Designing a solar/battery backup for refrigeration

Discussion in 'Off Grid Living' started by Wrenchbender, Jun 15, 2012.

  1. Wrenchbender

    Wrenchbender Monkey


    I have been pondering this project for some time. In event of tropical storms, hurricaines, etc we have power outages. Yes, a generator would be cheaper up front but when the power is out for days you can't always get fuel to run it nearby.

    I have a 17 cu ft refrigerator with top mount freezer and a 5 cubic foot chest freezer. The manuals do not give me any good information as to how many watts they draw. The do however tell me not to climb inside.......

    I am envisioning a couple panels feeding a battery maintainer, a bank of deep cycles, and an inverter robust enough to handle it all. Obviously, this would not be intended for indefinite use. I have good mechanical and fab/wiring skills, and a mudroom with laundry that is separated from the main house by an exterior door to place the equipment in.

    WHat I need to figure out is how many batteries, and what type of panels. I see sailboats with this stuff all the time, a small DC wind generator and some panels. If a rag bagger can do it why not a landlubber?
  2. Nadja

    Nadja RIP 3-11-2013 Forum Leader

    Yes you can do it. The first thingI would recomend would be a "kill-a-watt" meter. That will tell you what your appliances are actually drawing.
    oldawg and BTPost like this.
  3. TnAndy

    TnAndy Senior Member Founding Member

    Last edited by a moderator: Jan 25, 2015
    dragonfly and BTPost like this.
  4. dragonfly

    dragonfly Monkey+++

    We are running a large fridge up in northern Az right now, on a bank of 4 deep cycle batteries and 2, 135 watt solar panels....In fact on the same system we run all the lights ( 110), a window size A/C unit when it gets nasty hot, a flat screen TV, 2 satellites dishes for TV and internet, and the lap top computer. 2 weeks ago we ran everything ( turned it ALL on) from 10 am to 2 am and it never faltered once.
    The readings stayed in the 12.3 volt range and I have to say I was really impressed at that test! By 10 am the following day, the voltage was back up to 14.4 and it never blinked. I am not sure about the exact size of my fridge, but it's a double door type with a BIG freezer side! I took it from my house in Phoenix to my place up north and it beats the heck out of that tiny 2 cu ft thing in my travel trailer!
    it's now a permanent item, as most people have propane fridges that cost in the range of $1,100 to well over $2,400! In the summers they go thru propane like a drunken sailor and money!
    ditch witch likes this.
  5. Tikka

    Tikka Monkey+++

    Here is some information on batteries

    "a 12-volt battery will measure at about 12.9 volts when it’s fully charged and about 11.4 volts when it is fully discharged. That’s a total of 1.5 volts that represents the full range of charge on a 12-volt battery.


    % Charge = SOC

    % Charge = ((Measured Battery Voltage – 11.4 volts) / 1.5 volts) x 100​

    "That seems easy enough. So what’s the catch? In order for this formula to work, the battery must be in a rest state. In other words, the battery should not be supplying power to any type of load.

    The experts say that the battery should remain at rest for at least 24 hours to get an accurate measurement, but in a pinch a couple of hours are good enough to make a reasonable guess."

    Frequently Asked Questions -

    If you read voltage while the battery is being charged, you are reading the charging source's voltage not the battery's voltage. Try this with a battery you know is dead. then turn off the charger.
  6. Wrenchbender

    Wrenchbender Monkey

    Thanks, I ordered a kill-a-watt meter last night. Once I have a general idea, I'll take if from there. Good battery info as well. I know the batteries will be the most expensive part of the deal.
  7. Nadja

