Small Fridge Math - Help please ! :)

[DigitalMind]

Hello alternative energy guru's,

    I have another quick question, this time involving the math to calculate fridge uses for my sisters off-grid house that will soon run on Solar.

She's looking at a 3.3 cu ft fridge by Danby (Model DAR340BL) that is "Energy Efficient".  

I called Danby to get the power specs :

Average Yearly Consumption : 325 kilo watts

Start Amps : 6

Running Amps : 1.3

Voltage : 115 volts (Canada)

So, I may have misunderstood, but this is what I came up with  :

325,000 Watts / 365 Days a year = Average of 890 Watts per Day

I pretty much ignored the other specs since it should still all come out to an average of about 890 watts a day right ? (Adding up the start up of the compressor, and then the lower running amps while cooling)  

Her battery bank will consist of 6 golf cart batters (220ah - setup to have 660ah, and this is the only thing that will always NEED to have enough power all the time)

So, this post is pretty much to find out if i'm doing my math right, or if i've completely misunderstood how this works.

Thanks for all help and suggestions !

[americanreman]

Ok, you know what your daily usage is going to be, you do have the battery capacity to handle it, now how are you going to put back (into the batteries) what you used on a daily basis?

How many watts of solar are you generating? You'll need a good size array to get that used power back in to the batteries on a daily basis from the sun alone.

[Volvo farmer]

Your math is correct.

Be advised that a refrigerator five times the size of that Danby could use only 25% more energy. My 18CF energy star Kenmore uses 1.1 to 1.2 KW/day, both stated from the manufacturer and observed by myself.

You might just be OK with those batteries. You essentially have 330 ah to play with if you don't discharge them over 50%. I calculate the fridge to use 75amps/day (should that be ah/day?}, not including losses. Probably will use slightly over 100 amps/day with inverter losses. That's only 1/3 of your usable battery capacity. Sometimes it will be running when the sun is out too so that helps some. Two cloudy days in a row will likely have you running the generator, but you might get by with those batteries and that fridge, depending on your other loads and your solar charging capacity. I reckon you'll need at least 250W of panels, assuming 5 sun hours, just to run the fridge.

[ghurd]

"suggestions"

The 'watts/year' is dependent on the ambient temperature.

Colder takes less. Warmer takes more (a Lot more).

Place the fridge in a mostly unheated place?  The house doesn't look set up like that.

Maybe get a SunDanzer unit?  They are pretty impressive by my standards.

Only thing is they are chest units, and the choice is fridge OR freezer, big or small.

Certainly worth looking into as an option.  

Might save more money on the system than the fridge costs.

I know of a large SunDanzer fridge in NE OH that ran all summer from a dedicated 70W 12V PV, with a cheap 140AH 12V department store battery, and the battery never went below 12.5V that anyone saw.  Sometimes the PV was used for charging other batteries too.

Not bad for a total investment of ~US$1650, IMHO. (fridge, PV, battery, labor & everything else)

Just a thought.

G-

[wdyasq]

WOAH.....

Before you buy an overpriced piece of "Solar Hardware", consider this:

http://www.mtbest.net/chest_fridge.html

Averages 100W/day .... and about $200-250.(Doing the quick mental math) 4 hours X 50 watt panels ~= $250 .... $500-600 total being liberal with costs.

900W/day divide by 4 hours sun needs 220W panel X $4 watt = $900 + cost of $olar Frig.

I'd at least look at a chest freezer to refrigerator. But, your sister, your game, enjoy.

BUT, like the Farmer of Volvos, I'd buy a real regular refrigerator and panels enough before I would buy special stuffs.

Ron

[Volvo farmer]

Chest fridge is truly an economical way to cool food. Problem comes when someone wants to cook food and has to bend over, dig, move beer, dig some more, remove eight items, replace eight items, just to find the butter.

My chest freezer has 80 lbs of local grown ground beef in it, 30 lbs of green chiles and a few other things. Pretty easy to find what I'm looking for in a chest with only  a half dozen items.

Personally, I think chest fridges are for bachelors. It's hard enough keeping a woman happy off-grid without putting them through that nonsense.  

