Mixing panel wattage

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System
System Posts: 2,511 admin
in Solar Beginners Corner #1
I know that the answer is in here some where, I just can't find it. Please explain it again.

I have 4 SolarWorld 175watt panels paired as 48vdc. I ordered 2 more, planning on changing to 72vdc. When they arrived, they were 165watt. NON-RETURNABLE!

Question: Is it best to hook them up 175+175+165 each string? Or would I have less of a loss of power if I went two strings ie: 175+175+175 and 175+165+165? System is FX3048, MX60, 8 golf batteries and a combiner box. If it's best, I could go with three strings ie: 175+175 175+175 165+165 and add another 15 amp breaker to the combiner box.
I live in Hawaii, 5 miles from the most active volcano in the world. Low temperature is not a problem for the MX60.
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  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    #2
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    Re: Mixing panel wattage

    Where do you plan to install the system (~city and state)? A "72 V" array (three 24 V modules in series) would probably be a problem in a cold environment.

    More later,
    Jim / crewzer
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #3
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    Re: Mixing panel wattage

    What are you using for a charge controller, that's what the voltages need to match.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

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  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    #4
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    Re: Mixing panel wattage

    Cryobear,

    The answer you’re looking for probably goes something like this:

    Modules wired in series will operate at the lowest module current and at the sum of the module voltages. Modules (individually or in series strings) wired in parallel will operate at the lowest module (or string) voltage and at the sum of the module (or string) currents.

    Module specs ( http://lib.store.yahoo.net/lib/wind-sun/SW-175-Mono.pdf ):

    SW175
    Voc: 44.4 V
    Vmp: 35.8 V
    Isc: 5.30 A
    Imp: 4.89 A

    SW165
    Voc: 44.0 V
    Vmp: 35.3 V
    Isc: 5.30 A
    Imp: 4.68 A

    You have three practical options for your module inventory:

    Option A, two series strings wired in parallel:

    Series string 1: 175 + 175 +165
    Series string 2: 175 + 175 +165

    Array spec:
    Vmp = 35.8 V + 35.8 V + 35.3 V = 106.9 V
    Imp = 4.68 A x 2
    Array STC Power = 106.9 V x 4.68 A x 2 = 1000.6 W


    Option B, three series strings wired in parallel:

    Series string 1: 175 + 175
    Series string 2: 175 + 175
    Series string 3: 165 + 165

    Array spec:
    Vmp = 35.3 V + 35.3 V = 70.6 V
    Imp = 4.89 A + 4.89 A + 4.68 A = 14.46 A
    Array STC Power = 70.6 V x 14.46 A = 1020.9 W


    Option C, three series strings wired in parallel:

    Series string 1: 175 + 175
    Series string 2: 175 + 165
    Series string 3: 175 + 165

    Array spec:
    Vmp = 35.8 V + 35.3 V = 71.1 V
    Imp = 4.89 A + 4.68 A + 4.68 A = 13.85 A
    Array STC Power = 71.1 V x 13.85 A = 984.7 W

    Option B will deliver the highest power. I do not typically recommend “48 V” arrays for 48 V nominal flooded-cell battery systems. However, the combination of the relatively high array voltage (70.7 Vmp STC) plus the mild average daytime high temp (Honolulu = 89 F) may allow for an operational array voltage that’ll be high enough to charge and EQ the batteries.

    If Option B’s summer operational Vmp is not high enough, you’ll have to consider Option A. Its winter Voc should be just below the MX60’s operational limit of ~141 V.

    HTH,
    Jim Goodnight
    “crewzer”
    Eastern Regional Sales Manager
    OutBack Power Systems
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