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Can Two Inverters (One On-Grid, One Off-Grid) Be Connected to One Network?

Filipdz 30789 37
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Can I connect an on-grid inverter and an off-grid inverter to the same house network behind the meter?

Yes, but only with the right topology: the simplest arrangement is grid -> on-grid inverter -> off-grid inverter, with the off-grid inverter feeding dedicated house circuits and acting like a UPS while charging batteries from the grid and/or its own PV [#19896867][#20155811] If you want the off-grid inverter to be upstream of the on-grid unit or to pass on-grid power through it, you need AC coupling and phase compatibility, and only a few manufacturers support that properly [#19896393][#19896867] The PV panels should be separate for each inverter [#20354940] If the off-grid inverter is not certified/approved for grid feed-in, use it only as off-grid and do not export its energy to the grid [#20293447][#20292651]
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  • #31 20637222
    rob160
    Level 7  

    The inverter is three-phase, so production takes place on three phases...
    In the meantime, I think I found the problem. The inverter meter seems to be going haywire and/or the Luna battery and the inverter itself.
    At night, for example for the last 3 days at 8:33 p.m., the counter seems to be turning off??? Indicates 0 consumption. During the day it also seemed to reset along with the inverter.

    I think you need to check the counter somehow... communication cables, etc.
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  • #32 20638429
    rob160
    Level 7  
    Hello again, dear forum members.
    I think I found the reason. The UPS inverter (Easun) somehow disturbs the operation of the phase to which it is connected and the DTSU666 counter. After disconnecting the power supply, everything returned to normal.
    It seems that a solution such as in my case, i.e. on-grid inverter with BMS Luna and off-grid Easun SMG II as a UPS, is not possible.
    Maybe that's why there's been so little silence from the thread's author?

    I don't think I have many options here anymore.
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  • #33 20638444
    msr99
    Level 20  
    Maybe do more tests?
    e.g. EASUN without panels, without batteries.
    EASUN in different phases.

    Quote:
    example for the last 3 days at 20:33

    and this is not the moment when the sun has just set (actually, a small amount of sun causes MPPT in EASUN to see too low power and then turns off
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  • #34 20641200
    rob160
    Level 7  
    After switching to phase 2, the DTS counter continues to go crazy. It does not see power at night and Luna is not used.

    Do you know any device that will turn on the power using, for example, RS485?
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  • #35 20884712
    tomek0035
    Level 11  
    Hello. I recently have a Fox 3600 1-phase on-grid inverter and a 10 kW ESB inverter connected to the off-grid network, which has an exit to the house. Of course, it is known that each inverter has separate sets of panels. And as I have already written, everything from the power box to the devices in the house is on phase 1.
  • #36 21062491
    cdtdsilva
    Level 10  
    tomek0035 wrote:
    Hello. I recently have a Fox 3600 1-phase on-grid inverter and a 10 kW ESB inverter connected to the off-grid network, which has an exit to the house. Of course, it is known that each inverter has separate sets of panels. And as I have already written, everything from the power box to the devices in the house is on phase 1.


    What's your question then? Do you want to combine both systems? If so your best bet would be to use a transfer switch - Use grid while the grid tie inverter is running and swapp back to your 10KW inveter during other periods.
  • #37 21785290
    andrzejspieczny
    Level 1  
    >>20153842
  • #38 21852503
    tamer1281
    Level 1  
    >>19896197 This is the only way to do it! Tested and works.
    Offgrid 1-phase hybrids are normal consumers to which the whole building installation can be connected.
    You can, but also don't need to synchronise them as there are no 3-phase motors, and who in the house has one?
    The advantage is that the installation can be expanded in stages and the consumers can be switched to offgrid while maintaining the prosumer contract and the original ongrid installation.
    Another advantage is that there is no need to install grid-generator switches, because the hybrid is not a source of energy for the grid - it is a consumer. Safety rules are therefore maintained.
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Topic summary

✨ The discussion revolves around the feasibility of connecting two inverters—one on-grid and one off-grid—within a single network. Users explore various configurations, emphasizing the importance of AC coupling and compatibility between the inverters. It is noted that while connecting an on-grid inverter after an off-grid inverter is preferable, it requires careful consideration of phase compatibility. The conversation also touches on the need for dedicated circuits for off-grid loads and the potential for hybrid systems that utilize both inverters effectively. Users share experiences with specific inverter models, addressing concerns about energy management, battery charging, and compliance with local regulations regarding energy export to the grid.
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FAQ

TL;DR: Connecting an on-grid and off-grid inverter to one installation is possible; just respect the 3.68 kW per-phase export cap [PSE, 2023] and remember “Of course you can” [Elektroda, jurek.parus, post #19896867] Plan the order, certificates, and backup loads.