[DanB]

I think all women should come with a KWH/month rating....  just like appliances - some should have 'energy star' ratings.

[ruddycrazy]

We're totally off the grid here and we went for a 'Consul' gas/240 volt fridge. When using it off the batteries it draws about 250 watts but on gas it will run off a 9kg bottle for nearly a month. It's only 190 litres but that was the biggest fridge on the market that was gas when we bought it. It's been going now for nearly 4 years with no problems. If your using a gas stove then a gas/electric fridge might be your best option.

Cheers Bryan

[ghurd]

Ron- Let's not forget the cost of a reliable inverter capable of starting it, and not overheating the motor.

VF- Remember, she's been off grid with no electric for a year, and she's looking at a 3.3 CF model.  It won't take much to make her happy.

DigitalMind- Look around for other units.  

Example: Danby D9501S or D9504W, 9.5 CF, upright, top freezer, 285 KWH/year

No idea what it costs.

It still leaves the possibility of needing an expensive inverter.

http://www.energystar.gov/index.cfm?fuseaction=refrig.display_products_html

G-

[wdyasq]

Gurd,

You are correct. I figured she would have an inverter anyway.

One of the unsaid things about this thread is several 'similar but not equal' solutions to the lady's problems. I know if it were 'my project'. and it isn't, I would have planned for an outside vented 'Refrigerator cabinet'. I actually plan on such for my refrigerator and freezer on my 'next house'. It will be in a location far rougher for refrigeration than Canada.

I've always wondered why 'government housing' here doesn't use solar 'tempering' as part of the design. Then I think of the 'collective' thinking ability of those who reside in government housing. AND, the collective intelligence of those administrating government housing and I know why.

Ron

[DigitalMind]

Thanks for all the comments !!

Americanreman : She will be getting between 2 - 4 X 120 Watt Solar Panels

Volvofarmer : Thanks for advise ! Good to know she has the option of a bigger fridge if she should choose.  This one costs about $100, and considering she has nothing now, and she doesn't eat any meat, she doesn't need much room.

ghurd :  I read that article about the chest freezer converted to a fridge, and it seems great and has explained the greatness of chest fridges over the typical vertical fridge, however she doesn't want to buy an expensive 12volt fridge, i don't wanna do the work to convert a freezer to a fridge (looks simple at first, but seems to get more complicated as you read how to do it!)   I will look into Sundazer products though.

wdyask:  Same as above  

DanB :  HAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHH.  If only !

Ghurd : (again)  While on the subject of the proper invert so it can start, I just need to get one that will support the STARTING AMPS right ?  

Thanks for everything guys !! Really Really appreciate it !

[finnsawyer]

Shouldn't that be 325 Kilo-watt-hours?  That would mean an average of 0.89 KWH per day or .037 kilo-watts of power used on average.  That's 37 watts, which is possible if the unit doesn't run very often.  And why should it?  Or have they changed the meaning of words again?

[Bruce S]

digitalmind;

Wanted to jump in here for just a short moment and say that what I found back during or ice caused outage , was to make sure and find out if the new fridge has a start/run cap either built in or external to the motor. IF so you'll need to make sure the inverter has at minimum a 5x rating of it's start needs. More than likely it'll need more so be sure to get an inverter and keep the receipt!! and give it a try with fully charged batts.

Look like you have most of the other stuff pretty much figured out.

Best of luck!!

Bruce S

[ghurd]

"While on the subject of the proper invert..."

You would think so, but no.

The surge seems to be considerably higher than the name plate 'starting amps'.

This was a while ago and just our experience with a couple different units, so take it with a grain of salt- 115V 3.0A(run) unit nearly instantly blows all the fuses of a 750W / 1500W surge inverter.  

The 1200W / 2400W surge inverter goes into LVD before the compressor is running, then resets, then LVDs, etc.  That's with a fully charged 440AH 12V battery bank, maybe ~3' of #8 cables were a bit undersized.

Both of the inverters are modified sine wave, and a cheap brand name that we have good luck with.

Next problem is the type of inverter.  I read the motor will overheat with modified sine wave, shortening the motor life.  

