MPPT Sizing Factor

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Scottg4001
Scottg4001 Solar Expert Posts: 47 ✭
in Advanced Solar Electric Technical Forum #1
Hey everyone, great site and I have learned much through this forum. My question: is there a equation or multiplier I can add to my stand alone sizing spreadsheet to factor in a MPPT charge controller to calculate the amount of panels needed with use of a MPPT? Most MPPT manufacturers state 30% more energy harvest than traditional non MPPT. Has anyone done this, is it even feasible? Or are there just too many factors? My understanding is that it will reduce the number of panels needed, but I would like to know how much; a tenth, half a panel? Thanks in advance, Scott
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  • Windsun
    Windsun Solar Expert Posts: 1,164 ✭✭
    #2
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    Re: MPPT Sizing Factor

    Essentially, for MPPT sizing you can just use the panel specs - that is the max power in watts, divided by the minimum battery voltage.

    For example, a pair of 140 watt panels feeding a 24 volt battery bank would be about 280 watts/22 volts (the lowest ever expected battery voltage), which gives about 12 amps as the smallest MPPT controller you could use.
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  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    #3
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    Re: MPPT Sizing Factor

    On average you might see 10% more output from an MPPT CC. At times you may see 30% + but only in the early morning
     
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  • Scottg4001
    Scottg4001 Solar Expert Posts: 47 ✭
    #4
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    Re: MPPT Sizing Factor

    WindSun, thanks for the reply. I am not specificaly asking what size MPPT to use but more about factoring in the 30% extra energy production that a MPPT will give you into your sizing calculations.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #5
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    Re: MPPT Sizing Factor

    The 30% factor generally depends on subfreezing weather and refection of sunlight from snow.

    For warm weather, I like to derate the solar panel+charge controller by ~0.77 .... This allows for the fact that Pmaxpower=Vmp*Imp where Vmp falls with increasing panel temperature (plus some wiring, controller, and dusty panel losses).

    Remember too that a MPPT charge controller operates a bit like a self adjusting transformer (really a buck mode power supply--but don't know if you want that level of detail)--Where Power In = Power Out:
    • Power = Volt * Amps
    • Say Pin = 100 volts * 14.5 amps = 1,450 watts

    Then Power out for charging a 12 volt battery at ~14.5 volts would be:
    • Power out = Power In (*0.95 if you want some losses);
    • Iout = Pout/Vout = 1,450 watts / 14.5 volts charging battery = 100 amps into battery

    Note that while 100 amps out > 14.5 amps in -- In reality, the power is the same--Pin=Pou=~1,450 watts

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Photowhit
    Photowhit Solar Expert Posts: 6,003 ✭✭✭✭✭
    #6
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    Re: MPPT Sizing Factor
    westbranch wrote: »
    On average you might see 10% more output from an MPPT CC. At times you may see 30% + but only in the early morning

    ALso note that many people are using Grid tie speced panles for off grid use, these panels which fall between the standard charging voltage for 12V (@17.5VMP) and 24V(@35VMP) greatly improve with the use of MPPT charge controllers.

    Other reasons for using MPPT charge controllers have to do with long wire runs.

    This from someone who doesn't get much help from MPPT CC as my heavy load is in Summer running an AC, (they help more in winter when panel temps are lower, hence the voltage from panel is higher.) ...but I'll need them when I hook up my 29VMP panels.
    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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  • Scottg4001
    Scottg4001 Solar Expert Posts: 47 ✭
    #7
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    Re: MPPT Sizing Factor

    I believe I made this more complicated than it seems. The bottomline is my four 6v batteries require 2.75 240 watt panels, 24 VDC nominal. Obviously that equals three 240w panels. With 25% extra, do I need to spend the extra for a MPPT or would a standard PWM charger work fine. If you need more details to help me out I will be gladly list everything. Thanks,
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #8
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    Re: MPPT Sizing Factor

    What is the Vmp of the panels?

    Basically, I would recommend Vmp~35-38 volts for a 24 volt battery bank... To fully charge a battery bank, you need Vmp~35-38 volts minimum to recharge a 77F battery bank with Vbatt-charge=29 volts and Vbatt-equalize=30+ volts.

