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Thread: Adding mono to poly string or 60 cell to 72 cell string

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    Adding mono to poly string or 60 cell to 72 cell string

    Want to upsize a system built in 2015.
    They have 300W, 72-cell panels. 8.4 Imp.

    Any reason not to add mono to the poly string?
    Any reason not to add 60 cell to the 72 cell string?
    Also confirming: new panel Voltage can be mismatched as long as I stay under 600VDC total?

    I will match or exceed (E) panel Imp.

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    Quote Originally Posted by Zee View Post
    Want to upsize a system built in 2015.
    They have 300W, 72-cell panels. 8.4 Imp.

    Any reason not to add mono to the poly string?
    Any reason not to add 60 cell to the 72 cell string?
    Also confirming: new panel Voltage can be mismatched as long as I stay under 600VDC total?

    I will match or exceed (E) panel Imp.
    Maybe yes, maybe no. There is no pat answer.

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    As a general rule you can add a different panel to a series string going to an MPPT input as long as the Imp of the old and new panels are within 5% of each other. Making the new panel greater Imp means you will not lose any of the original string output, but may not get the full benefit of the new panel.

    What may make it a little sticker is if the old and new panels have a different off-axis response and so will be farther off Imp equality in morning and evening hours.
    Unfortunately you do not necessarily have the full information to be able to predict.

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    If you have more voltage headroom to increase the length of the strings, can you reorganize and lengthen them to free up a MPPT to use for the new panels?

    I just commissioned my system a few days ago and I am already getting greedy and thinking how I would add more panels. I have 6 MPPT's and for me it works very well to reorganize them to five, using the 6th for a new strings. My DC/AC ratio is already high enough though that it may be prudent to get an additional inverter.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

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    Quote Originally Posted by GoldDigger View Post
    As a general rule you can add a different panel to a series string going to an MPPT input as long as the Imp of the old and new panels are within 5% of each other. Making the new panel greater Imp means you will not lose any of the original string output, but may not get the full benefit of the new panel.

    What may make it a little sticker is if the old and new panels have a different off-axis response and so will be farther off Imp equality in morning and evening hours.
    Unfortunately you do not necessarily have the full information to be able to predict.
    There are 2 strings. I will be adding 1 to 2 (maybe 3) panels to each string of 6.
    I think it will be fine, since today's panels are all higher W and thus higher Imp.
    In this case, 8.9A new vs 8.4A existing.
    All I lose is that little extra A. I figure it also gives me wiggle room for any variance in off-axis response.

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    Quote Originally Posted by electrofelon View Post
    If you have more voltage headroom to increase the length of the strings, can you reorganize and lengthen them to free up a MPPT to use for the new panels?

    I just commissioned my system a few days ago and I am already getting greedy and thinking how I would add more panels. I have 6 MPPT's and for me it works very well to reorganize them to five, using the 6th for a new strings. My DC/AC ratio is already high enough though that it may be prudent to get an additional inverter.
    Good idea, and I know what you mean.

    In this case it is the opposite. There is only 1 string of 12 panels. No more can be added to string as it is at 600VDC limit. (actually calcs out at 604 V max sys V! altho not with actual temp coefficient, but using NEC multiplier of 1.11)
    I will split it, creating two strings at 300Vdc each. There is a dual MPPT inverter.
    Then I will add a couple panels to each string.

    (option: If he wants at least 5 new panels then i might add em all together in their own new string. I need min. 5 panels to hit min operating V of Inv. at warm Temps.)

    BTW - What is your DC to AC loading on inv? Over 120%?
    Last edited by Zee; 03-12-19 at 07:08 PM.

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    Quote Originally Posted by Zee View Post
    Good idea, and I know what you mean.

    In this case it is the opposite. There is only 1 string of 12 panels. No more can be added to string as it is at 600VDC limit. (actually calcs out at 604 V max sys V! altho not with actual temp coefficient, but using NEC multiplier of 1.11)
    I will split it, creating two strings at 300Vdc each. There is a dual MPPT inverter.
    Then I will add a couple panels to each string.

    (option: If he wants at least 5 new panels then i might add em all together in their own new string. I need min. 5 panels to hit min operating V of Inv. at warm Temps.)

    BTW - What is your DC to AC loading on inv? Over 120%?
    Ok gotcha.

    My dc/ac is 1.18. Between it being cloudy and my travels, I actually haven't been able to play with/observe the system hardly at all yet, but the one time I did, the inverters were putting out max. I would have to do more detailed analysis to see if it's worth the $$ for another inverter or just add more panels to existing.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

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    Quote Originally Posted by electrofelon View Post
    Ok gotcha.

    My dc/ac is 1.18. Between it being cloudy and my travels, I actually haven't been able to play with/observe the system hardly at all yet, but the one time I did, the inverters were putting out max. I would have to do more detailed analysis to see if it's worth the $$ for another inverter or just add more panels to existing.
    Cool.
    I go up to 1.2 as a rule of thumb when designing new systems. (If it makes sense e.g 120% rule etc.) No appreciable loss of power annually. I think it is less than 1%.

    I would just add panels.

    I have added panels to get an existing system of 2.2 kW-DC expanded to 6.1 kW-DC on an existing 3.8 kW inverter!
    Yeah that's a lot, It is an outlier.
    But in this one case, it made sense given the conduit run, install labor, cost of new inverter, second inverter space requirement and ugliness on wall, and the fact that the existing inverter was installed last year, and it is a good one.
    Also.... I am not saying I wouldn't pull a permit for such an expansion.......... but the inverter and AC inverter output circuit and PV breaker certainly were not touched or altered.

    Pros and cons change if your inverter is near expiration or problematic.

    I wonder what her bell curve will look like in June. Probably a square curve, a flattened top hill.......

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    Yes I suspect it will be more cost effective to add more modules to the existing inverters. I could add another row of 10, which would bring me up to1.375:1
    That's kinda high but not anywhere near ridiculous. PV syst estimates clipping losses, I should see what it says, although my free trial expired
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

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