Plus, IIRC, I read some of the fancy new models have control circuits that won't work with modified sine wave inverters.

That means a lot of work or guessing what plays well together, buy it, and hope for the best.

OR a $1000 pure sine wave inverter, extra solar to cover it's large losses, etc... PLUS a refrigerator.

Or just buy a SunDanzer for $1000, open the box, and connect it to the battery.

It became a no-brainer after a couple weeks of trying (and buying) standard refrigerators.

Two extra items.

Google around for problems with SunFrost before buying one. I'll save you some time and show you one... Good reading for other options with a large system too.

http://www.windsun.com/General/Sunfrost.htm

A post about inverter and refrigerator difficulties,

http://www.fieldlines.com/story/2007/4/24/161341/327

G-

[ghurd]

Like Fin said, it takes an 'average' of 37W for that fridge.

Say you get an Exeltech XP1100 inverter to run it. The inverter takes 20W not doing anything.

Then instead of an average of 37W, it's 57W, plus the inverter inefficiencies.  Meaning it throws off the math by over 54%.  That's a lot of extra solar panels and battery.  Plus the inverter setup, plus the fridge itself.

All of a sudden the price tag on the 12V fridge looks a lot better?

G-

[wdyasq]

Nah, just need to add more panels.........

Actually Gurd, advice from one who has been there. Probably could have bought a bunch of nice shirts from what it cost to learn that. -

Thanks,

Ron

[DigitalMind]

Finsawyer :

"Shouldn't that be 325 Kilo-watt-hours?  That would mean an average of 0.89 KWH per day or .037 kilo-watts of power used on average.  That's 37 watts, which is possible if the unit doesn't run very often.  And why should it?  Or have they changed the meaning of words again?"

ACTUALLY, I think you're right. I'm looking at the sheet where I was writing my notes when calling Danby, and I DID in fact write 325 Kilo Watt Hours per year (average)

... to be honest I don't quiet understand the difference, (I'm still on a learning curve here, been reading almost non-stop for a few weeks to re-learn basic electronics and how solar works)  but you're telling me it adds up to only 37 watts per day ?!

That would be great ... kinda why I was posting what I was thinking, i'm glad you picked up on my mistake.  

[ghurd]

And I didn't even get into how a 1100W sine wave inverter running at 150W will have an efficiency of ~50%(?), so the 37W goes up to 74W!

I can't afford a shirt.  Sears won't take those fridges back.

[DigitalMind]

Wow,

   Finsawyer, Ghurd,

     I had done my math all wrong.  I guess my electronics lesson for tonight will be learning to understand the difference between kilo-watt hours and kilo-watts !!

Seems like if it uses only 37 watts a day, it makes things alot easier (considering I was counting 890!) , although I will make sure to get an overly powerful inverter to avoid problems.  (hopefully)  

[Gary D]

 It's heading into winter fast. Perhaps an ice chest to keep milk, eggs, butter/ margerine would work this year? Let mother nature freeze 2 gallons of water each night, swap them out for 2 in the cooler on the way back from an early morning outhouse call?

 A root celler/ under the floor type cooler could keep most other things happy ...

 That way she could get used to watching a battery setup and figure out the basics and what is truely a necessity in the spring expanding as necessary? Is a bit off what you were asking...

 Just a thought...

[wdyasq]

Reread - he did NOT say 37 watts a day. NONE HAVE, it figures 37W X 24 Hours - 959W/day - 959 X 365.25 is 350kWh/yr. Just a few watts a day can make that go 300-400 kWh/yr.

These are all 'fuzzy numbers'. 10% energy variation is common on appliances.

Gurd is one who KNOWS of what he speaks. Some here, me included only dream of being off grid. For me it will be again, as I have lived several times off-grid.

Good luck,

Ron

[ghurd]

Hold on, you have that backwards!

It is using an "Average" of 37W All The Time!

Meaning it is about the same as running a 37W light bulb Day And Night.

I'll not go into much fancy math...  Or fancy circuits that could (with enough time to figure it out) considerably reduce it.  Or explaining fancy controllers that can reduce the needed amount of panels.

