24V panels.. 30VMP or 36VMP??? (VOC comes into play also)..

ywhic
ywhic Solar Expert Posts: 621 ✭✭
in Off Grid Solar & Battery Systems #1
Down to the last questions and concerns..

I know photowhit mentioned in another thread that 36VMP panels are better than the 30VMP panels.. but the VOC is like 44.5 on the 36VMP panels then the VOC on the 30VMP panels which is 36-37 VOC..

The MorningStar Pro 30 controller limit for VOC is 50V.. the Xantrex C35 Controller is 55V for VOC..

The lower panels with 36-37VOC would be at 44.4VOC on -5 to -13 F day (using 1.2 as a correction factor).. the higher VOC 44.5 panels would be at 53.4VOC.. and over the 50 amp limit for the Morningstar Pro 30 controller I keep looking to..

Photowhit also mentioned the 30VMP panels are more suited to/for gridtie systems and may not charge thru the PWM controller enough..

The 24v (30VMP) panel at 100'F would at optimum produce 27.36 (using 0.08 degradation number I found online * 33'F change to get to 100'F..) which I would think would make the controller still charge the battery bank..

And the 24v (36VMP) panel at 100'F would at optimum produce 33.36.. (using same 0.08 number)..

The only downside I see on the Xantrex C35 is the lack of a heatsink.. and minding the fact TX gets to 100F during the summer..

The other issue is that it does go to 0' F where I am at also and I'd like some power from the panels in the winter for lighting and water pump.. Heat is by a woodstove.. I am totally off grid by 7 miles..

What 24V panels should I focus on and what controller??

The proposed bank of 24v battery will be 440ah..
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Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: 24V panels.. 30VMP or 36VMP??? (VOC comes into play also)..

    Okay, with a PWM controller the Voc doesn't really matter.

    What will really matter is the 30 Vmp is not enough to charge a 24 Volt system using a PWM controller.

    Most of the time your concern will not be with cold temps and Voc, it will be with hot temps and Vmp. Considering the best the 30 Vmp panels can do is not sufficient to charge a 24 Volt system, when that hot Texas sun gets involved you are going to have undercharged batteries.

    Your best bet to cover all angles is the higher Voltage panels and the Xantrex controller. Although I am not at all sure that the 1.2 compensation factor is necessary for 0F.
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  • ywhic
    ywhic Solar Expert Posts: 621 ✭✭
    #3
    Re: 24V panels.. 30VMP or 36VMP??? (VOC comes into play also)..

    Why is the C35 and C60 the same rated 55 VOC and the C40 rated at 125 VOC in the book??
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
    Re: 24V panels.. 30VMP or 36VMP??? (VOC comes into play also)..
    ywhic wrote: »
    Why is the C35 and C60 the same rated 55 VOC and the C40 rated at 125 VOC in the book??

    You'd have to ask Schneider/Xantrex that. :D
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  • Photowhit
    Photowhit Solar Expert Posts: 6,006 ✭✭✭✭✭
    #5
    Re: 24V panels.. 30VMP or 36VMP??? (VOC comes into play also)..

    The c40 will work for 48v systems, anything lower than 34 volts is NOT made for 24 volt systems with out running pairs through an mppt charge controller. They have been mislabled by the seller IMHO.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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  • ywhic
    ywhic Solar Expert Posts: 621 ✭✭
    #6
    Re: 24V panels.. 30VMP or 36VMP??? (VOC comes into play also)..

    Think I am going with the C35 or C40.. gives me wiggle room and with meter option front gives me about the same as the MS 30 Pro..

    For a 24V system with 950 watts (potential) coming in at 21 CMP would this be my settings for it?? (in the manual)..

    24Vsetup.JPG
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  • Photowhit
    Photowhit Solar Expert Posts: 6,006 ✭✭✭✭✭
    #7
    Re: 24V panels.. 30VMP or 36VMP??? (VOC comes into play also)..

    Check with the battery manufacturer, if they don't post any thats a good place to start.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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  • 65DegN
    65DegN Solar Expert Posts: 109 ✭✭
    #8
    Re: 24V panels.. 30VMP or 36VMP??? (VOC comes into play also)..

    If you are in a hot climate I would go with single crystal panels as they have less derating under hot conditions.
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  • jagec
    jagec Solar Expert Posts: 157 ✭✭
    #9
    Re: 24V panels.. 30VMP or 36VMP??? (VOC comes into play also)..
    ywhic wrote: »
    Why is the C35 and C60 the same rated 55 VOC and the C40 rated at 125 VOC in the book??

    Maybe they got a good deal on 125V FETs when they were producing C40s.

    I always suspected that "max Voc" ratings could be mildly exceeded, since the charger should pull the panels down by several volts even in float (and there's no real power at Voc anyway), but I'm certainly not willing to put that theory to the test.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #10
    Re: 24V panels.. 30VMP or 36VMP??? (VOC comes into play also)..
    jagec wrote: »
    I always suspected that "max Voc" ratings could be mildly exceeded, since the charger should pull the panels down by several volts even in float (and there's no real power at Voc anyway), but I'm certainly not willing to put that theory to the test.

    This is true depending on the controller. Some PWM will clamp the Voc down immediately, others have less input tolerance and will either shut down or fry. There's a pretty wide range of controllers out there now so I wouldn't hazard a guess as to how any of them perform.

    An MPPT type controller will not like too high a Voc no matter who made it; they function differently, so to speak.
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