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  • Too many panels ?

    Hello all. New here but see a wealth of knowledge so here goes. I have had a 9.9kw system installed. There are 2 Fronius 5kw inverters. My question is. One inverter has 20 x 300 watt panels running to it and the other has 13 x 300 watt panels. Is this setup wrong in that we are wasting 1kw more than the inverter can use and under using the other.

    Thanks in advance to any answers and for what it's worth this setup is in Brisbane Australia.

  • #2
    Do all the panels face the same direction ? Perhaps wiring was simpler the way it was done ? Maybe shade was an issue, why install panels where they are shaded
    Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
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    • #3
      Panels are all on the same plane, North facing and no shade at all.

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      • #4
        It sure sounds like someone took a shortcut. Of course if you are in a warm climate its probably rare that your panels will be clipping as the output of the panels drop the warmer its gets. If its a roof mounted array, design details can also impact the temperature of the panels. Ideally the edges should be open on 4 sides but some installers put seal strips on the edges to improve the aesthetics. Its also done to exclude animals that may nest or do damage. In either case is further raises the temperature of the panels as there is no air flow behind them. Roof mounted arrays maybe low cost but not ideal in hot climate compared to a pole mount.

        As long as the inverter on the big string is not clipping you are not losing production. If you have wide range of yearly temps than there could be issues with either array going outside to the voltage range of the inverter but in theory the installer ran the configuration software to check for that.
        Last edited by peakbagger; 07-27-2020, 07:03 AM.

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        • #5
          Throughout the day, as the sun comes across the sky (so to speak), the panels get early dim light, more light, peak light, then less light and no light. It will be roughly a sinewave of light vs time, producing a crude sinewafe of power vs time. In your case, it is likely that at high noon, the 20-panel string will be able to produce more energy than the inverter can handle, and the inverter will throttle back (some call it clipping), like in the plot below. In this plot, I'm showing a real system example where the panels are capable of ~20% more power than the inverter.

          For a short amount of time, you may not be getting your money's worth out of those panels. But is it a significant loss? Look at the sketch. To my eye, in this case, there is not enough energy lost to worry about. Yes, it could be better, but if you have to pay a person to rewire the array, it may take a long time to recoup the money you paid the worker for their time and materials.
          lost-energy.jpg

          Are you able to use the Fronius monitoring software to see a plot like above? That will give you a real indication of how much energy is being lost.
          7kW Roof PV, APsystems QS1 micros, Nissan Leaf EV

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          • #6
            There is one more detail worth considering. Solar panels degrade slightly with time. They may lose roughly 1% output per year. After 5 years, the above plot would show a much smaller red area, so even if you were able to improve production by 30kWh this year, you might .only improve production 5kWh in the fifth year. (Those are ficticious numbers included to give perspective.)
            7kW Roof PV, APsystems QS1 micros, Nissan Leaf EV

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            • #7
              Originally posted by Whitey View Post
              Panels are all on the same plane, North facing and no shade at all.
              How old is the system ?

              Is, or has, the system shown clipped output at any time ?

              I'd agree w/ Peakbagger that it looks like someone may have taken took a shortcut or maybe screwed up in the wiring hookup somehow.

              Depending on the panel voltages and current characteristics, that's kind of a bastard number of panels to work with even if it's not a prime number. Might have been easier with, say one 10 kW inverter and 32 panels in 4 strings as only an example.

              Why two inverters ? Nothing inherently wrong with 2 half size vs. 1 bigger inverter, just not too common. Anything about the site/situation that would require 2 inverters ?

              On clipping, a 9.9 STC kW system will rarely, if ever, put out 9.9 D.C. kW to an inverter. The inverter wired to the 6 STC kW input might see some minor clipping 1X/awhile, but at 5/6 =.0833, I doubt you'll see much, and less as the system ages. My highest measured clear day system output ratio using max. power into the inverter on a cold day with a clean array was 4,882 W/5,236 W = 0.932. That happened once with most max. clear sky values falling in the 4,500 to 4,600 W range.

              A real SWAG on clear day max. quasi steady state power array output to the inverter for a well designed 9.9 kW system might be ~ 0.90 * 9.9 STC kW = ~ 8.91 STC kW, with most max. daily max. clear sky instantaneous ratios falling ~ 0.80 - 0.86 range or so.

              The wiring you report is funky, but before I went further I'd call the installer and ask the question : "Am I correct in what I think I'm seeing and if so, What were you thinking ?". Kind of hard to comment on a design without being on site. Not my design anyway. Maybe there's reasons for the inverter input mismatches.

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              • #8
                Thank you very much for the replies. A lot of it went over my head but I do understand the basics and I'm not overly concerned now. The system is only 5 months old and in summer I watched during a 37C / 98.6F degree day and the best seen on the display was 4487. If it has been clipping it must be rare.

                Again thanks, the main reason for the question was to see at an early stage if it was a problem and if so one worth inquiring with the installer. Your replies would tend to say leave it so that's what I will do.

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