The 890 watts should be 890 watt hours.  890 watt-hours / 24 hours = 37W 'average for 24 hours a day'.

An inverter big enough to start it is maybe using 20W. 20W x 24 hours = 480 watt hours, with nothing even plugged in.

The big pure sign inverter with a light load (150W) will be less than great in the efficiency department.  Meaning when the fridge is running, it takes double what the fridge takes.  So that 37W average just went to 74W average. (it will depend on the inverter)

So.  74W x 24 hours = 1776 watt hours per day.

The inverter will be loaded about 4 hours a day to run the fridge, so the 20W is for only 20 hours a day, 20W x 20 hours = 400 watt hours.

Together, 1776 + 400 = 2176 watt hours per day.

Guessing (because we don't know exactly where you are) the insolation is 3 hours per day.  You have 3 hours to generate 2176 watt hours.  2176 / 3 = 725 watts of PV output.

Output watts is different than Rated watts, and the output will need increased by ~40% to find the approximate Rated watts.

So 725W is about 1030W of Rated solar panel watts.

Now throw in at least 10% for inefficiencies, losses, a minute bit of safety, etc.

That means I would start looking at about 1200W (rated) of solar panels to run ONLY that $99 fridge.

And the $99 fridge is Still going to need getting up around $1000 for the inverter system.

And it will need a Lot more battery bank to efficiently accept the output from 1200W of panels.

Guessing you'll have $7.50 a watt in the panels. Panels, S&H, brackets, wire, connectors, stainless steel bolts, etc.

Controller for that much power, maybe $1250.  

Few $100 more for disconnects, boxes, etc.

Pure sine inverter, maybe $600, plus a couple hundred more for big giant wire, etc.

What?  15% PST, GST, and B.S. Tax?

My quick guess, DIY, about $13,000.  JUST to run the fridge!

And we didn't even get to the batteries yet!

OR $1000 for a fancy fridge, ~100W of panel, and battery.

One way or the other, the $99 fridge has to go.   It's not smart to leave it in the equation.

G-

[DigitalMind]

Ok ok ok .. I got it .. So I guess my math was more or less right.  I expect it to use more than it says.  

I didn't realize I would need a $1000 inverter though .. that's kind of a bummer ..

Thanks for info guys  !!

[DigitalMind]

After re-reading Ghurd's last post about 6 times to understand it .... i'm confused.

So basically my setup (even with 4 X 175 watt panels) will not run this fridge ??

... I wish I could talk her into a propane fridge or something, god knows i've tried, but she doesn't want it.  

[ghurd]

Tell her if she wants a fridge, $2200 for a solar, or $2200 for a propane.

Then tell her she has to buy propane.  Every month.  Forever.

Tell her 'size doesn't matter'.

(I'm so going to hear about that)

Like VF said, a big one is only a little more than a small one.  Or a big one could be less than a small one, like the above models.

Your math was right the first time, you just didn't understand everything you were getting into.  Like inverters. Surge. Efficiencies. Rated watts vs Real watts. Batteries. etc.

So no.  700W of solar won't run a $99 fridge.  Even if "mostly proper math" makes it look like it would.

However, a $1K fridge, including the necessary solar and battery, would cost less than running the $99 thing.

You win.  :-)

Please. Think about it this way.

The 700W planned install of PVs cost what, ~$4,500 for the PVs alone?

Maybe $1150 for the fridge which uses some power, say 200W of the rated solar.

It will leave 500W of solar power for "other stuff".

Maybe she would love a 500W microwave?

Maybe she would love to see the 'Blue Jays' via 'dish' on a 42" LCD TV?

Maybe she'd like to take the $1150 cost of the fridge off the ~$4500 solar panel cost?

I am running out of angles to explain it.

[Volvo farmer]

I think your math is a little off.

I just put up 680W of panels to compensate for my 1.2KW/day fridge. We have 5 hours insolation and I'm tracking, but I got way more panel than I needed for this job. I think I could have done it with 300W fixed.