Why it matters: A correctly wired hybrid keeps the lights on during blackouts while still earning feed-in credits.

Quick Facts

• Single-phase export cap: 3.68 kW per phase in Poland [URE, 2023]. • AC-coupled hybrids must meet PN-EN 50549 or VDE-AR-N 4105 [PSE, 2023]. • RfG/RfC certificate required before any grid injection; check the PTPiREE inverter list [Wykaz certyfikatów, 2024]. • Off-grid idle draw: 22-60 W (Easun SMG II datasheet, 2023). • Safe battery sizing: daily kWh × 1.2 for 80 % DoD [IRENA, 2022].

1. Can I connect an on-grid and an off-grid inverter to the same network?

Yes. Forum users have operated both topologies in one switchboard without damage [Elektroda, Filipdz, post #19897606] You must isolate AC inputs with separate breakers and follow local grid codes.

2. Which connection order is better: grid→on-grid→off-grid or grid→off-grid→on-grid?

Grid→on-grid→off-grid is simpler. If the grid fails, the on-grid stops, and the off-grid powers loads [Elektroda, jurek.parus, post #19896867] The reverse order needs AC-coupling logic to avoid battery overcharge.

3. Do the two inverters need separate PV strings?

Yes. Sharing panels can confuse MPPT trackers and void warranties. "You need to have separate panels for the off-grid inverter" [Elektroda, BrucieK, post #20354940]

4. What certificates allow an inverter to feed the Polish grid?

Your device must carry RfG/RfC conformity plus a test report on PTPiREE’s approved list [Wykaz certyfikatów, 2024]. Devices lacking this, such as many budget Easun models, must remain off-grid [Elektroda, Slawek K., post #20298757]

5. Is the 3.68 kW single-phase limit enforced?

Yes. DSO contracts cap single-phase export at 3.68 kW; higher power requires three-phase connection [URE, 2023]. Exceeding the cap can trigger inverter derating or DSO penalties [Elektroda, MichałS, post #20291916]

6. Will the off-grid inverter charge batteries during a blackout?

If wired grid→on-grid→off-grid, only the off-grid inverter remains live and will charge from panels or batteries [Elektroda, Filipdz, post #19897606] The on-grid inverter sleeps until grid returns.

7. How do I stop battery overcharge in an AC-coupled system?

Choose an off-grid inverter that supports frequency-shift control. It raises AC frequency slightly; the on-grid inverter senses this and throttles output [Victron, 2023].

8. Can an off-grid inverter disturb my smart meter?

Yes. A user reported a DTSU666 meter showing erratic imports because the UPS-style inverter distorted the phase voltage [Elektroda, rob160, post #20634958]

9. How much standby power will the off-grid inverter draw?

Datasheets show 22-60 W idle, roughly 0.5–1.4 kWh daily (≈4 % of 5 kW-h battery) [Easun SMG II datasheet, 2023].

10. How do I wire a basic hybrid safely?

  1. Install separate DC strings to each inverter.
  2. Feed on-grid AC output to the main breaker; route this line through an RCD.
  3. Connect off-grid output to a dedicated sub-panel labelled “Backup”. Follow IEC 60364 clearances. Test transfer before energising.

11. What happens during a prolonged grid outage?

On-grid shuts down instantly. Off-grid keeps critical circuits alive until batteries reach the low-voltage cut-off (usually 44 V for 48 V packs) [Easun manual, 2023].

12. Can I start small and add batteries later?

Yes. Hybrid inverters can run in UPS mode without PV; you can add panels and batteries later [Elektroda, XeNeS, post #20155811]

13. Recommended off-grid inverter for a Solis 3-phase on-grid owner?

Look at budget Easun iSolar SMW (up to 8 kW on one phase) or a three-phase Deye SUN-5K-SG03LP1 for full symmetry [Elektroda, XeNeS, post #20154283]

14. Edge case: what if both inverters push power simultaneously above contract capacity?

Your bidirectional meter sums exports; exceeding the contracted 10 kW can void settlement and force disconnection [Elektroda, waltergraham72, post #20292863] DSOs check this via 15-minute telemetry.

15. "My off-grid inverter always imports 300 W even when I export 6.9 kW—why?"

Likely meter phase mis-alignment or inverter waveform distortion. Re-check CT orientation and try moving the inverter to another phase; this fixed the issue for rob160 [Elektroda, 20637222]
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