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Choosing which string to add additional panels to?

  • 01-12-2021 2:18pm
    Registered Users Posts: 331 ✭✭

    Hey all

    I recently installed a 7.5Kw PV system with a 5KwH pure drive battery and 6Kw solis inverter. I have 14 panels on East facing and 8 on west facing (with optimisers due to a small bit of shading).

    I believe I can add more panels to this system - but what would the max be?

    It would be great to determine which of the strings is giving me a better return on panel to power ratio - is there any way to understand from the inverter how much power is being generated from each string? I seem to only be able to get a combined total


  • Moderators, Home & Garden Moderators Posts: 4,458 Mod ✭✭✭✭graememk

    Your gonna be at or very close the voltage limit on the 14 panel string.

    So.. your only option is the 8 panel string.

  • Registered Users Posts: 331 ✭✭peustace

    Thanks @graememk the installers mentioned that also. I thought my options would be either to add 6 panels to the roof with the existing 8, or add 6 to the roof with 14 panels but wire them to the 8 panel string (they would need optimisers) - if this is doable then being able to tell which roofside is most efficient will help me decide which option to go with.

    what would be the max load this inverter could take?

  • Registered Users Posts: 2,224 ✭✭✭bullit_dodger

    Dunno which exact model inverter you have, but most of them operate to about 600V as a max on a string

    You'd have to look you what the nominal voltage is for each panel. Usually its about 38-40V (or thereabouts?), so if you have 14 panels, that will be 560V (it'll depend a little on the specific panels)

    Although while I think that limit is your first hurdle, adding 3.0Kwp if you have 7.5Kwp already , your probably going to max out at 6Kw for much of the summer months even though you have 10+Kwp on your roof. Inverter is fine, it will pull up to your max (6Kw) irrespective of how much you have panel wise. So for a few months with that single inverter you maybe "wasting" a little. What it would do though is improve your generation for a lot of the "marginal" days for many of the cloudier months.

    You ever hit the 6Kw limit currently?

  • Registered Users Posts: 331 ✭✭peustace

    Yeah, I’ll hit the 600V limit with 14 panels per string.

    I haven’t hit the 6Kw limit yet as the panels are only recently in and I won’t hit peak production until spring or summer months. Increasing my production in the winter months (even marginally) is my aim. But I’m still hoping to figure out which roof would be optimal for any additional panels. Is there a way to see what each string is producing individually?

  • Registered Users Posts: 2,224 ✭✭✭bullit_dodger

    I'm on the same path myself peustace. I've 4x panels east and 10x panels west. Very much thinking of adding 2x panels to my 4 east ones to boost the production there. Unfortunately the 100ft high trees across the road mess up my production west from 4pm onwards, so getting the east ones boosted would really improve things for me. The east one is messy as there are velux windows that I have to fit them in about - wasn't possible to do as a part of the initial install due to the BER requirements.

    So I'll 6.1Kwp on a 5Kw inverter, but I don't think this will be a problem as it's an east/wests split, so unlikely to be full generation.

    IN terms of production per string, it should be available via the inverter. But someone with a Solis one would have to tell you. I've a givenergy one and I can chart it ....

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  • Moderators, Sports Moderators Posts: 17,888 Mod ✭✭✭✭slave1

    That 5kW inverter should be good to go for another 4 panels if you can fit them

  • Registered Users Posts: 2,224 ✭✭✭bullit_dodger

    True - although I have only space for 2x more in my case. Sadly the price for 2x panels will not be cheap. It's typically a fixed cost to get installers out. So installing 10 panels (hardware aside) doesn't cost much more than 2 panels as you have to pay for them to come out to you.

    Suspect those 2x panels will be the guts of €700-800 euros to get installed.

  • Moderators, Sports Moderators Posts: 17,888 Mod ✭✭✭✭slave1

    You got to question that payback so...

  • Registered Users Posts: 2,224 ✭✭✭bullit_dodger

    Yeah, there is that slave, but for me payback isn't the highest priority. Sure I like to crunch the numbers (when in Uni I did a degree in math) but really it's more about doing the right thing for the environment for me - with some nod to my wallet of course! Can't have all me fun money gone on panels!

    Probably will wait and see what the production is like in reality in the brighter months.

    If only I could get 1.5+Kwp south facing........with a high slant angle.

  • Registered Users Posts: 704 ✭✭✭buzz11

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  • Registered Users Posts: 331 ✭✭peustace

    Thanks @buzz11. Do you know if there is a way to export the figures for total daily and cumulative monthly production figures from each string?

