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Expanding 1200Wp PV Installation: Combining 3-Phase & Single-Phase Inverters for 4kWp Setup

kamilboski 23145 39
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Can I expand a 1.2 kWp PV system to about 4 kWp by combining a 3-phase inverter with a single-phase inverter, and will that be acceptable for the grid paperwork?

Yes, but the single-phase inverter should stay within the DSO limit of about 3.68 kW; otherwise use a three-phase inverter and file the update with the distributor [#19566157][#19566384][#19566444] You can add a second inverter, and the forum says no extra device is needed for the inverters to “see” each other [#20319063] If you use a single-phase inverter, connect it to the most heavily loaded phase [#18924050] If the grid voltage is already around 250 V, a single-phase inverter can push one phase over the limit and cause shutdowns, so a three-phase inverter is the safer choice because it spreads power over all three phases [#19567432][#19568595][#19569563] For a complicated roof, another suggestion was to use an inverter with two MPPTs so multiple panel rows can be connected without mixing orientations [#18924184]
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  • #31 20015709
    grochowiakdariusz
    Level 2  
    Posts: 4
    Board Language: polish
    Thanks for both options - that is, in principle, I can do both - we'll see what the installer will advise - probably what will be the best for him :) will probably bet on the variant with the least amount of work.
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  • #32 20015860
    kosmos99
    Level 38  
    Posts: 4188
    Help: 338
    Rate: 815
    Board Language: polish
    DEFENCE wrote:
    it will be possible for the power company to remotely switch off installations above 10kW


    It doesn't matter how many inverters there are.
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  • #33 20020792
    grochowiakdariusz
    Level 2  
    Posts: 4
    Board Language: polish
    As expected, the installer persuaded me to set up a new installation, motivating the fact that it is not known how the old one will continue to work, so it's better that there is no circus on the new inverter (that one string does not work or works poorly due to aging panels). All in all, let everyone be responsible for their part of the work - and the old panels are now 6 years old, later they will be 10, etc., while the new ones are only 4 years old ... The only thing that worries me is whether the Panasonic 9kW 3-PHASE pump - which will be installed by another company will not be stupid and, for example, turn off because it will receive 3kW more electricity on one phase? Maybe some, I don't know, voltage stabilizers are required or something.. ?? Regards
  • #34 20020805
    Slawek K.
    Level 35  
    Posts: 3015
    Help: 259
    Rate: 1297
    Board Language: polish
    grochowiakdariusz wrote:
    As expected, the installer persuaded me to set up a new installation, motivating the fact that it is not known how the old one will continue to work, so it's better not to cause a circus on the new inverter (that one string does not work or works poorly due to aging panels).
    All in all, let everyone be responsible for their part of the work - and the old panels are now 6 years old, later they will be 10, etc., while the new ones are only 4 years old ...
    The only thing that worries me is whether the Panasonic 9kW 3-PHASE pump - which will be installed by another company will not be stupid and, for example, turn off because it will receive 3kW more electricity on one phase? Maybe some, I don't know, voltage stabilizers are required or something.. ?? Regards

    Buddy, the pump will take as much energy as it needs, it will not "get" anything.
    If there is an excess of energy from PV production, the difference will be sent to the grid, if there is not enough, you will have to collect the difference from the grid - that's how photovoltaics work.

    Regards
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  • #35 20296220
    ukopec34
    Level 6  
    Posts: 26
    Rate: 2
    Board Language: polish
    Hello, I have 2 pv installations on 2 inverters, 1 gas. The question wants to expand these 2 installations, can I buy a 3-phase inverter or just sell the 1-phase one and install a 3-phase one in the 2nd installation. The point is that the entire 2nd installation after the extension will have about 12kW.
  • #36 20296394
    Slawek K.
    Level 35  
    Posts: 3015
    Help: 259
    Rate: 1297
    Board Language: polish
    You cannot have 12kWp on three single phase inverters, you will exceed the max. power for a single single phase inverter. Buy one 3 phase inverter.

    Regards
  • #37 20296476
    ukopec34
    Level 6  
    Posts: 26
    Rate: 2
    Board Language: polish
    Now on 2 inverters in total I have 6.2kW on one I have 3600 and on 2 I have 3000
    Only in separate vases. There must be 2 inverters here because I have panels on 2 different buildings.
  • #38 20297046
    Slawek K.
    Level 35  
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    Do this third installation on a 3-phase one, because you can't connect 6kWp to a 3.6kW single-phase inverter because it doesn't make sense, you will lose a lot of energy.

