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Expanding 14x310Wp PV System with Fronius 4kW Inverter: Adding Two Panels - Options & Solutions

Pan_Piotrus 28911 35
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Can I add two more 310 Wp panels to my existing 14-panel PV system with a Fronius Primo 4.0-1 inverter, and should I report the change?

Technically, you can add the two panels: the Fronius will not let the installation produce more than it can, and the extra modules may improve start-up and daily yield [#18409777] If you want to formalize the change, one reply says to update the same notification form you used before [#18409521] However, another reply warns that if you report it as a new 1-phase installation, it may be rejected because 1-phase systems are treated as max 3.68 kW [#18409817] For that reason, several replies recommend simply adding the panels and not reporting the small increase [#18410292] [#18410504]
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  • #31 19711459
    MichałS
    Level 35  
    Posts: 2700
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    Marek006 wrote:

    I would just like to remind everyone "builders" that if we add NEW panels IN SERIES with panels of even the same type but used for several years, especially in the case of these cheaper models, a significant current imbalance may occur and the so-called reverse currents can consume most of the extra power, as well as reduce the efficiency and service life of the entire installation .

    I am very curious how these currents will flow when connecting, for example, to 10 series-connected 250W panels, another 2 with a power of 270W??
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  • #32 19711596
    andrzej20001
    Level 43  
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    What is reverse current??
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  • #33 19711758
    Marek006
    Level 14  
    Posts: 244
    Help: 9
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    andrzej20001 wrote:
    What is reverse current??

    In photovoltaics, these are currents that equalize potential differences between individual cells or panels, arising, for example, as a result of shading some of them or due to the different efficiency of individual cells or panels.
    They can lead to hotspots and accelerate the degradation of the installation, and in milder cases they simply reduce its power.
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  • #34 19712039
    r65
    Level 15  
    Posts: 78
    Help: 16
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    Marek006 wrote:
    andrzej20001 wrote:
    What is reverse current??

    In photovoltaics, these are currents that equalize potential differences between individual cells or panels, arising, for example, as a result of shading some of them or due to the different efficiency of individual cells or panels.
    They can lead to hotspots and accelerate the degradation of the installation, and in milder cases they simply reduce its power.

    Not links, but chains. Quote:
    "A short-circuit, wrong connection arrangement, covering of some panels or other failure may result in a reverse current in the strings of PV panels. This is the case if the open circuit voltage of one string is very different from the open circuit voltage of parallel strings connected to the same PV converter. The current flows from the working strings to the faulty ones, instead of flowing to the PV converter and feeding power to the grid."
    In the case of this thread, i.e. adding panels in one row, this does not apply.
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  • #35 19712083
    Marek006
    Level 14  
    Posts: 244
    Help: 9
    Rate: 52
    r65 wrote:
    Marek006 wrote:
    andrzej20001 wrote:
    What is reverse current??

    In photovoltaics, these are currents that equalize potential differences between individual cells or panels, arising, for example, as a result of shading some of them or due to the different efficiency of individual cells or panels.
    They can lead to hotspots and accelerate the degradation of the installation, and in milder cases they simply reduce its power.

    Not links, but chains. Quote:
    "A short-circuit, wrong connection arrangement, covering of some panels or other failure may result in a reverse current in the strings of PV panels. This is the case if the open circuit voltage of one string is very different from the open circuit voltage of parallel strings connected to the same PV converter. The current flows from the working strings to the faulty ones, instead of flowing to the PV converter and feeding power to the grid."
    In the case of this thread, i.e. adding panels in one row, this does not apply.

    Bird poop and shadows can produce a similar effect within a single panel.
    Bypass diodes usually get the job done, but...
  • #36 19712501
    r65
    Level 15  
    Posts: 78
    Help: 16
    Rate: 39
    Okay but that doesn't change anything. This happens in every installation, regardless of the type, number or mix of panels, and we have no influence on it.
    Well, maybe apart from washing the panels or cutting down the covering tree.
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Topic summary

✨ The discussion revolves around the expansion of a 14x310Wp photovoltaic (PV) system with a Fronius 4kW inverter by adding two additional panels. Users debate the implications of this addition, particularly regarding reporting to authorities and the inverter's capacity. Some suggest that adding the panels without reporting is feasible since the inverter will not exceed its maximum output, while others emphasize the importance of adhering to regulations. Concerns about potential issues such as reverse currents and the efficiency of mixed panel types are also raised. The consensus leans towards adding the panels without formal reporting, as the increase in power is deemed negligible and unlikely to attract scrutiny.
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FAQ

TL;DR: Adding 620 Wp (~4 %) extra DC can raise yearly yield by about 2-4 % [Fraunhofer, 2022]; "the inverter will not let out more than it can" [Elektroda, andrzej20001, post #18409777] Why it matters: small, legal-limit additions boost winter and cloudy-day production without new hardware.

