If you have a new account but are having problems posting or verifying your account, please email us on for help. Thanks :)
Hello all! Please ensure that you are posting a new thread or question in the appropriate forum. The Feedback forum is overwhelmed with questions that are having to be moved elsewhere. If you need help to verify your account contact

Second String. Second Inverter??

  • 19-05-2022 3:47pm
    Registered Users Posts: 10

    Hi All, I got PV panels installed in March. I have 14 x 370w giving me 5.18kW along with a 4.8kW battery and a 6kW Solis inverter. All the panels are on a south facing roof which is grand but I've realised that I should have put some on the west facing roof to take account of the evening sun. The last few nights the sun has been shining on our west roof till 9pm or later and our existing panels are pretty much shaded by that stage.

    If I was to add a small number of panels 8-10 on the west facing roof, would I then need a 2nd inverter as I'd be exceeding the 6kW of my current one? I believe that the 6kW Solis has dual input so a second string shouldn't be an issue from that perspective.


  • Moderators, Education Moderators, Home & Garden Moderators Posts: 8,170 Mod ✭✭✭✭Jonathan

    If you have a second MPTT free, use that. Much easier than trying to apply for NC7 approval.

  • Registered Users Posts: 2,529 ✭✭✭bullit_dodger

    As Jonathan says most inverters support 2x strings, so if you have a spare string your laughing! (look on your inverter and see if you have used both).

    The problem is though, is that it's "unlikely" that you have a string free currently. Panels are about 40-45v (open circuit) and most inverters have a max voltage input of 600v, meaning that your typically max'ed out at 12 (possibly 13) panels in a string before you hit the voltage limit. The fact that you have 14 panels on that south roof would suggest (but not definitively state) that the installer has used both strings on that roof. Makes sense - as no reason why they wouldn't.

    Your not goosed though. What I'd do is take two panels off your south roof, so now you have 12 there and then place all 12 of these south panels on 1 string. You now have a spare string in the inverter for your west roof and you can use the 2x panels that you have moved from the south roof, along with some additional ones. It's important to get the same type/make of panels as you had originally, or if not.....make sure that they have the same voltages/internal resistances on the new panels as the old ones as in theory all the panels on a string should be the same. Otherwise you might be forced to use optimizers on that west facing array (even though you don't have any shading)

    The fact that you have only recently done the install would make getting the same panels relatively easy. If this was a couple of years later.....manufacturers move on and it may be difficult/impossible to get the same and you'd be forced to either scrap the 2 panels you moved from the south, or use optimizers

  • Registered Users Posts: 10 Fandango1976

    Thanks bullit_dodger. Very comprehensive reply 👍

  • Registered Users Posts: 2,529 ✭✭✭bullit_dodger

    Your very welcome - let the forum know what you do. Always good to hear what people decide.

  • Moderators, Sports Moderators Posts: 19,024 Mod ✭✭✭✭slave1

    I agree with Bullit that your current system uses both "strings". Your inverter can handle up to about 8kWp so you have loads of room to leave existing panels as is and just put new panels (up to 8kWp in total as a rough guide) on your unused roof aspect. I'd also fill up your roof and if this only requires a small number of panels on top of your existing Inverter you could look to microinverters to avoid outlay on a second inverter

    My stuff for sale on Adverts inc. EDDI, hot water cylinder, roof rails...

    Public Profile active ads for slave1 (

  • Advertisement