    Regards
  • #39 20318125
    quanhellneo
    Level 2  
    Posts: 2
    Board Language: polish
    Hello, I have 3.84kWp for 3f the growatt 4000 inverter wants to add 6kWp. I wanted to add a growatt 6000 or 5000 inverter (I don't know if it can handle less than 6kWp) in order to turn off smaller installations in the event of a failure or high voltage to reduce the voltage on the phases. will the inverters not "bite" or do they have to be connected together by some additional device so that they can see each other? finally, I know that there is an option to buy 9000kWp and run old and new panels in 2 strings.
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  • #40 20319063
    Slawek K.
    Level 35  
    Posts: 3015
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    Board Language: polish
    You can add a second inverter, nothing will bite and you do not need any additional devices. I would recommend this 5kW for a 6kWp installation.

    Regards
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Topic summary

✨ The discussion revolves around expanding a photovoltaic (PV) installation from an existing 1200 Wp setup to a 4 kWp configuration by combining 3-phase and single-phase inverters. Participants emphasize the importance of not exceeding 3.68 kW per phase to comply with energy regulations. Suggestions include using a 4 kW 3-phase inverter alongside a 1 kW single-phase inverter, particularly on the heavily loaded phase. Concerns about voltage increases when connecting single-phase inverters to a 3-phase system are raised, with recommendations to monitor voltage levels and consider using multiple single-phase inverters to balance loads. The conversation also touches on the implications of connecting multiple installations to a single meter and the need for proper documentation and compliance with local energy distributor regulations.
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FAQ

TL;DR: Polish DSOs cap single-phase inverter output at 3.68 kW [Elektroda, Slawek K., post #19566384] and, as one installer says, "Above 3.68 kWp go 3-phase" [Elektroda, Cytro12, post #18923192] Combine a 4 kW 3-phase plus 1 kW 1-phase inverter only if each phase stays under the cap [Elektroda, Tech132, post #18924050]

Why it matters: Oversized or badly balanced systems trigger grid-side shutdowns and void approvals.

Quick Facts

• Single-phase limit: 3.68 kW per phase (all Polish DSOs) [Elektroda, Slawek K., post #19566384] • Grid over-voltage trip: 253 V according to EN 50549-1 “PV inverters” • Smallest 3-phase inverters: typically 3–3.3 kW AC [Elektroda, kosmos99, post #19570039] • Remote curtailment planned for systems >10 kW [Elektroda, DEFENCE, post #20015017] • PV modules lose ≈0.6 % output per year [NREL, 2022]

Can I run one 3-phase and one single-phase inverter on the same connection?

Yes. DSOs allow multiple inverters as long as each phase sees ≤3.68 kW AC and the total contracted power is respected [Elektroda, ksysju, post #18923184] Balance the single-phase unit on the most loaded or lowest-voltage phase to avoid asymmetry [Elektroda, Tech132, post #18924050]

How do I pick the phase for a single-phase inverter?

Measure phase voltages at peak generation. Choose the phase showing the lowest daytime voltage or highest household load; this reduces the risk of 253 V trips [Elektroda, anaba255, post #18924184]

Do multiple inverters need to communicate with each other?

No. They all synchronise to the grid, not to each other. You can simply parallel AC outputs at the main board; no hub or modbus link required [Elektroda, Slawek K., post #20319063]

How can I calculate safe extra PV power before hitting 253 V?

Use the three-step rule:
  1. Measure open-circuit phase voltage U on a clear day.
  2. Obtain loop impedance Z from an electrician.
  3. Compute P = (253 − U) × 253 / Z [Elektroda, 3301, post #19571636] If P is 2 kW, adding 3 kW will likely trigger trips.

What happens if my total inverter power tops 10 kW?

DSOs plan remote curtailment for systems above 10 kW and may move you to the new net-billing regime [Elektroda, DEFENCE, post #20015017] Staying below 9.99 kW avoids that switch and extra charges.

Will mixing 255 W and 450 W panels on one inverter waste power?

Only if you place mixed panels on the same MPPT string. Separate strings keep each array at its own optimal point, so 450 W modules deliver full output [Elektroda, kosmos99, post #20014751]

My roof faces three directions. How do I handle multiple orientations?

Pick an inverter with two MPPTs and add a small extra single-phase unit for the odd-angle array, or install microinverters. Four orientations need four MPPT channels for peak efficiency [Elektroda, anaba255, post #18924184]
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