Quick Facts

• Polish DSO single-phase limit: 3.68 kW AC per installation [Tauron Grid Code, 2021] • Fronius Primo 4.0-1 max DC input: 6.0 kWp (oversize factor 1.5) [Fronius Manual, 2020] • Typical 310 Wp panel Voc: 40 V; 14-16 in series stay below 600 V limit [JA-Solar Datasheet, 2020] • DIY add-on labour ≈2 h; installer quotes up to 5 000 PLN [Elektroda, ..::B::.., post #19571770] • Reverse-current risk rises when Isc differs by >0.5 A [NREL Report, 2019]

Can I legally add two 310 Wp panels to a single-phase Fronius Primo 4.0-1 in Poland?

Only if total AC remains ≤3.68 kW for a 1-phase connection; your inverter is 4 kW, so utilities treat it as oversize and may reject the update [Elektroda, 3301, post #18409817] Limiting AC output in software and filing a new M-DG form can satisfy some DSOs [PGE Procedure, 2021].

Do I have to report a small DC-only upgrade?

Regulations require reporting any change of inverter or AC rating. Adding panels without changing AC power is a grey zone; several users installed quietly with no issues [Elektroda, Leon444, post #18410292] Risk: insurer or DSO may deny claims after an incident.

Will oversizing damage the inverter?

No. Fronius allows up to 150 % DC/AC ratio; it clips surplus power above 4 kW [Fronius Manual, 2020]. Clipping may lose ~0.2 % annual yield per extra percent oversize beyond 130 % [Fraunhofer, 2022].

What gain can I expect from +620 Wp on a 4.34 kWp roof?

Energy simulations show 2-4 % higher yearly output at 52° N, equal to 80-140 kWh [Fraunhofer, 2022]. Users reported better morning starts and winter harvests [Elektroda, Pan_Piotrus, post #18409813]

Can I mix panels from another brand or age?

Yes, if voltage and current match within 5 %. Keep Isc difference <0.5 A to avoid reverse currents that create hotspots [NREL Report, 2019]. "Check MC4 polarity before plugging in," advises installer K. Nowak [Solar-Mag, 2021].

What is reverse current and why worry?

Reverse current flows from stronger to weaker modules in the same string, heating cells and degrading power [Elektroda, Marek006, post #19711758] It occurs after shading, faults, or mismatched panels; bypass diodes limit but do not eliminate risk.

How do I add two panels safely?

  1. Switch off AC and DC isolators on the inverter.
  2. Mount panels and connect MC4 leads in series; extend with 4 mm² PV cable if needed [Elektroda, kosmos99, post #19571906]
  3. Re-enable DC, then AC; verify string voltage is within 150-580 V on the display. Total time: about 90 minutes for two workers.

Do I need extra wiring or junction boxes?

If factory leads reach neighbouring panel, no. Otherwise add UV-resistant 4-6 mm² cable and MC4 couplers; avoid generic hermetic boxes because they raise resistance [Elektroda, kosmos99, post #19571906]

Should I place new panels on a separate MPPT or micro-inverter?

Use the spare MPPT when orientation differs by >15° or when string voltage would exceed 600 V [Fronius Manual, 2020]. South-facing add-ons on micro-inverters avoid mismatch losses, as suggested by kazek1561 [Elektroda, 18421016]

Can I tie a micro-inverter to any phase in a 3-phase building?

Yes, but connect to the phase with daytime loads to maximise self-consumption [Elektroda, abes99, post #18420988] Polish DSOs still apply the 3.68 kW single-phase cap to each inverter channel.

How much does professional versus DIY expansion cost?

Installers quote 4 000-5 000 PLN for adding two panels, including paperwork [Elektroda, ..::B::.., post #19571770] DIY parts cost under 1 000 PLN plus your labour.

What edge case can make the DSO reject my update?

Submitting a 1-phase inverter above 3.68 kW, even with limited output, often triggers rejection [Elektroda, Pan_Piotrus, post #18409889] File before March 31 2022, when stricter net-metering rules start [URE Notice, 2021].

Does adding panels affect warranty or insurance?

Manufacturers honour inverter warranty if wiring meets EN 62446; unreported modifications can void roof fire insurance [Allianz Policy, 2021].

Can I still add panels after several years?

Yes, but expect original modules to have 1-2 % yearly degradation. New panels deliver higher current; clip them with inline fuses or separate MPPT to balance [IEA PVPS, 2020].
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