    Nadja RIP 3-11-2013 Forum Leader

    Your batteries must be able to get a full charge from your solar panels. I don't quite know how the dragon is doing all of what he says on such a little system, but don't be fooled into thinking it is going to be inexpensive, as it won't. Do it right the first time by asking what type of batteries, charge controller and inverter to buy before you just jump on the wagon and spend sheckles on something which may not be the right way to go , only to have to do it all over again.
    TnAndy and hank2222 like this.
  8. dragonfly

    dragonfly Monkey+++

    I can tell you this...the 4 new deep cycle batteries, 2-new 135 watt panels, and a new digital controller/invertor set me back a nice chunk of change! Now I am working on a 2nd system...smaller and less expensive but still expecting to spend at least $1,000...
    That's just for internet and satellite tv service....Nothing is cheap IF you expect to have a good system that is reliable. Since I use very little 110 ac up there I manage very well.
    Most of what I have to use is run off a generator: power tools.
    Lights and such are very minimal usage.
    If I chose to go all out and have the same ability as a house in the city, it would be so expensive to the point it would have to pay for itself in about 20 years!
  9. Nadja

    Nadja RIP 3-11-2013 Forum Leader

    A system capable of running a house as to living in a city would cost upwards of 25-30k minimum. Even up here, without ac etc and all the frills, you can expect to spend about 20 k by the time all is said and done. And that is if you do all the labor yourself.
    Guit_fishN likes this.
  10. techsar

    techsar Monkey+++

    Been thinking about this for a while, and the math doesn't add up. 855 watts (approximated) for 16 hours (13680 w/hr) cannot be replaced by 270 watts in 8 hours (2160 w/hr). This is not even counting conversion losses. I suspect the figures given (most likely the time) are in error...
  11. Cruisin Sloth

    Cruisin Sloth Special & Slow


    Post some of your location , because it helps Us/Me do some slight light time's etc.
    Since the forum has been reworked , much of the info ,plus where to look has been moved.
    Getting to be work here . And your correct techsar with the equations , even the abacus needle is pointing off ;) ..

    Reading the running watts of the fridge/freezer is no true amount that it takes on startup draw. So there is no set rule of thumb, and since they make these items to be cheap , and only really report the lowest power consumption you can ever expect in Antarctica ;) .

    Im sure you understand. Just to get you close to understanding , Kill-a-watt reads 140 watts running , so just to be safe , times that by 4 to 5 (i'll split it) for start up surge 140X4.5 = 630 watts & you said you had two beer coolers , so thats 1260 watts & to round it up 1300 watts Min inverter, NOW into Sine-Waves , Pure/true are the only ones that are guaranteed not to do damage to any electrical device . MSW,SSW Modified,square,adjusted any new buzz word that is not True/pure sine wave will let the smoke out. So now you just tripled the $ from the pos MSW units . Now after all that , have a 50%+ more cushion for safety , 2000W true sine inverter out back unit 48 volt . Now you can figure light hours to amount of panels required at the shortest day charge the batterys so they don't get used down past 30 % there limit. MATH to the max.

    Lots of things to consider , best is to know that you MUST work within your Ambient areas first, after that , working adjustments along the planning. If you figured XX$ then your 5 to 10 times low. But on the Plus side , It's much cheaper that it was years ago..

  12. BTPost

    BTPost Stumpy Old Fart,Deadman Walking, Snow Monkey Moderator

    Sloth, You made ONE assumption that inflated your numbers significantly.... You have two Beer Coolers, that have Run Power Draws of 140 Watts each, and a Starting Draw of 630 Watts each, correct. So, the chances of BOTH Beer Coolers starting up at the same time is NEVER 100%. it is actually closer to 5-20%. Therefore, assuming that both Beer Coolers, only run 50% of the time, which is a very High Percentage, in real life, You would have a startup draw on the first Beer Cooler, of 630 Watts, for say 10 seconds, and then it would drop to 140 Watts. Then say the second Beer Cooler starts up, and the draw would be 630 Watts + 140 Watts or 770 Watts, for the next ten seconds. After that Startup Time, the draw for BOTH Beer Coolers would be 280 Watts, until the first Cooler came down in temp, and shut Off. Then the draw would be 140 Watts, until either the second Beer Cooler, shut down, or the first Beer Cooler, restarted again. a 1Kw Inverter, with a Surge Starting capability of 1.3Kw for 10 seconds, would be fine. Also Note here, that MOST, if not ALL Sealed Freon Refer Compressors will run just spiffy, from MSW, and even Squarewave Inverters. Yes, they will get a bit hotter, because of the nice Square Edges of the waveform, but those Square Edges are dissipated in just a bit more heat, Not a life threatening condition for a Seal Freon Compressor Units. If your Beer Coolers are Thermoelectric Types, then they have no Start Up surge at all. Also just about ALL Inverters, even cheap Chinese Junk Inverters can handle a 125% surge for 30 seconds, without letting the Magic Smoke out. ..... YMMV.....
  13. Cruisin Sloth

