Lets Talk About Refrigerators.

Discussion in 'Off Grid Living' started by Kingfish, Jul 23, 2012.


  1. Kingfish

    Kingfish Self Reliant

    I am looking at a 5 star Kenmore refrigerator. 18.2 cubic feet upright. Energy use is 383 kwh per year and the unit draws 6 amps while it runs. Using a 12 volt battery bank and a 2500 watt inverter how many Amp hours (size ) bank will I need to run the frig 4 times per day at 1/2 hour times 4 days. This would be a total of 2 hours at 6 amp draw per day times 4 days. Total 8 hours at 6 amps. I would not want to go under 11 volts or past 50% discharge. Im not talking solar panels here. Just the bank.
     
  2. Nadja

    Nadja RIP 3-11-2013 Forum Leader

    Kingfish, why don't you take a kill a watt meter to the store and plug it in to the refer and then of course plug in the unit and see what is really what ? My refer only draws a couple of amps max after the miniscule start up . That doesn't sound right to me.
     
  3. ghrit

    ghrit Ambulatory anachronism Administrator Founding Member

    Start with the fact that 6 amps at 120V is a long way from the same amperage draw at 12V. Power = Volts X amps in DC. Power = Volts X amps (X a factor which I've forgotten that corrects for the sine wave) when you are dealing with 120V AC. So, you can't do a direct calculation without corrections.

    You can use the killawatt meter for real numbers, but for sizing your panels, controller, and inverter, use the nameplate data for "close enuf" estimates and add a factor for age, motor starting draw, and deterioration. It is imperative that you understand the difference between amps DC and amps AC at any particular voltage.

    Now, is that 2500 watt inverter rated DC or AC? It makes a world of difference. Likewise, it makes a difference if that is watts input, or watts output.
     
  4. Kingfish

    Kingfish Self Reliant

    The 2500 watt inverter will make about 17 to 20 amps of 120 volts ac. I am going to invert or convert 12 volt to 120 same as Nadja. I know the inverter will handle the load. The battery bank must be designed to last at least 4 days with no recharge. The frig will be on a timer so it cant run more than 4 times per day. I need to know how big my battery bank has to be. Nadja told me to find out what the unti draws. I did that. 6 amps. It is rated at 383 kwh per year. I am assuming that is with the door shut. I really dont know how they rate them but it is the most efficient Kenmore made. lowest kwh per year at 383. That equates to just over one kwh per day.
     
  5. Kingfish

    Kingfish Self Reliant

    I guess I just have to buy the meter. Ill be back on this thread after I get one and Take it to the store.
     
  6. BTPost

    BTPost Old Fart Snow Monkey Moderator

    Ok Kingfish 6 Amps @ 120 Vac for 8 Hrs per day total = 48 AmpHours/Day @ 120Vac. Figure your Inverter is say 85% Efficient, (Low side) That would be 48 AH * 1.15 = 55 AH @120 Vac * 10 = 550 AmpHours @ 12 Vdc Now figure that to keep your Battery Bank above the 20% Discharge Rate, and recharged once every 24 Hrs, with a Charging Efficiency of 80%. 550 AH * 5 = 2750 AmpHours Total Battery Capacity. 550 * 1.20 = 660 AmpHours of Recharging required to recharge the Battery Bank back to Full Float, over say 10 hours, which would be 66 Amps Minimum Charger capability.

    That is one heck of a Battery Bank that would be required... I would be looking at raising the Battery Bank voltage for such a system to 24 Vdc Minimum, and more likely 48 Vdc. At 24 Vdc the Total battery capacity would be 1375 AmpHr, and @ 48 Vdc would be only 687 AmpHrs, with the corresponding Recharge Amps @ 24Vdc, 33 Amps, and @48 Vdc, 16.5 Amps. But by shorten the Charging Times to 5 hours, you would be at 66 Amps @24 Vdc, and
    33 Amps @48 Vdc.

    The 1375 AmpHr @ 24Vdc is very similar to what I run for my whole Cabin. I run two 5 Hour a day Recharge Cycles, per day. 8Am to Noon, and 5Pm to 10Pm. So your Refer, will be very similar to my whole Cabin Power System. ..... Kingfish
     
    TheEconomist likes this.
  7. Kingfish

    Kingfish Self Reliant

    Doug at Deka told me to go to 48 volt as well. Im looking into what is going to be the best cost effective way to go. I just ordered two kill a watt meters and as soon as they arrive I am going to find out exactly what I need. Thanks guys.
     
  8. TnAndy

    TnAndy Senior Member Founding Member


    Using a Kil-a-watt meter will give you more exact numbers.....saying you're gonna run it 4 times a day for 1/2 hour is merely guessing.

    Most fridges are gonna run about 30 minutes out of an hour. You're figuring 360w/hrs/day in your (1/2hr x 6amp x 4 runs/day ) ( see my math below).....but the energy figure off the yellow tag for the fridge already told you OVER 1,000w/hr ( 383kw/hrs/yr......divide 365 days into that, and you get a bit over 1kw/hr/day.....a kw/hr is 1,000watt/hr )....so you're off by a factor or 3-4 right off the bat. Those yellow tags give you a starting point to compare appliances, but just like the mileage sticker on a car....YMMV.....Your mileage may vary......due to local temps, number of times you open it, amount of food you keep in it, and so on.....A meter is the only way to get real world figures in YOUR house.

    BUT, just as an exercise:

    Figure the watt/hrs you need:

    120v x 6 amps = 720watts

    Run that 1/2 hour, and you multiply 720 x .5 = 360 watt/hrs.

