Wire question

SystemSystem Posts: 2,511 admin
Hi all, I am new to solar power and have just purchased a kit for my cabin. I have 3 - 20 watt panels and a 4 watt controller going to 2 - 6 volt batteries. My problem is that I have about 70 feet from my main usage area (fridge and lights) to the solar panels. Am I better off with the controller and batteries closer to the fridge or closer to the solar panels, or does it make any difference at all?

Thanks in advance.


  • icarusicarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Wire question

    Let me first answer you basic question.

    The shorter wire runs the better. The higher the amperage,, the bigger the voltage drop over distance. (not strictly true,,, but given a choice between the high amperage draw close and the low charging amperage farther away,,, I would choose the former)

    Now, to answer a question that hasn't been asked (but will IMHO).

    With 60 watts of panel,, there is no way on gods green earth that you are going to run ANY sort of fridge off of solar panels! As many of us have suggested in many ways on this forum,,, Do the math! Calculate your loads/charging capacity and see how much you can draw.

    A short rule of thumb,, sort of down and dirty calc, is if you take 50% of your panel capacity, multiply by 4 (or maybe 6,, the number of hours of perfect sun) and that gives you a ROUGH number of watt/hours that you can expect to be able to pull out of the batteries on a perfect day. This takes into account panel efficiency, charge controller ef, battery charging ef, inverter ef, and wiring loses.

    So in your case,, your 60 watts of panel, multiplied by an optimistic 6 hours would give 360 wh,,,less ~50% losses giving you somewhere around 180wh a day,,,, if you are lucky. (Smaller systems tend to be way less efficient than larger ones for a variety of reasons,, charge controllers mostly).

    So your 180 watt hour would run 1 15 watt cfl bulb for ~ 11 hours. It would run my propane fridge with a 350 watt element ~ 1/2 hour. I doubt it would even START a compressor type fridge.

    Now if you are planning ANY reserve capacity,, you will have to cut those numbers further. For example it is never advisable to draw a battery down more than 50% and many people, myself included suggest drawing it down no more than 20%. An other rule of thumb is the three day rule. That is having enough battery capacity to run your expected loads for 3 day without the need for any charging. It is then nice to have enough panel capacity to get that three days back in one day of sun.

    May I suggest that before you spend any more money on your system, you spend a bunch of time reading all you can here and elsewhere about small PV systems. (There are some very smart folks here,,,(I am not including myself in that camp however) who have forgotten more about PV solar than most of us will ever know. The wheel has been invented before,,, take advantage of that knowledge).

    One other thing to note. I assume that your batteries are 2 T-105 type golf cart batteries,,, with perhaps 225 amp/hours each. That would be a good start to a system. The general rule of thumb however is that your panel capacity should be ~5-13% of your battery bank for proper charging,, something above 11 amps. Your 60 watts of panel would likely only put out somewhere in the neighborhood of 3-4 amps on a perfect day,,, into 12vdc. You will also need a way to top up these batteries,,, and a auto type charge is NOT the way to do it.

    Good luck,, ask as many questions as you need to,,, and welcome to the club,


    PS I assume that you meant to say you had a 4 AMP not 4 WATT controller,,,,
  • System2System2 Posts: 6,290 admin
    Re: Wire question

    thanks for your help. My fridge is propane but requires 12v for electronic ignition. It doen not have the heater element for defrosting.(it is a little older). I also only use this during summer when I am getting at least 12 - 14 hours of sunlight. I am in a dessert climate.
  • icarusicarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Wire question

    Got it,

    That's what I get for making assumptions! I too run propane fridges,,, but they have heating elements that would allow them to run on 120 vac or 12vdc.

    A Dometic propane fridge will draw ~.05 amps on standby,,, maybe an amp for a few seconds during the ignition phase. In this case, the draw on the batteries is so small it doesn't really matter where you put the batteries and the controller. I have run a dometic control board 150' from the battery with a piece of #12 wire!

  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Wire question

    you still need more pv to properly charge the batteries you have as tony pointed out (and don't sell yourself short tony for you know more than you think), but you'll also have need of a larger capacity charge controller. i recommend you search our host's webstore for pvs and a controller. http://store.solar-electric.com/ also read up on things here too so you can get a better idea of what it is you are doing and may want to do.
    package deals aren't really deals as they fit few right on the button and one can piece together something better most times. if it is at all possible for you to return those items, with the exception of maybe the batteries, i'm sure everybody here can advise you on something that will work out even better for you.
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