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Thread: Small system to power freezer and refridgerator

  1. #1
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    Default Small system to power freezer and refridgerator

    Very new to solar but not new to conservation. I want to build an independent battery backup system that can run my 5 cubic foot freezer and 18 cubic foot fridge (energy star). I have been confused by some of the varied information out there on manufactures and what they want to sell you. Manufacturer states both together use 625 kw a year. 1.7 kw per day. I have read that a 48v system is better than 12 due to wiring cost and mppt charge controller is the best. Want to use flooded batteries and quality inverter. I live in arkansas and plan on installing panels on ground mounted, adjustable tilt brackets instead of roof mount. Have nice south facing location. Now question is how many 230v panels, and 12v deep cycle batteries in series would it take to build system to do this and interested in having it be expandable in future.

  2. #2
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    Small is not relevant to what you want to do. It is actually quite a large expensive system. It all depends on how watt hours you need in a day to determine the array wattage, Battery capacity is fixed at daily watt hour usage and voltage. and charge controller is a function of panel wattage, controller type, and battery voltage.

    So first you need to nail your daily watt hour demand. Without that you can go no where. So I will use your 1.74 Kwh or 1740 watt hours you quoted and assume you live in Ft. Smith Arkansas.

    So in winter FTSM Solar isolation at optimum tilt angle of Latitude is 3.7 Sun Hours. Assuming you go with a MPPT charger you need [1740 wh x 1.5] / 3.7 Hours = 705 watts. Add 10% for Cloudy Day recovery and round up and you get 780 to 800 watts

    For A PWM controller you need [1740 x 2] / 3.7 = 940 watts. Add 10% for cloudy day recovery and round up to 1050 watts

    For battery capacity you need 1740 wh x 5 = 8700 watt hours of capacity. So at:

    12 volts you need 8700 wh / 12 volts = 725 Amp Hours
    24 volts you need 363 Amp Hours
    48 volts you need 182 Amp Hours

    For a MPPT controller running 800 watts input the controller needs to be at least:

    12 volts = 800 watts / 12 volts = 66 amps or an 80 amp MPPT controller. Can grow to 4000 watts at 48 volt battery.
    24 volts needs a 33 amp or an 40 amp MPPT controller can grow to 2000 watts @ 48 volt battery.
    48 volts needs a 17 amps or a 20 amp MPPT controller can grow to 1000 watts

    For a PWM controller Panel current = output current: I am not going to run through this drill you can figure it out.
    MSEE, PE

  3. #3
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    Actually, I think you need twice the amp/hr batteries above for 5 days of no sun, since you don't want to discharge the batteries below 50%.

    So, buy 6 L-16 400amp/hr batteries ( Local Deka or Trojan dealer ), wire in series for 48v, 400amp/hr battery bank.
    About $1500

    Buy a pre-built Outback power center, that includes the inverter, (spec an FM80 charge controller on it and a 48v inverter) and all the breakers, both DC and AC you need, hang it on the wall and plug in your panel input, and battery leads, and you're ready to go.
    About $4000 (Photo of one I set up for a guy in 24v )

    I only use Outback stuff, so I can't comment on other brands.



    Panels: 8 to 10 230w panels will do you.....or get 9, wire in 3 strings of three panels in series, and that will give you about 90+ volts in on the charge controller, meaning lower line loss.

    Make the mounts ( I'm assuming you fabricating locally ) so they will tilt as much as 50 degrees off horizontal for the winter, and back to 20-25 degrees for summer. Adjusting monthly or quarterly will up your production some for sure. You can watch the screen on the Outback charge controller to hit the peak numbers at any angle at any given time.

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    Quote Originally Posted by TnAndy View Post
    Actually, I think you need twice the amp/hr batteries above for 5 days of no sun,
    Sorry Andy but that is a ridiculous design goal for a emergency system. I mean you are talking about $4000 worth of batteries that will likely never be used, and end up in the garbage pile in 5 years.

    Even my recommendation is silly. I only did it to make a point of how silly it is. There is no need for solar period in this application. It can all be done with small generators and small batteries with a standard AC charger for less than $1000. To do it completely with solar he is looking at $5000 to $6000, if done with 5 real day battery capacity he is staring at $8000 for something rarely if ever used.
    MSEE, PE

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    Well, Derick, it all depends on what your view of the world is, and where it's headed.

    He asked, I responded......I didn't climb on a soap box and preach it.

    IF what I suspect is coming actually happens, 8,000 bucks in the bank won't be worth diddly, and fuel for a generator will be dang hard to come by.

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    Quote Originally Posted by TnAndy View Post
    Well, Derick, it all depends on what your view of the world is, and where it's headed.

    He asked, I responded......I didn't climb on a soap box and preach it.

    IF what I suspect is coming actually happens, 8,000 bucks in the bank won't be worth diddly, and fuel for a generator will be dang hard to come by.
    Thanks Tnandy, we think that if things do get bad and if you only have panels and tied to grid and the grid goes down then you only have power when sun is out. My main concern is to keep the most important thing on line and I feel it is refrideration, maybe i could get by with only the refriderator and forget the deep freezer if it does happen. also we are getting a nice wood heater to heat house and cook if needed. if nothing does happen then at least i have some nice panels. just trying to think ahead thanks again

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    Quote Originally Posted by ken1645 View Post
    Thanks Tnandy, we think that if things do get bad and if you only have panels and tied to grid and the grid goes down then you only have power when sun is out. My main concern is to keep the most important thing on line and I feel it is refrideration, maybe i could get by with only the refriderator and forget the deep freezer if it does happen. also we are getting a nice wood heater to heat house and cook if needed. if nothing does happen then at least i have some nice panels. just trying to think ahead thanks again
    Actually you have nothing as the inverter will shut down.
    Rich
    WWW.solarsaves.net

    NABCEP certified Technical Sales Professional

    http://www.solarpaneltalk.com/showth...Battery-Design

    http://www.calculator.net/voltage-drop-calculator.html (Voltage drop Calculator among others)

    www.gaisma.com

  8. #8
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    Yep.....exactly what Rich said. Panels and a grid tie ONLY setup ( which most grid tie setups are ) means without the grid, you have no power.

    Virtually ALL solar setups require some kind of buffer, either in the form of batteries, or the grid ( which acts like a big ole battery ). Panels alone, even with a grid tie inverter, won't work.....sun or no sun.

  9. #9
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    Quote Originally Posted by Naptown View Post
    Actually you have nothing as the inverter will shut down.
    That is why im looking at a battery type system, i know its expensive but like you said money in a bank thats closed is nothing, didnt meen to sound ignorant but we feel that things have a high probabilty of getting bad. so we just want help with a system to run either both appliances or maybe just the fridge which also has a small freezer, if things get bad people relying on just panels or generators for back up might be in for a shock. just need some help building a system with or with out the freezer thanks again

  10. #10
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    Trying to prepare for the end of the world as we know it makes no sense. Where are you going to get the things to put in the fridge? Where to buy medicines too many have to have to live?

    If people spent more time worrying about how to get ahead we would all be better off.

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