  • Moderators, Education Moderators, Home & Garden Moderators Posts: 7,826 Mod ✭✭✭✭Jonathan

    Yes, you can see these details on

  • Registered Users Posts: 1,484 ✭✭✭MAULBROOK

    Add to the west string, very handy during the winter.

  • Registered Users Posts: 331 ✭✭peustace

    Thanks @Jonathan - can you point me to where this is pls? I found the graph that allows me to see daily/monthly or yearly production per string but when I try and export it I hit an error saying no data selected

  • Moderators, Education Moderators, Home & Garden Moderators Posts: 7,826 Mod ✭✭✭✭Jonathan

    Reports -> Inverter Reports -> String 1/2 Voltage/Current details.

    There was a way to craft your own graphs similar to the SolisCloud app, but I can't see that option available any more.

  • Registered Users Posts: 331 ✭✭peustace

    Thanks again, here's the report that I can export - I think this is what you are referring to. Do you know if there a way to get a daily cumulative total per string?

  • Registered Users Posts: 2,224 ✭✭✭bullit_dodger

    From the (small) screenshot above you have the voltage and the amps..... so P(w) = Volts x Amps and you have an instantaneous wattage every 5 mins.

    Zip that into Excel, and just a matter to integrate that over time - should give you the power / day. No?

  • Registered Users Posts: 331 ✭✭peustace

    Resurrecting this thread to get some guidance.

    I got a 7.5kw system installed last year with a 5KwH pure drive battery and 6Kw solis inverter. I have 14 panels on East facing and 8 on west facing (with optimisers due to a small bit of shading). Very happy with the system and what it produced. I've been told that I can add 6 more panels.

    If it becomes possible to add a 2nd inverter, that's something I'd like to do in time. My question is - should I add 6 more panels on the existing system now and use up roof space or hold off and wait until panels become more efficient and inverters cheaper and then install a potentially bigger system (Knowing that I'll prob be waiting up to 5 years for this)?

    Would love to hear if others had a similar conundrum and what path they choose?

  • Registered Users Posts: 1,130 ✭✭✭DC999

    Could I ask why you're considering adding more? Because the installer said you can when you didn't ask (so a bit of a sales push maybe)? You're buying more power than you expected? Battery runs out quickly (maybe it's another battery or a DIY 20kwh one and sell 5kwh one)? Energy usage is house has gone up, and why? Do you use a heap of energy. Got a new EV / might get one. Got a heatpump. Want to maximise the FIT payment. Want to give a neighbour free power at times... So what's the need first - if you get me?

    So would be going from 7.5kwp to 9.5kwp but you have a limit of 6kwh on the export back to the grid. So you'd be losing a lot at times (would never make it to the grid) unless you have EVs to soak it up, & large hot water tank and eddi to heat it up. 5kwh battery would be filled in jig time.

    Looks like they are 340w panels (7.5kwp / 22 panels) so would still get that same size now I'm sure.

    Hope you don't think I'm being a pain here. Asking as it means your payback gets longer. So it's all good to get more if you know why and are happy to do that.

    Others can answer 'how to do it' - design side is well above my paygrade at this point :) First is the 'why' do it. Or at least that's how my head works

  • Moderators, Home & Garden Moderators Posts: 4,458 Mod ✭✭✭✭graememk

    I put mine in with the ethos of build for the average day not the good ones and don't worry about the export.

    Even with my oversized array 8kwp on 6kw inverter, I don't hit my inverter cap in August only may/June.

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  • Registered Users Posts: 2,224 ✭✭✭bullit_dodger

    So you have 14 panels and 8 panels. How are they setup with respect to the strings. I assume that you have 14 on one string and 8 on the other?

    That would make the most sense - although 14 panels (normally) is a bit-of-a-push to fit onto a string. Most inverters max out at 600v dc and with panels in about the 45v range each, 12 or 13 panels is all you typically can get on a string. The fact that you have 14 is unusual. Not unheard of though as it could be that your panels are closer to 40v each, so 14x40=560v......or perhaps your inverter can handle more than 600v. Throw up the make/model of inverter and panels and we can look up the specs for you.

    So the 6 that you've been told you can add will have to go on the west roof. DC999 raises some good questions to keep yourself honest. The why behind things is always good to clarify for yourself.

    Me - I'd add the panels on the existing system. Sooner you do it, the sooner you start your payback - although 7.6Kwp is a fair whack already, are you using all that?