My overnight usage (from the time the batteries go below 26.8V to when the sun comes up) went from approximately 40Ah without the fridge to 80Ah. 300W of panels would recharge 40Ah in slightly more than three hours, minus maybe 15% charging losses?

You do make some good points. I wouldn't recommend a 120V electric refrigerator for a small solar system either, especially if they are planning using a modified squarewave inverter. My system is big enough that I just eat the inverter losses and it doesn't end up making much difference. I'm out of bulk, through a 2 hour absorb and into float by 1PM this time of year, and that's all that matters to me. I'm actually at the point where the more electricity I can use at once, the better my inverter's efficiency becomes.

For this little house, I'd consider a propane fridge as well. The little 8CF Danbys are less than $1000. Sure, you've got to buy the gas but they're dead silent and no extra solar is needed at all. $400 saved on a 100W panel, plus some savings on a battery. You end up giving it back to the gas company, but only a little at a time.

[Volvo farmer]

I think  that 700W of solar and six golf cart batteries might run that fridge. I don't think there would be much left over for other loads though, and if it got cloudy, you'd be running for the generator switch in about 48 hrs.  What is your daily solar insolation (sun hours)? Got any shading problems or is the whole place open to the sky? You going with a fixed array or tracking?  What is your planned inverter? Ghurd is right that you need a $1000 plus inverter to run a fridge. I'd argue $1500+ actually after seeing RCPilots Prosine go up in smoke twice.

I ran my whole house off of 680W of solar for about a year. no electric fridge, but I was usually only 1KW into the batteries overnight. I'm now a little more than 2KW into them with the electric fridge. 700W of panels might just keep up with my demand if I could get six sun-hours. I bet I could live off 700W of solar, with my fridge and other loads April to September. December would be another story though

[americanreman]

As I said earlier in this thread, getting the power used back into the batteries is your main concern. 2000 watt vector inverter and a no frills fridge will do the job.

Plan on getting additional charging through gas gen though for sure, you'll need a backup to charge the battery bank unless you get a loan for the solar panels.

Been there and done it about 20 years ago in an off grid cabin and today on my my boat...times have not changed much except inverters are alot cheaper, paid 400 dollars for my first 350 watt inverter in 1982, thought I died and went to heaven, still look back at it as the best money I ever spent in my lifetime.

[finnsawyer]

Watts is power or energy per second.  That is, one watt is one joule per second.  So, to get the energy used in some time period you must multiply times the time in seconds to get joules or watt-seconds.  For appliances that would give rather large numbers, so they divide by 3600 to get watt-hours, and then by 1,000 to get kilo-watt-hours.  Example:  Take your 37 watts times 3600 seconds in an hour times 24 hours = 3,196,800 watt-seconds or joules per day.  I realize this is rather cumbersome, but that's the way it is.  Your fridge needs 0.89 KWH per day.  A panel that averages 200 watts for four hours a day would come close to this figure (.2kilo-watts times 4 hours).  So, two panels should do it.  The 6 amp starting current only lasts for a few seconds, so you don't need a very large battery either.

[DigitalMind]

You guys rock.   Thank you so much for all the useful advice and time spent explaining this to me.  I will start looking at the $1000 fridges.  What am I looking for ? 12 Volt chest fridges  

She won't go for the proprane or gas or whatever, never ...  

[ghurd]

Yes.

http://www.thesolar.biz/SunDanzer%20Refrigerators%20and%20Freezers.htm

[alibro]

If you only need a small fridge why not get one out of a mobile home/caravan. They are designed to run on mains, 12V and bottle gas. We used one (bought 2nd hand for around £100 stg or $200) for over a year running off propane gas with no problems at all and a 10Kg (22lb) bottle lasted about 3 months. It wasn't very big but was great for every day basics. I never ran it off 12V so I don't know what power it took but I guess the manufacturers should be able to tell you.

Good luck

Alibro

[Volvo farmer]

Those absorption fridges are horribly inefficient on electricity. Somewhere in this thread, someone said that their little unit like yours used 250W when running. My 18CF one uses about 100W. An electric heat pump is inherently more efficient than using an electric heating element in an absorption system. But if you want to cool food with heat from a flame, absorption is the only way to go.