    A MPPT charge controller can only "down" convert the voltage... It cannot step up the voltage from low Vmp-hot panels.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Scottg4001
    Scottg4001 Solar Expert Posts: 47 ✭
    #9
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    Re: MPPT Sizing Factor

    Vmp= 30.2 volts, but I have yet to see that in real world conditions. Highest I've seen is around 28-29. The setup would basically be three panels in parallel= 30.2 Vmp, 23.85 Imp.
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #10
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    Re: MPPT Sizing Factor
    Scottg4001 wrote: »
    I believe I made this more complicated than it seems. The bottomline is my four 6v batteries require 2.75 240 watt panels, 24 VDC nominal. Obviously that equals three 240w panels. With 25% extra, do I need to spend the extra for a MPPT or would a standard PWM charger work fine. If you need more details to help me out I will be gladly list everything. Thanks,

    I think we need the specs on your panels. If your panels are 24 volt nominal you might be able to hook them up in series if you use an MPPT controller. There will be some advantages to that in cost, performance, and expandability.
    --vtMaps

    edit: I didn't see the last two posts before I responded.
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • Scottg4001
    Scottg4001 Solar Expert Posts: 47 ✭
    #11
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    Re: MPPT Sizing Factor
    vtmaps wrote: »
    I think we need the specs on your panels. If your panels are 24 volt nominal you might be able to hook them up in series if you use an MPPT controller. There will be some advantages to that, in both cost, performance, and expandability.
    --vtMaps

    Trina 235 watt panels. 7.81 Imp, 30.1 Vmp. Total of three panels
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #12
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    Re: MPPT Sizing Factor
    Scottg4001 wrote: »
    Trina 235 watt panels. 7.81 Imp, 30.1 Vmp. Total of three panels

    My opinion is get a midnite MPPT controller. Hook up your panels in series. You will not need the three fuses/breakers that you would need with a parallel setup. If you decide to buy a 4th panel you can rewire to be two parallel strings of two panels and you still won't need fuses/breakers.

    I don't know your distance between panels and controllers, but whatever it is you will also save on copper by using a smaller gauge than with a three in parallel configuration.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #13
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    Re: MPPT Sizing Factor

    Yea--You are going to need to put 2 or 3 panels in series to have reliable/full charging of a 24 volt battery bank and get a MPPT type charge controller.

    Midnite, Outback, MorningStar all make good Vin~140-150 input charge controllers (Voc-cold).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Scottg4001
    Scottg4001 Solar Expert Posts: 47 ✭
    #14
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    Re: MPPT Sizing Factor
    vtmaps wrote: »
    My opinion is get a midnite MPPT controller. Hook up your panels in series. You will not need the three fuses/breakers that you would need with a parallel setup. If you decide to buy a 4th panel you can rewire to be two parallel strings of two panels and you still won't need fuses/breakers.

    I don't know your distance between panels and controllers, but whatever it is you will also save on copper by using a smaller gauge than with a three in parallel configuration.

    --vtMaps

    Well, I currently have a 15amp Sunsaver MPPT on a two panel setup. Can I hook up the two panels with the same specs in series and still get enough voltage for a moderatley quick charge. I guess I have gotten mixed up with the current vs voltage for batteries. From what I have been reading, voltage is more important than current for battery charging.

    I also have a 45A MPPT TriStar CC still in the box.
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  • SolaRevolution
    SolaRevolution Solar Expert Posts: 410 ✭✭
    #15
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    Re: MPPT Sizing Factor

    IMO the higher output with MPPT will be seen when most typical off-grid systems need it most. In effect, when the batteries are at a low state of charge and the winter weather is colder than STC rated tempuratures.

    Spreadsheets for off-grid systems often reflect the highest expected loads and power production during the worst possible times. I would suggest using a multiplier which takes into consideration the effect on PV of the average daily low tempurature of your site. In warm weather it could be a bost as low as +/-5%. In cold ambient temps it could be boost as great as +/- 38%.

    If your spreadsheet is for on-grid, the only fair comparison between MPPT and PWM charge controllers would be using modules with nominal battery charging voltages (36 or 72 cells) In grid-tied systems the batteries are usually at float voltage while "selling" power. The difference would generally be that non-MPPT arrays would be at float voltage while MPPT arrays would be at the maximum power point. A difference of as little as +/- 7% in warm ambient temps and as great as +/- 28% in cold ambient temps.