    People will differ on this, but I'd be 100% in behind Graeme too - you build your system for march and October. I like to think of my production from Nov->Feb (inclusive) as a "bonus" :-)

  • Moderators, Sports Moderators Posts: 17,888 Mod ✭✭✭✭slave1

    I'd go now while panels exist that will complement your existing setup, do not hold out for a more efficient panel as the efficiency gains are minute year on year, where you see larger wattage panels this is due to physically larger panels 99% of the time.

    Over size to take into account the dodgy weather we get (along with late Autumn, Winter, early Spring) and in the days were we are basking just let it go to the grid (FIT or not)

  • Registered Users Posts: 331 ✭✭peustace

    Super questions @DC999. I have an EV car due in January, and I'd like to be in a position to charge the car with any export. I'm not hugely interested in the FIT - anything I get from that is solely a bonus. I'm also not hugely concerned with the payback period either - energy prices will only continue to increase and any investment here will easily payback over a short time especially with rising fuel prices also. I already have an Eddie with a good size tank which is giving me great amount of hot water during the good days. I'd also like to extend into the winters months where I could be more 'off-grid' as much as possible. With current setup, I hit about 60% reliance on the grid in Dec/Jan (an approx figure) - adding panels would reduce this reliance.

  • Registered Users Posts: 331 ✭✭peustace

    I have 14 panels on one string (west facing) and 8 panels on 2nd string (each facing).

    The inverter I have is a Solis 6kw RHI-6K-48ES and the panels are JA Solar 345W -

    My thinking was to split it and put the additional 6 across evenly on east and west - but tie them to the string that only have the 8 panels on it right now.

    7.5kw is a large system but because they're split across E & W I'm rarely getting above 6 at any moment in time. Going for additional will give me a bit more headroom to charge the incoming EV car (And who knows in time maybe help with a heat pump in the house)

  • Registered Users Posts: 2,224 ✭✭✭bullit_dodger

    If I'm reading this correctly (and I'm pretty sure I am) your inverter is rated to take a max DC input voltage of 600v

    And your panels have an open circuit voltage of 46v

    So 14 x 46.6v = 652v.

    I'm sort of surprised that your not getting over voltage alarms/errors on that string to be honest. Could you be and not know about it?

    My thinking was to split it and put the additional 6 across evenly on east and west - but tie them to the string that only have the 8 panels on it right now.

    Are you saying above you'd add 3x panels on the east and 3 panels on the west? If you were doing it that way, you would have to have optimizers on all 14 panels on that string then. Alternatively, if you have the roof space, you could install all new 6x panels on the west roof along with moving 1x panel from east to west. this would mean that your string would have 7 panels on east, and 7 panels on you could parallel wire them. That would eliminate the need to have optimizers, but you would have to get the same panels as what you already have. Those panels are older as I recall?

    Aside: Your inverter is rated to handle 8Kw max. Note sure if it's wise to massively break that limit. I think it's ok, but I'm not a spark, I wouldn't exceed it by much. 1Kw max is all I'd push it.

  • Moderators, Sports Moderators Posts: 17,888 Mod ✭✭✭✭slave1

    I have similar setup, a 6kW inverter fed by two strings but as Bullit says I have not exceeded to voltage handling, this is not good (so I am told).

    House first, then water, then in a year I put 820kW surplus PV into the EV, so around 5,280kms into either a Kona or Tesla Model 3 which achieve similar range given their battery sizes. Hopefully this will gain some insight into what you are after.

  • Registered Users Posts: 331 ✭✭peustace

    Thanks all, again some great insights. I already have optimisers on the front 8 panels due to some shading to my thoughts were I would split the potential 6 more and put them in along with optimisers again. Having said that, I now need to check if I can actually add in 6 more panels. I want to avoid any damage to the inverter by overloading it.

    For this year alone todate I’ve exported 2400kws, so I do see a lot of potential in dumping that into an EV. Now my house consumption will increase (bigger fam, more Electrical appliances etc), and again perhaps I could use some of that for a heat pump

  • Registered Users Posts: 1,130 ✭✭✭DC999

    As a benchmark, our city EV (which does low mileage) uses an average of 7 kwh a day. So if you have an EV plugged in during the day on a large PV system, it's gonna charge direct from solar easily for a lot of the year. We'll never have an ICE car again. Only have our EV a few months but it's a game changer.

  • Registered Users Posts: 779 ✭✭✭hick

    Only found this thread by chance when looking for something else. A great resource for lots of folks. I have a similar challenge/opportunity see thread….

    Only HW differences is I have JA 375W

    (I wonder was it the same Enniskillen installers🤔)

    @peustace how did you get on? Did you do the upgrade?

    @bullit_dodger thanks for the reply on my thread and I can see a lot of the same applies here

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