GT Only vs. GT with Battery Backup

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DC Builders
DC Builders Registered Users Posts: 13 ✭
in Solar Beginners Corner #1
If I want to run a 48 volt battery UPS GT system, with 14 210 WATT Solar Panels, Vmp 26.6; Imp 7.90.

Instead of one string at 372 Volts, 7.9 amps to the inverter for GT Only........

I would run 7 strings of 2 panels for 53.2 volts and 55.3 amps to my charge controller then to batteries and on to inverter?

What is the best way to do this?

Thanks
Dan
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,448 admin
    #2
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    Re: GT Only vs. GT with Battery Backup

    Is there anyway you can
    1. run 15 panels--2x string of three
    2. run a XW 4024 hybrid inverter
    Your issue is, when using the MPPT charge controller--you need a minimum voltage at the battery bank of around 52-54 volts DC (to float them).

    Your 2x Vmp 26.6 = 53.2 vdc on a cool sunny day... On a windless sunny 95F summer day--your panel Vmp is going to be around 42 volts--way too low to charge the battery bank and run Grid Tie.

    So, either you need 3 panels in series or a Vbatt of 24 volts to operate with two panels in series.

    Where will the installation be, what brand/model of panels, what Hybrid inverter and MPPT Charge Controller are you thinking of using?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • DC Builders
    DC Builders Registered Users Posts: 13 ✭
    #3
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    Re: GT Only vs. GT with Battery Backup

    Thanks Bill,

    I'll check into it...I am on the right track lowering the voltage?
    I deleted my post about single line drawing. It was too snobbish.
    We are here to help and get help, not to turn solar energy into a litigious night mare.

    Thanks again,
    Dan
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,448 admin
    #4
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    Re: GT Only vs. GT with Battery Backup

    It depends on what you need...

    A "typical" GT inverter operates around 200-600 VDC input.

    A typical Hybrid inverter runs on a 24 or 48 volt battery bank. And the typical MPPT charge controller runs on Vbatt-Charging+2 volts to around 140-150 VDC maximum.

    And, from what I have looked at the Vmp/Voc specs. of solar panels--Vmp hot : Voc cold (minimum operational to open circuit below freezing voltage) range was almost 1:2...

    It works out, roughly that Vmp nominal at STC is around ~100 VDC maximum. If Vmp>100 VDC by much--sub zero open circuit voltage may exceed 150 VDC. And for hot weather/climate on a battery bank--you are looking at nominal temperature Vmp of the string of around 72 VDC minimum... (based on 48 volt bank).

    At least for now (until the whole Xantrex name change / website update is done)--this link still works to configure the strings for the XW 60 Amp charge controller.

    There are other inverters / hybrid inverters (in and outside the US)--Do you have a particular hardware configuration you are looking at?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • DC Builders
    DC Builders Registered Users Posts: 13 ✭
    #5
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    Re: GT Only vs. GT with Battery Backup

    Bill,

    Class had holiday this weekend. I am a little a head of what we have been studying.
    Can see that once batteries are introduced the situation is more complex.

    Checked that 4024 you recommended it looks like it does a lot.

    I am just finally understanding the algebra on kWh/battery voltage.

    volts * Amp hrs (watt hours)
    volts

    volts over volts = 1, so answer is in Amp Hours

    Not stupid just slow,
    Dan
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