Help with small project,Please

dlonedlone Registered Users Posts: 15
I have 10/100 watt 12 volt panels. My invertor is also 12 volts. My controller is an 80 amp Outback.

My question is what is or would be the best wiring configuration to use? Also if I bump up the voltage to 24 v how do you wire the panels together? Will I have to have 5 strings or is there another way? I'm new to this guys,so be easy on me.
Thanks n advance


  • AntronXAntronX Solar Expert Posts: 462 ✭✭
    Re: Help with small project,Please

    Provide more details. Are panels identical? What is their Vmp, Voc? Length of wire between array and FM-80. Wire gauge. Battery type and amount. Your location. Panel orientation angles. Inverter brand, model. Amount of load to be powered.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Help with small project,Please

    I believe Outback actually has a string configuration tool on their site. Not certain.

    There are some fine details that would make a difference, such as actual Vmp & Voc of the panels as well as length of wire run from the array to the charge controller and not least your ambient temperatures (panels put out more when cold, less when hot).

    The simple answer, if it were me doing it, would be five strings of 2 panels each: the two in series giving "24 Volts". There are other potential configurations, but the further from "12 Volts" the array is, the more down converting the controller does and the efficiency drops a bit.

    You could also do it "the other way 'round" and put five in series. This would probably be a Voc of less than 120 Volts, well under the FM80's max, but it depends on the specific panel and temperature conditions. Then you'd have two parallel sets of 5.

    With multiple strings in parallel, be sure to fuse each string.
  • BB.BB. Super Moderators, Administrators Posts: 32,027 admin
    Re: Help with small project,Please

    Welcome to the Forum!

    Not a trivial question at all... The Vmp-array (Voltage maximum power) needs to be a few volts higher than the battery charging voltage and Voc (Voltage open circuit) when cold needs to be lower than the maximum input voltage of the charge controller.

    Basically, this means for a 12 volt battery bank, Vmp of the array will work from ~17.5 volts to ~100 volts... And you achieve those voltages by placing one or more solar panels in series (+ to - to + to - etc.)--Just like placing batteries in series for a 3 D Cell flashlight.

    Next, comes the question of why 17.5 or 40 volts or 100 volts... Basically for the Outback MPPT (Maximum Power Point Tracking) Charge Controller--its optimum range for a 12 volt bank (most efficient) will be from ~17.5 volts to 35 volts for Vmp array.

    If your array will be a long distance from the battery bank/charge controller--then you may want to look at the 90-100 Volt Range for the Vmp-array operating voltage.

    If you remember your electrical math: Power = Voltage * Current

    If you make the voltage 5x higher (for example, Vmp=17.5 volts * 5 series = 87.5 volts), then your current from the array to the charge controller will be 1/5th as large--And you could spend 1/4 the price on copper wiring from the array to the charge controller. Note, the charge controller will be a little bit less efficient--but the savings in copper costs usually more than makes up for the percent or so extra losses.

    To do this exactly, can you tell us which brand/model number of panels (or what their Vmp/Imp/Voc rating is)--And also where you live (very cold panels will have their Voc rating go up quite a bit).

    One of us can help give you a couple configurations to meet your requirements.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • dlonedlone Registered Users Posts: 15
    Re: Help with small project,Please

    Thanks, glad to be here,and find a site to help us newbes out.

    My array will be about 25 ft of wire away(Havent purchased wiring yet).My panels are Silicon solar Model LS 100 Monocrystalline. VMP18v IMP5.55a AM1.5 1000/m @25 degree c (I have 10 of these panels)

    My invertor is a Xantrex 1512, OutBack 80 amp Controller,With 250a Midnight Solar Plus DC 250 Plus.

    Battery bank. I dont have them yet. They will be on the other side of the wall ,of my Electronics. I'm in Indiana and have a clear view of the sun for most of the day.Panels will be mounted stationary,for now. I know this is a small unit,but figured I would start small. Hope this helps. Panels are 12 volts also
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Help with small project,Please

    Actually, 1000 Watts of panel is good for a "12 Volt" system and is about the limit the FM80 can handle in that system configuration (I think they recommend 1400 Watts maximum). It should be capable of around 3 kWhrs per day or more if sun/temperature are good.