    Alex Aragon


    PS:
    To all you number crunching MPPT detractors:;)
    I was using battery nominal voltage as lowest voltage for off-grid, 55vdc (if it were a 48 volt system) for float for on-grid and temp correction factors of +/-0.5% per deg C with 50c for high module operating temps and 0c for low module operating temps.
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  • Scottg4001
    Scottg4001 Solar Expert Posts: 47 ✭
    #16
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    Re: MPPT Sizing Factor

    Sorry everyone you have opened my eyes but I want to be on the same page. So let me start fresh:

    I currently have two 235 watt panels, 30.1 Vmp, 7.81 amps wired parallel. So I am getting about 27 volts and 14.4 amps on a sunny day. This is to charge (4) 6v 224Ah batteries. Now, you all have mentioned I should switch the wiring and use series to get about 54 volts and about 7.2 amps and this will charge the batteies quicker. Correct?
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #17
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    Re: MPPT Sizing Factor
    Scottg4001 wrote: »
    Well, I currently have a 15amp Sunsaver MPPT on a two panel setup. Can I hook up the two panels with the same specs in series and still get enough voltage for a moderatley quick charge. I guess I have gotten mixed up with the current vs voltage for batteries. From what I have been reading, voltage is more important than current for battery charging.

    I also have a 45A MPPT TriStar CC still in the box.

    I don't offhand know what is the max voltage the sunsaver can handle. You mentioned the Vmp of the panels, but not the Voc. Voc is what counts and you must also account for it being higher than specified when the temperatures are below standard conditions. Even a moment at too high a voltage can ruin your controller.
    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • Scottg4001
    Scottg4001 Solar Expert Posts: 47 ✭
    #18
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    Re: MPPT Sizing Factor
    vtmaps wrote: »
    I don't offhand know what is the max voltage the sunsaver can handle. You mentioned the Vmp of the panels, but not the Voc. Voc is what counts and you must also account for it being higher than specified when the temperatures are below standard conditions. Even a moment at too high a voltage can ruin your controller.
    --vtMaps

    Sunsaver is 75 volts max. Voc of panels is 37.1, so its borderline. And I am in south Florida, so the system will never see freezing temperatures.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #19
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    Re: MPPT Sizing Factor

    Because you are using a MorningStar 15amp Sunsaver MPPT solar charge controller--No, you should not do that. On cold days, the Vmp-array may hit 80 VDC which is over the controller's 78 volt max input rating (I used freezing--depends on how cold it gets in your area).

    You need to look at a Morning Star TSS MPPT family (45 amp or 60 amp) -- Or other brands like Midnite Classic and Outback which support Voc-cold>140 VDC.

    For example, you can plug in your information into a String Calculator (here is MorningStar's) to see exactly what would work or not.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Windsun
    Windsun Solar Expert Posts: 1,164 ✭✭
    #20
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    Re: MPPT Sizing Factor

    Vmp is the max power point, and is specified at 25 deg C and full sun, and a perfectly matched load. You will almost never see that in real life. But that is the main purpose of an MPPT controller, it tracks the actual real-world Vmp under varying conditions.

    Basically an MPPT controller has to be rated (1) at the max possible current output of the panels in MPPT mode, and (2) 10% or so above the Voc as listed in the specs. Those are the two that you have to watch, all else is based on that. If the Voc is exceeded, it is possible to blow out the controller input.
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #21
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    Re: MPPT Sizing Factor
    Scottg4001 wrote: »
    Sunsaver is 75 volts max. Voc of panels is 37.1, so its borderline. And I am in south Florida, so the system will never see freezing temperatures.
    Its not borderline. Its over the line. Maybe you don't get freezing temps, but even cooler than 70° F temps will ruin your sunsaver controller. What's the max voltage on your tristar that is still in its box?
    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • Scottg4001
    Scottg4001 Solar Expert Posts: 47 ✭
    #22
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    Re: MPPT Sizing Factor
    BB. wrote: »
    Because you are using a MorningStar 15amp Sunsaver MPPT solar charge controller--No, you should not do that. On cold days, the Vmp-array may hit 80 VDC which is over the controller's 78 volt max input rating (I used freezing--depends on how cold it gets in your area).