    Do that 4 times a day, and you get 360 x 4 = 1440w/hr

    Do that 4 days and you get 5,760w/hrs

    Now let's take a pair of Deka L-16's rated at 400amp/hrs. They are 6 volt each, so we'll wire a pair of them in series to give us 12v, but that keep the amp/hr rating the same ( series wiring ) ( parallel wiring keeps the voltage the same, and doubles the amp/hrs )

    How many WATT/Hrs can be taken out of that battery bank and discharge it to 50% ? Answer: 200amp/hrs x 12 volts = 2400watt/hrs.

    Since, as Bruce points out above, you're NOT gonna convert DC to AC without some losses, knock about 20% off that figure on conversion......so say you're down to about 2100 watt/hrs, or less, that you have to work with. Those 2 L-16 batteries would do you ONE DAY ( or little over ) on your (1/2hr x 4) days without any recharging......meaning you'd need 4 times that ( or a total of 8 batteries ) to pull of 4 days of autonomous use ( no recharge ) pulling your batteries down to 50%.
     
    Nadja likes this.
  9. Cruisin Sloth

    Cruisin Sloth Special & Slow

    TN Andy has the closest DART. But the board is not always round.
    Killawatt = the first thing for YOU to understand, here : China/usa

    I have a few just to measure & check.
    Whatever you think of the energystar ratings , they are only close if your board is like there sample testing area.
    Use real meters & math with a knowledgeable mind.
    You will be totally floored how much it takes.
    Check this site out: NAWS
    Don't give up , but buy one unit and start to see the correlation.
    Spend your franklins wisely ..

    Sloth
     
  10. Kingfish

    Kingfish Self Reliant

    That is what I am looking for. An Idea of what I am going to need. I feel much better now. Thank you much guys.
     
  11. Kingfish

    Kingfish Self Reliant

    So batteries in series add voltage but the amp hours or watt hours stay the same. Those same hours add in parallel. I am sure we will find the new refrigerator is going to use less then 6 amps as Nadja says. Like Nadja we are going to put timers on the freezer and refrigerator . We can set them to run when we want them to.. We can control how many times a day it is opened as well. I should have those meters today or tomorrow. I am starting to see the outline of my system.
    I think you misunderstood me. 1/2 hour at 6 amps 4 times per day is 2 hours per day. times 4 days making it 8 hours over a (4) day period. Not 8 hrs per day.
     
  12. TheEconomist

    TheEconomist Creighton Bluejay Site Supporter+

  13. BTPost

    BTPost Old Fart Snow Monkey Moderator

    Yep, I got that wrong... but the rest of the math is Ok, so you could recalculate using the same formulas and get the correct answers.....
     
  14. -06

    -06 Monkey+++

    Before investing that much why not just go to a Propane unit. A big fridge will cost you $15-1800 and all you need is a propane tank that will last you for many months. The Ammonia based refrigerant is much more efficient than the ones used in electric units.
     
  15. Cruisin Sloth

    Cruisin Sloth Special & Slow

    If you get a electronic control refer unit , and your switching power to the unit on & off , they have a bug that they like to start on the defrost mode (heater to melt the evap ) then they cool. So you could be costing yourself much more grief. I use energy star units but all my stuff is clunk & click (mechanical timers ,old school) They wont surprise you with a 300 bean board that failed or has been obsolete after warranty. The so called "green" stuff has been made for a set TIME-X till you need to replace it.

    There is always lots to consider.
    Sloth
     
    Gafarmboy and Kingfish like this.
  16. Kingfish

    Kingfish Self Reliant

    Well? good questions. First we are on the grid now. My wife is not going to go for any alternative appliances but she did agree to an energy star rated one. I looked at Sunfrost and one other very expensive low amperage unit and the cost just plain stopped us. After talking with Nadja I dont believe we need to get that efficient. I tried the chest style refrigerator and she just plain said no. I realize chest style refrigerators are way more efficient as they don't dump the cold air every time you open it. Chest freezer and an upright refrigerator. So now Im off to find the ones that are stingy on power but still have a good track record. So far I have a Kenmore and a Frigidare with the same ratings only the frigidare is about 100 bucks cheaper. The same went for a propane frig. Our propane is dedicated for long term water and power tools. So I am stuck using modern appliances that fit in the same places as our old ones. These are getting changed out in the next three weeks. Then Im buying an inverter and batteries. Then a charging system in that order.
     
  17. Kingfish

    Kingfish Self Reliant

    It could just be that we need to limit how many times we open it. Ill have to see what they use first then go from there.
     
  18. Cruisin Sloth

    Cruisin Sloth Special & Slow

    Well then If you store the beer in the fridge, your going to make it very hard to keep that door shut. use a marking system with paper on the fridge door to see how many times it's opened ! there is another uncontrolled number..

    Sloth
    Im betting 30 times a day here, British but like cool beer ;)
     
  19. TnAndy

    TnAndy Senior Member Founding Member

    One thing I suggest you do.....pick up an external thermostat control so you can use a freezer as a fridge IF the SHTF. Wife may not "like" it, but I guarantee SOME refrigeration beats NO refrigeration !

    I have two of them sitting on the shelf.

    With a thermostat control like this:

    http://www.kegman.net/products/9025-Freezer-Temperature-Controler-"Freezer-Tamer"-%2d-9025.html

    9025__21927_zoom.


    You can turn a 7-9cuft chest freezer into a DANDY 200watt/hr fridge ( 1/5 the power requirement of that upright energy star you have )
    9025__21927_zoom. 9025__21927_zoom.
     
    Gafarmboy, TheEconomist and Kingfish like this.
  20. TnAndy

    TnAndy Senior Member Founding Member


    The problem, WC, is then you're totally dependent on propane for your refrigeration. I'd imagine SHTF, and propane trucks ain't gonna run.
     
    Cruisin Sloth likes this.
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