    25 feet of run between the array and the charge controller is not difficult to handle. In the 5 rows of two configuration it would be about 30 Amps. Use the Voltage Drop Calculator to get an accurate fix on your wiring.

    You could probably charge around 500 Amp/hrs of battery with this set-up, which would give you approximately 3 kWhrs per day to work with (neatly coinciding with the above harvest figure) maximum. This begs the question: what do you plan on using it for? It is always best if you determine your loads first, and then design the system to accommodate them (or modify them if supplying the required power becomes too problematic).

    So ... Application(s)? :D
  • dlonedlone Registered Users Posts: 15
    Re: Help with small project,Please

    Just want to start with enough to run some lights,coffee pot,possibly T.V.or frig in the event of a power outage.I have a 4k generator,but its expensive to run 24 hrs a day. We have lost power around here for a week or more the last two years. With all the limbs hanging over the power lines,its only a matter of time before it happens again. Again thanks for the help.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Help with small project,Please

    Okay, a couple of the things you mention are big power users: coffee pot (heating element) and refrigerator.

    That said, I run a refrigerator and much more on far less than 3 kWhrs per day. But I'd skip the coffee pot: probably 1200-1500 Watts right there. Even if only for a few minutes a day, it will suck up a lot of power. My refrigerator uses roughly 1.2 kWhrs per day (roughly; temperature and humidity and how often you open the door makes quite a difference).

    Add some batteries and I think you've got a viable small off-grid system.

    One potential problem: I believe your inverter is an MSW type, and the refrigerator won't "like" that. It may not run at all, or most likely it will draw more power and shorten lifespan. For occasional use though, if it runs it at all it should be fine.

    Don't lose the generator; even the best off-grid system needs back-up because the sun doesn't always shine! Right now, I can't see across the street for the smoke from forest fires. :cry:
  • icarusicarus Solar Expert Posts: 5,433 ✭✭✭✭
    Re: Help with small project,Please

    Nix on all the resistance heating element appliances (Coffee pots, hair driers etc)

    There are tons of alternatives available that don't use electricity. For example, Coleman makes a very nice stove stop "Mr. Coffee" that works as well as any plug in variety.

    Start you system design first by getting a good handle on the loads, and look to conserve every KWH possible. A few quick rules of thumb, first, off grid battery based systems will yield ~1/2 of name plate rating, averaged out over about 4 hours of sun per day.

    Name plate rating/2*4=Daily KWH harvest.

    People usually over estimate their potential solar harvest net/net, at the same time they under estimate their loads, leading to a system that doesn't initially perform very well. To avoid this, and to avoid the "ready, fire, aim syndrome," do the best homework you can to minimize your chance to spend money on a system that doesn't perform as it should.


    PS Also consider how you are going to use this system. If it is a part time (cabin?) system, you might find that a Propane fridge for example comes a a better cost curve than a conventional fridge. That curve changes as the house gets used full time however (in favor of conventional fridge).
  • dlonedlone Registered Users Posts: 15
    Re: Help with small project,Please

    I bought one of those Kill a watt meters a few weeks ago, to check out a few things.
  • dlonedlone Registered Users Posts: 15
    Re: Help with small project,Please

    Thanks for all the help guys,Spent around 4000 so far,didnt realize I needed a controller,such a big DC box. But hopefully I have everything sized right,to be able to upgrade.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Help with small project,Please
    dlone wrote: »
    Thanks for all the help guys,Spent around 4000 so far,didnt realize I needed a controller,such a big DC box. But hopefully I have everything sized right,to be able to upgrade.

    Ah, upgrade. Tricky thing, that. :roll:

    Your 12 Volt system is about at the limit. If you need more Wattage capacity you either have to install another separate system or switch out inverter & batteries to go to higher Voltage. Something to think about.

    The good news is that an Outback FM80 will handle 80 Amps at 12, 24, or 48 Volts so you shouldn't have to buy a new controller if you decide to up the system Voltage.
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