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Panel limit per string

  • 23-08-2021 10:34am
    Registered Users Posts: 1,880 ✭✭✭

    Quick theoretical question on the max panels you can have per string for a Solis 6kw hybrid RHI-6K-48ES-5G.

    Here's my setup:

    String 1 - 12 x 335w Qcells all black G9 - max voltage around 400-410v

    String 2- 9 x 335w Qcells all black G9 - max voltage around 300-310v

    I checked the Solis designer but they don't have this exact panel on their list so I picked the closest 330w one I could find there and it's telling me max panels per string is 13 which doesn't make sense to me if each string can take up to 600v

    Was thinking of maxing out string 2 by adding 5 panels to the back of the house roof but apparently I can only add 4 as the Voc value above goes over with 5 panels but I

    Does that actually make sense? I'm thinking 5 should work as well as these panels would be N facing and all existing ones are S facing.

    As you can see the actual Voc spec of these panels is 40.78 so 14 panels x 40.87 is 572.18 which is still within the 600 limit. Which probably means the designer is wrong as it does not have this exact panel spec on the list.


    1. Any reason why I cannot extend string 2 to 14 panels?
    2. Will the 5 extra N facing panels cause any issues in the bad months with startup voltage, keep the S facing ones from producing early? These 5 will all have optimizers on them.


  • Moderators, Home & Garden Moderators Posts: 4,421 Mod ✭✭✭✭graememk

    You need to take into account the worst case scenario

    The colder the panels get the higher the voltage is.

    Got this formula when i was looking to oversize my array. 12 was my limit.

    Worst scenario at -20 degrees is 40.8(voltage) x 12 panel x 1.1215 temp coeff x 1.03 tolerance = 565.55V

    13 pushed me over.

    If all the north panels have optimisers on them, they wont effect the south array..

  • Registered Users Posts: 1,880 ✭✭✭SD_DRACULA

    Hmm maybe that's how the Solis designer is calculating it where 13 is the limit according to it.

    Might send Solis an email to confirm.

  • Moderators, Sports Moderators Posts: 17,883 Mod ✭✭✭✭slave1

    Surely optimisers are irrelevant when it comes to start up voltage, each string would be 100% independent.

    My NE were frozen 24/7 last Winter and no impact on SW string

  • Registered Users Posts: 1,880 ✭✭✭SD_DRACULA

    So the string with the extra N panels would have 9 facing S and 4 (or 5) facing N, it wouldn't be N on it's own string hence the optimizers?

    I know I could probably just put in a 2kw inverter or something but wouldn't be worth the expense for N only.

  • Moderators, Category Moderators, Motoring & Transport Moderators, Regional East Moderators Posts: 62,620 CMod ✭✭✭✭unkel

    Same here. 12 was my maximum. I was tempted to go 14 as I have the space for it and the extra install would have only taken me 30 minutes at minimal extra costs. But it would have sent me over the maximum voltage per string limit.

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  • Moderators, Home & Garden Moderators Posts: 4,421 Mod ✭✭✭✭graememk

    Well.. we were using the same inverter and the same panels! I would hope it would be the same 😂

  • Registered Users Posts: 1,880 ✭✭✭SD_DRACULA

    Hmm my installer said 14 or 15 should work - I'm inclined to think that makes sense if 4/5 panels are N facing they will never output more than 50% so maybe?

    Waiting on a reply from Solis...

  • Moderators, Home & Garden Moderators Posts: 4,421 Mod ✭✭✭✭graememk

    Voltage could be high and not generating any current. My highest voltage spikes are just after startup, and just before shutdown.

    Are you talking about 14 or 15 on a single string, or 14 or 15 in total (i have 2 strings of 12)

  • Registered Users Posts: 1,880 ✭✭✭SD_DRACULA

    12 one string and 14 on the second.

    Yeah the voltage spikes when going from clouds to full sun which makes sense.

    But as you can see the current 9 panel string hovers around 300 with the odd spike (yesterday to today)

    And this is the current 12 panel string which hovers at around 400

    If you can read it, the compression on boards is an absolute joke