series vs parallel panels

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bmet
bmet Solar Expert Posts: 630 ✭✭
in Solar Beginners Corner #1
With the recent price reduction in panels, I'm shopping for an upgrade, but haven't found anything in the class of my 80 W Sunwize. Small panels seem to be the same cost.

If I go larger to take advantage of price, what is the rule of thumb for combining panels of different wattages? I'm staying with a 12V upgrade because I don't have the budget for a complete redesign. The new panel would be double the Imp of my Sunwize. Is it safe to parallel a 5 and a 10 Amp panel together, and is the 'tie-in' made at the panels or the CC? Thank you.
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  • Seven
    Seven Solar Expert Posts: 292 ✭✭
    #2
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    Re: series vs parallel panels

    I would like to know that answer also. It would see on the surface that if you put two of the small panels in series and they were within 10% of the ratings on the larger panel, that you could then put them in parallel. I do not know what happens to Imp in series though.

    Good question.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #3
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    Re: series vs parallel panels

    For paralleling panels, matching Vmp within 10% should work fine.

    And for series connections, again matching Imp within 10% will work fine too.

    Note, if you have small+large panels in parallel, you will need to check the series protection fuse rating for each parallel string connection... Larger panels will usually use larger rated fuses.

    If you do not use series protection fuses (or use the wrong rating), you do run the risk of fire if there is a a short in the array or one of the solar panels.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Seven
    Seven Solar Expert Posts: 292 ✭✭
    #4
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    Re: series vs parallel panels

    Bill, I have a side question on this.

    I have read alot about fusing panels and I would like a bit of clarification on this. If I am running strings of three panels each(Kyo215) and I have each string going into a combiner box with a breaker on each string,(15a breaker,~8a panel output), do I need any more fuses/breakers on the panel side? I know about breakers going to the CC, batteries, etc.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #5
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    Re: series vs parallel panels

    The problem with layered safety and safety factor upon safety factor, it is very easy to lose your way.

    So first, fusing/breakers are there to protect writing. So look at the wire run from the combiner box to charge controller. What is the maximum rating of the wire?

    Then compare with both the total circuit breakers (add them up), and add up the total:

    Breaker = Isc x 1.25 x 1.25 (nec safety factor twice--if I understand nec derating of solar pv circuits)

    And one last check, reveiw the charge controller manual for maximum input current.

    If any of the above exceed the wire run capacity or exceed the charge controller maximum input current rating, the you would need another breaker to protect against excessive current.

    More or less, how I explain the process.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Seven
    Seven Solar Expert Posts: 292 ✭✭
    #6
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    Re: series vs parallel panels

    Thank you for that answer. It explains the reasoning well.

    Do I need fuses in between the panels in the string?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #7
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    Re: series vs parallel panels

    If you have three or more strings of equal sized panels in series, then each parallel string needs a series fuse.

    Or, if you have a large panel with Isc larger than the series protection fuse of the smaller panel.

    You are protecting a shorted string from being fed too much current from the other panels in the array.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #8
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    Re: series vs parallel panels

    You do not need more than one fuse/breaker per series string.

    -
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Seven
    Seven Solar Expert Posts: 292 ✭✭
    #9
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    Re: series vs parallel panels

    That is what I needed to know, thanks!
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  • bmet
    bmet Solar Expert Posts: 630 ✭✭
    #10
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    Re: series vs parallel panels

    The Vmp between the 5 and 10 amp pnels would
    Be within 10%, but not the Imp. So it is a
    Bad idea to parallel them together?
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #11
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    Re: series vs parallel panels
    bmet wrote: »
    The Vmp between the 5 and 10 amp pnels would
    Be within 10%, but not the Imp. So it is a
    Bad idea to parallel them together?

    they should parallel just fine. matching the imp is for placing the pvs in series, not parallel.
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  • bmet
    bmet Solar Expert Posts: 630 ✭✭
    #12
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    Re: series vs parallel panels

    Thanks for the clarification. I planned to fuse both panel's positive leads, but wasn't sure if the combined current also needed to have a fuse prior to the input of the CC.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #13
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    Re: series vs parallel panels
    bmet wrote: »
    Thanks for the clarification. I planned to fuse both panel's positive leads, but wasn't sure if the combined current also needed to have a fuse prior to the input of the CC.
    If you only have two modules you don't need any fuses at all in the PV as long as the conductors can carry the worst case current.
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  • Electricsuperduty
    Electricsuperduty Registered Users Posts: 8 ✭
    #14
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    Re: series vs parallel panels
    niel wrote: »
    they should parallel just fine. matching the imp is for placing the pvs in series, not parallel.

    So when you parallel different amp panels, the voltage remains 12 and will the amps be 15 (5+10) OR 10 ( the higher panel rate) or will the little panel drag the amp rate down to 5 into the CC?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #15
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    Re: series vs parallel panels
    Electricsuperduty wrote: »
    So when you parallel different amp panels, the voltage remains 12 and will the amps be 15 (5+10) OR 10 ( the higher panel rate) or will the little panel drag the amp rate down to 5 into the CC?

    Photovoltaic panels are funny little things. They're current sources, not Voltage sources. Put a 10 Amp panel in parallel with a 5 Amps panel (both with similar Vmp) and you get 15 Amps. Not like with batteries which have quite a different set of rules when it comes to Amps out - and in.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #16
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    Re: series vs parallel panels
    Electricsuperduty wrote: »
    So when you parallel different amp panels, the voltage remains 12 and will the amps be 15 (5+10) OR 10 ( the higher panel rate) or will the little panel drag the amp rate down to 5 into the CC?

    i think you're getting it as parallel adds the currents together and keeps the voltage the same. series adds the voltages together and keeps the current the same.

    when paralleling you want the pv voltages to ideally be identical, but if they are in the neighborhood it is fine. some feel within 10% and i feel within 5%, but both are arbitrary figures. same goes for putting pvs in series as the currents then ideally should be identical and the same arbitrary percentages for excursions away from one another apply.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #17
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    Re: series vs parallel panels
    Electricsuperduty wrote: »
    So when you parallel different amp panels, the voltage remains 12 and will the amps be 15 (5+10) OR 10 ( the higher panel rate) or will the little panel drag the amp rate down to 5 into the CC?
    When you put dissimilar modules in series, the current from the string will be limited by the lowest current module in the string. When putting dissimilar modules in parallel, the voltage of the system will be limited by the lowest voltage module in the arrangement. It is hard to say exactly what the resultant voltage or current will be because when the MPPT device receiving power from the modules varies the load looking for the "sweet spot", the mismatched modules will see different loading and move to different spots on their IV curves.

    Obviously, the best scenario is for the modules all to be exactly alike, but the more similar the mismatched modules are, the less this affects the overall output of the array. If they are more similar in voltage than in current, it is better to put them in parallel with each other. If in current, series.
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