-21-2006, 12:03 PM   #9
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Let me just say...the way 'fridge manufacturers provide their model's kwh/year is almost useless. What they need to tell is how many watts the compressor uses while running.

It makes calculating power usage nearly impossible. Wonder why they do that?

Now back to the math frenzy already in progress.
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Old 04-21-2006, 01:08 PM   #10
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Thank you for covering Ohm's Law in every conceivable way (and he power consumption equations from that), Jason.

Now....as for the ratings....I'm not sure if this is true or not, but it seems to me that the ratings are for the power the fridge actually uses as it cycles on and off. The idea is to make a fair comparison between different models. For instance, one model might draw a much greater amp load, but if it's insulated better and doesn't have to run nearly as often it will probably use less kilowatt hours per year. Basically the energy star ratings don't tell you what the maximum instantaneous consumption of the machine is. That information is found on the UL tag on the back of the machine and will post information such as "120vac @ 60hz, 1.4 amps" for instance. Aside from making sure that your inverter can handle the surge load I don't think you need to be concerned with instaneous draw.

Basically what I'm trying to say is don't try and factor it by saying it will only cylce this many times per day. The number of watts per day as you deducted is the number of watts it will draw over 24 hours of normal start and stop operation.

Like I said, I'm not sure about this. It is just what makes sense to me so if someone else out there knows better, please feel free to correct me as I'd like to know for sure one way or the other
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