re: Fridges

[captainkf]

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

[the_experience03]

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

[captainkf]

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

[phillbus914]

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....

[SeanF]

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.

[Eric von Kleist]

Originally Posted by the_experience03

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.

[captainkf]

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

[lapeer20m]

[quote=Eric von Kleist]

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.

[SeanF]

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.

[the_experience03]

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

[shaggy]

There is a dude named Steve that sells reconditioned propane/electric fridges in the 8.5 cubic foot range for 400 to 500 bucks with a 3 year warranty. I bought one from him. I think he even joined this board. My question is does it have to be enclosed between walls to be sturdy and to keep from tipping over?

Tim

[Les Lampman]

I've got to head to work so I have to keep this short...

4 amp/hr is not a particularly effecient model of refrigerator.

Jason nailed it...watts is watts.

Somewhere on the unit (and usually in the manual) there *has* to be the load rating for the applicance; in other words, how much power it draws while running. This is manadated in regulations so it has to be there somewhere (and must be on a plate/sticker on the unit).

Find out what it draws when it's running on AC power; multilply amps times volatge and you have watts. Takes those watts and divide by 12 (close enough) and you know what the load in amps is on the DC side.

Then...depending on where you live you can guess as to how long the refrigerator will run each day (based on air temp, how often you open and close the thing, insulation around it, etc). If you decide that's 50% of the time (12 hours per day) and you're pulling 4 amps when it's running then you need 48 amps from your batteries to support the refrigerator each day. To do that you should have a 137 amp-hour battery; probably a group 31 deep cycle would get the job done in a pinch.

Obviously if the unit only runs 30% of the time (about 7 hours) you'd only need 28 amp hours and that only needs an 80 amp-hour battery (about a group 27).

Most high efficiency units now use the Danfoss compressors; the smaller refers typically the model 35 (there are others on the market now). They'll pull as little as 2.5 to 3 amps on the DC side. They're used in the Norcold DE series which has no propane system. They also don't require the unit to be level; usually only within 35 degrees or so (it's only the absorption model refers that have to be level, or nearly so). They aren't inexpensive but neither are the high efficiency home units based on the same compressors.

Late...gotta go!

[lapeer20m]

Originally Posted by SeanF

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.

if you're looking at the fridge in person, there is almost always a tag that tells either the amps @ 120 volts, or the wattage of the unit. This little tidbit doesn't really help if you're looking at them online though....The amount of current on the label does not reflect how many watts are required for startup either. A compressor can take 2 or 3 times the normal operating current during startup.

[captainkf]

Thanks for the info on the tagw. I looked at a bar size danby fridge and it uses 80 watts or 1.2 amps AC power. I then looked an apartment sized fridge with exposed coils on the rear and it used about 130 watts or 1.8 amps. Not a whole lot of difference. However after looking at brands like Norcold, nova kool and the prices that go with them I have decided that a power savings of .8 amps ac is not worth the extra $$. I think I can get it to run pretty consistently if I add extra insulation and keep the fridge low (under counter) and out of direct sunlight. I have seen a few used in the local paper for under $50. So we'll see how it works out.

Thanks for the info all!

-Richard

[Les Lampman]

Originally Posted by captainkf

Thanks for the info on the tagw. I looked at a bar size danby fridge and it uses 80 watts or 1.2 amps AC power. I then looked an apartment sized fridge with exposed coils on the rear and it used about 130 watts or 1.8 amps. Not a whole lot of difference. However after looking at brands like Norcold, nova kool and the prices that go with them I have decided that a power savings of .8 amps ac is not worth the extra $$. I think I can get it to run pretty consistently if I add extra insulation and keep the fridge low (under counter) and out of direct sunlight. I have seen a few used in the local paper for under $50. So we'll see how it works out.

Thanks for the info all!

-Richard

It doesn't make any difference what they draw on the AC side if you're connected to shorepower (at least not this small difference). But if you're running on an inverter small changes add up...

The 80-watt unit needs a whopping 7 amps DC to run (about 5 hours on a Group 31 deep cycle battery) and the 130-watt unit will draw almost 12 amps; with the same Group 31 battery the larger unit can go about 3 hours. [This is calculated with inverter efficiency factored in and drawing the battery down to it's 50% level (after which damage occurs).]

So the change in wattage affects the DC side of the situation to a very large degree...in fact, in this case it represents a 74% increase in electrical usage (the 80-watt unit vs the 130-watt unit).

It may still work out just fine; this information is just so you know going in what to expect.

[captainkf]

that is quite a bit of juice, however I just can't shell out over $900 for a 2.7 cuft norcold or Nova Kool. They're much more efficient, true. But I am planning on at least 6 6volt tgo8 batteries which should keep some juice flowing for a bit, that and a bit of solar. I know it's not optimal, however I think it should work.

Worst case scenario I pull it out, use it as a "recreational beverage" cooler on my deck and throw a three way in down the road. I'll run gas lines and power just in case.

Thanks for all the info everyone!

-Richard

[Eric von Kleist]

[quote="lapeer20m
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.) [/quote]

DOH! I did it again. That's right, of course.

[Jody]

The sticker on the back of my little dorm fridge says:

115V AC 60HZ ...... 1.25 amp

According to lapeer20m, that means my fridge requires 143.75 watts

143.75 watts @ 12 volts = 14.3 amps

So basically the refrigerator that I own may be too much of a drain on the batteries to use with an invertor, but shouldn't be too much of a drag when plugged into a campground receptacle or powered by a generator.

I am a professional firefighter and a Bluegrass musician... and this electrical stuff is new to me. Thanks for sharing the knowledge.

[lapeer20m]

A chick firefighter....now that's hott! lol

I have been a paid on call firefighter for quite a few years, until recently. I work as a full time paramedic now.

do women firefighters tend to be pyro's like most male FF's?

as an example: I have a 30 foot flame thrower

[Jody]

I am not a female, and I'm not a pyro. I have been fighting fire for 18 years, and If I never saw anything else on fire (besides a campfire) that'd be fine by me.