    Cruisin Sloth Special & Slow

    Well your figuring is also ONE assumption. There is Line loss / voltage drops to Power factors and the true efficiency of items claimed and what they normally can produce. cutting the design and build to the just good enough is for fools who whine about how they were ripped off.

    I was just giving the original poster something more to really think about. Since he purchased a kill-a-watt unit , he is on the correct path.
    Visit here OP :
    You standing beside that fixed wing should know that safe limitations makes a safe and long life.

  14. Wrenchbender

    Wrenchbender Monkey

    WOW, I was away on vacation for a week! Look what got posted!

    In any case, the kill a watt has been hooked up to the 5 cu ft chest freezer for over 650 hours now. I measure 97.7 watts and .84 amp when I can hear the compressor running. The freezer is outside in a non air conditioned porch/laundry room, and it is about 100* in there as I speak. I understand there will be inrush amps etc. Should I unhook it for a while, then plug it in with an inductive amp clamp on the cord to read inrush current? Now, I have yet to connect it to the refrigerator, which is inside the air conditioned house., but suspect it will draw similar power.

    Now, I live at 32* lattitude, Savannah GA area. I have a very good south/west exposure to place a panel or two, and can do so on a post I can manually adjust. I live at the edge of a residential, older neighborhood and am zoned commercial/residential. In other words, I'm not too worried about putting out a panel due to codes.

    Once working, I plan to just let this thing run the fridge and freezer all the time. I plan to buy quality shit up front, not looking to go cheap on the important stuff.

    Thanks everyone for the responses, I shall check the links. :)
  15. BTPost

    BTPost Stumpy Old Fart,Deadman Walking, Snow Monkey Moderator

    If you are looking for "quality" then look no further than OutBack Power Systems... They build the BEST s**t on the market. These same guys, designed ALL the Trace Inverter SW, TR, and DR Series Inverters, before the company was sold out from under them. Then they walked "Out" the "Back" Door, down the street a couple of blocks, and started their OWN Outfit, named OutBack Power Systems. ..... YMMV.....
  16. Kingfish

    Kingfish Self Reliant

    I just found out people from Trace also built Magnum inverters. Magnums have all the same fets and other components as Outbacks just not the huge aluminum case.. Magnum designed a charge controller that will work better with the Nickel Iron Batteries accepting voltages all the way down to 9 volts before shutting off. They did this because you can run these batteries down farther then lead acid. Also running Nickel Iron down does not hurt them. SO you get more power, longer discharge time. Im still researching this new system. KF
    BTPost likes this.
  17. -06

    -06 Monkey+++

    A bud in central NC has six panels, propane fridge, on demand gas hot water heater, and is totally off grid. This year he added a standby propane generator for the AC and long term rainy weather. He had a small diesel genny but it had to be started manually. They live very well, watch TV/etc. all they want. They pump their water to a tower but added an RV pump for the shower for more pressure. We are talking about building trailers to house panels that can hook to our homes or be towed for remote power. The bottom will be storage space with a separate vented area for the batteries/controller.
  18. Wrenchbender

    Wrenchbender Monkey

    OK, now I know WHERE to buy the stuff, but I still have to calculate how many watts of panel and how much battery I need.
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