    You need to look at a Morning Star TSS MPPT family (45 amp or 60 amp) -- Or other brands like Midnite Classic and Outback which support Voc-cold>140 VDC.

    For example, you can plug in your information into a String Calculator (here is MorningStar's) to see exactly what would work or not.

    -Bill

    Bill, thanks. I am just north of Miami. Average low is 60 degrees.
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  • Scottg4001
    Scottg4001 Solar Expert Posts: 47 ✭
    #23
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    Re: MPPT Sizing Factor
    vtmaps wrote: »
    Its not borderline. Its over the line. Maybe you don't get freezing temps, but even cooler than 70° F temps will ruin your sunsaver controller. What's the max voltage on your tristar that is still in its box?
    --vtMaps

    TriStar is rated at 150volts. Sucks because the Sunsaver has LVD built it.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #24
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    Re: MPPT Sizing Factor

    You really need pretty close to the record low (at least in the last 10+ years)... Not the average low.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Scottg4001
    Scottg4001 Solar Expert Posts: 47 ✭
    #25
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    Re: MPPT Sizing Factor
    BB. wrote: »
    You really need pretty close to the record low (at least in the last 10+ years)... Not the average low.

    -Bill

    We have had a record low of 36
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #26
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    Re: MPPT Sizing Factor
    Windsun wrote: »
    Basically an MPPT controller has to be rated (1) at the max possible current output of the panels in MPPT mode, and (2) 10% or so above the Voc as listed in the specs. Those are the two that you have to watch, all else is based on that. If the Voc is exceeded, it is possible to blow out the controller input.

    Windsun, as I understand it there is no harm done if the MPPT controller is rated less than the max possible current output of the panels. You will not be able to harvest the excess, but nothing will be damaged.

    Generally, Vmax for the controller must not be ever be exceeded. BUT there is an exception to that rule: Midnite controllers have hyperVoc. When their input voltage exceeds their Vmax they shut down, but they are not damaged until the input voltage exceeds Vmax+Vbattery.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #27
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    Re: MPPT Sizing Factor

    VtMap,

    As I understand, you are correct... MPPT controllers will limit their output current to max rated--safely.

    However, for many controllers, if you exceed Vin-max -- They will log the voltage and invalidate your warranty (don't know about Midnite--but I think you are correct).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #28
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    Re: MPPT Sizing Factor
    Scottg4001 wrote: »
    TriStar is rated at 150volts. Sucks because the Sunsaver has LVD built it.
    Do you really need that feature? Do you have DC loads? If you are using an inverter, doesn't it have its own LVD?
    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • Scottg4001
    Scottg4001 Solar Expert Posts: 47 ✭
    #29
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    Re: MPPT Sizing Factor
    vtmaps wrote: »
    Do you really need that feature? Do you have DC loads? If you are using an inverter, doesn't it have its own LVD?
    --vtMaps

    Yes its strictly a DC load. No DC- AC inverters.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #30
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    Re: MPPT Sizing Factor
    Scottg4001 wrote: »
    We have had a record low of 36
    Then that's the number you should use. Refer to the module specs for the temperature coefficient for Voc, calculate the array voltage at that temp, and compare it to the CC's max voltage. This is the number you have to be the most conservative with in PV design because it is the one that will damage equipment and void warranties if you get it wrong.
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  • Scottg4001
    Scottg4001 Solar Expert Posts: 47 ✭
    #31
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    Re: MPPT Sizing Factor
    ggunn wrote: »
    Then that's the number you should use. Refer to the module specs for the temperature coefficient for Voc, calculate the array voltage at that temp, and compare it to the CC's max voltage. This is the number you have to be the most conservative with in PV design because it is the one that will damage equipment and void warranties if you get it wrong.

    OK, then for me to keep my current charge controller I will need to find panels with a lower Voc that will still be below the max charge controller voltage input with the temperature coefficient. I should still have plenty of real world voltage to be around the 35-40 volts to charge the batteries. It seems a pair of mono panels will do the trick.
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