Help choosing right power inverter!!!

marcini
marcini Registered Users Posts: 7 ✭
in Off Grid Solar & Battery Systems #1
Hi Guys,

I have 840watts off solar panels, and 25 12v 72a/h deep cycle batteries, charged by Morningstar MPPT 50, and I'm debating on what kind of inverter i should get 12 or 24vdc, i'm not looking to get anything crazy and expansive, just something for a backup in case of power failure, maybe something with charger and transfer switch built in. So what's the benefit of having 24v over 12volt one except lower amperage, 12 volters seem to be bit cheaper. Thanks in advance.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #2
    Re: Help choosing right power inverter!!!

    Half the current and twice the useful voltage drop allows you to transmit 4x the power on the same wire.

    Roughly 1,200 to 1,500 watts is a nice maximum size for a12 volt inverter.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • dwh
    dwh Solar Expert Posts: 1,341 ✭✭✭
    #3
    Re: Help choosing right power inverter!!!

    Lower amperage is pretty much it. You might save a few bucks on wire by rigging your 12v batteries in series pairs rather than running a bunch of equal length parallels.

    That's a lot of battery...1800ah at 12v...21,600 watt hours.

    Take 'em down 50% and that's 10,800wh. Take at least 3 days to recharge that off your PV *if* you got 5 hours/day of good sun.

    Even at a 5% charge rate you'd still need a 90a charger to plug in the wall - so an inverter with a built-in 100a charger @12v would be barely big enough.

    With a 24v rig, you could run 60a charger and do a better job of charging from the grid.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
    Re: Help choosing right power inverter!!!

    No offense, but this is going about things the wrong way. Usually you determine what loads you need to supply first. From that you can determine what size inverter you need and how big a battery bank. Then you know how big an array you need to recharge with.

    Those batteries aren't going to be of much use; trying to put 25 batteries in parallel is a really bad idea, and even upping to 24 Volts would leave you with a lot of parallel connections - which means difficulty in keeping current flow even between batteries.

    The choice of 12 or 24 Volt is based mainly on the consistent Wattage need. 12 Volts is good for perhaps 2 kW maximum. It gets troublesome at higher Wattage.

    You've got 840 Watts of panel? To work backwards that could supply:
    840 @ 80% efficiency = 672 "usable" Watts over 4 hours; about 2.6 kW hours per day
    "12 Volt" system:
    672/14.2 V charging = 47 Amps - enough for 470 Amp hours (6-7 of those batteries in parallel - don't do this!) which is about 2.8 kW hours.
    "24 Volt" system:
    672/28.4 V charging = 23 Amps - enough for 230 Amp hours (3 parallel strings of two of your batteries - doable) which is about 2.7 kW hours.

    Usual caveats: rough calculations for comparative purposes only; not actual system recommendations.
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  • marcini
    marcini Registered Users Posts: 7 ✭
    #5
    Re: Help choosing right power inverter!!!

    What do you guys think of this:

    http://sunelec.com/index.php?main_page=product_info&cPath=6_40&products_id=795

    I happened to get all those batteries from a slightly used building ups system, that's why i have so many, i would never purchased so many for such a weak solar output. That's why i'm asking you guys on how to set it up the best way. thanks again.
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #6
    Re: Help choosing right power inverter!!!

    Determine the loads, then work backward. You might find that more than one inverter suits your needs. A small one for everyday small loads, a bigger one for larger loads. Inverters are most efficient when they are loaded efficiently. (a 2000 watt inverter running a 200 watt load is very inefficient for example)

    Tony
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #7
    Re: Help choosing right power inverter!!!
    marcini wrote: »
    What do you guys think of this:

    http://sunelec.com/index.php?main_page=product_info&cPath=6_40&products_id=795

    Well I'd take issue with their claim of an MSW inverter working with inductive loads. Not a good match-up if that's what you're trying to run.
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  • dwh
    dwh Solar Expert Posts: 1,341 ✭✭✭
    #8
    Re: Help choosing right power inverter!!!

    It's a MSW inverter. 50a @ 24v is barely enough charger for a 900ah (@24v) bank, but it is enough. The 20v low voltage cut out is pretty low - take it down that low too many times and your batteries will be toast.

    This part...

    "The high power charger (120A) can charge a 12V/1600 AH battery bank in 14 hours. For example, a single unit of Inverter 2424 with a 1600 AH battery bank can supply a 2000W workload for over 8 hours after a charge of 14 hours."

    ...is all BS since the minimum input voltage is listed at 20v so this thing doesn't even do 12v.

    Says it does 220v or 230v, but is that two hots and a neutral ala US, or is that Euro type?
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  • marcini
    marcini Registered Users Posts: 7 ✭
    #9
    Re: Help choosing right power inverter!!!

    don't forget i also have my Morningstar MPPT 60 charger and to extra 300watt panels from Schott Solar that I didn't connect to anything yet, since i'm not sure what to do with them yet, because of their awkward voltage rating, that doesn't fit my system at the moment.
    Schott Solar Panels - they work, but the glass shield is shattered and that drags the efficiency down, but let's not digress, let's leave this for another occasion.
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