AC Coupling 2025: Complete Guide 6 Hybrid Inverter Configurations

AC Coupling: the flexible solution for adding storage

With the rise of solar energy storage in Belgium and France, AC coupling technology is emerging as a flexible solution for adding batteries to an existing photovoltaic installation. Unlike DC coupling, where the panels are connected to the batteries via a single hybrid inverter, AC Coupling involves connecting a (hybrid) inverter-charger on the AC side in parallel with an existing PV inverter.

In practice, the panel array remains connected to its original grid inverter (Fronius, SMA, Huawei, micro-inverters, etc.), and the hybrid inverter with batteries is connected to the electrical panel in AC parallel. This architecture allows you to keep your original PV installation without modifying the DC side, while adding storage and energy autonomy.

"AC coupling offers universal compatibility: all your inverters work together via the domestic network, with intelligent energy management for storage, backup and financial optimisation." – Wattuneed Technical Department

This comprehensive guide explores the six possible AC coupling configurations and scenarios in 2025, their advantages, technical challenges and solutions for optimising your self-consumption. Whether you have a single-phase or three-phase system, a network with or without neutral, or a mix of inverters from different brands (Deye, Sofar, Fronius, SMA, etc.), this guide will help you master AC coupling like a pro.

Configuration 1: Simple AC coupling grid-tie inverter + hybrid inverter

The first configuration is the most common and easiest to implement: adding a hybrid inverter to an existing photovoltaic inverter. If you already have a grid-tie or string inverter for your panels, you can add a hybrid inverter (or inverter-charger) connected to a battery pack.

How simple AC coupling works

The two inverters operate in parallel on the domestic grid:

• The PV inverter feeds solar energy into the AC grid and supplies the home's instantaneous consumption
• The hybrid inverter draws this surplus AC energy to charge the battery via AC→DC conversion
• In the evening or when needed, the hybrid feeds the stored energy back into the grid (DC→AC) to power the home
• Everything is synchronised, maintaining the 230V 50Hz of the internal grid

This solution is ideal for retrofitting an existing solar installation as it avoids touching the DC wiring of the panels. This gives you the best of both worlds: the high efficiency of the dedicated PV inverter (98-99%) and the flexibility of the hybrid inverter for storage and backup.

Advantages of simple AC coupling

• Quick installation: AC connection only, no modifications to the DC side of the panels
• Universal compatibility: Works with all PV inverters on the market
• PV inverter preserved: Warranty and operation unchanged
• Scalability: Battery capacity can be adjusted as needed
• Backup possible: The hybrid inverter can power essential loads off-grid

In practical terms, the hybrid monitors the voltage/frequency of the internal grid and charges the battery as soon as a solar surplus is detected. However, care must be taken to ensure that the hybrid inverter can handle the required charging power and that the two inverters are properly coordinated. 🔗 See Deye AC Coupling features

Configuration 2: Multi-inverter AC Coupling (several PV + one hybrid)

It is not uncommon to have several PV inverters in operation, for example two string inverters on different orientations, or a cluster of micro-inverters on a complex roof. AC Coupling offers high cross-brand compatibility: a Sofar hybrid inverter can recover energy from Enphase micro-inverters, or a Deye hybrid can work with a Huawei inverter.

Principles of multi-inverter coordination

As long as they are all connected to the same AC panel, the principle remains the same. The hybrid inverter will "see" the sum of the PV outputs and modulate the battery charge accordingly. The key is coordination and regulation to avoid conflicts.

Typical multi-inverter configuration

Residential example in Belgium:

• PV inverter 1: Fronius Primo 5.0 (5kW, south-facing, 12 x 415Wp panels)
• PV inverter 2: 6 Enphase IQ7+ micro-inverters (1.8kW, east-facing, 6 x 300Wp panels)
• Hybrid inverter: Deye SUN-6K-SG04LP3 (6kVA three-phase, 2 MPPT available for direct PV)
• Battery: 3× Pylontech Force-L2 (10.8kWh total, 48V 225Ah)
• Total PV power: 6.8kW AC coupled (1.13 ratio acceptable)

In this configuration, the Deye hybrid inverter recovers the cumulative production of the two PV sources via AC coupling. The surplus charges the battery with priority, and grid injection only occurs when the battery is full. The Enphase micro-inverters continue to produce even in backup mode thanks to Deye's frequency shifting.

Points to note for multi-inverters

• Island mode support: Check that your PV inverters accept variable 50-52Hz frequency
• Cumulative power: Do not exceed the AC load capacity of the hybrid (see section 4)
• Different orientation: Use separate MPPTs from the hybrid if possible for optimisation
• Communication: External Modbus meter recommended for fine coordination

Configuration 3: Special case of a 3×230V three-phase network without neutral (IT)

Some installations, particularly in Belgium or in older homes, are three-phase 3×230V without neutral (IT type network). This sensitive case requires special attention during AC coupling. In a 3×230V network, there is no neutral conductor: the 230V voltage is present between each pair of phases (instead of phase-neutral).

Inverter compatibility on IT networks

If your hybrid inverter or PV inverters are not designed for this mode, they may detect phantom faults or disconnect permanently. Fortunately, there are solutions for AC coupling in 3×230V without neutral:

Deye configuration on Belgian IT network

Deye SG04LP3 series hybrid inverters natively support 3×220V IT mode, provided they have the latest firmware (v1.46 minimum). Here is the configuration procedure:

1. Advanced Settings menu: Access the advanced network settings
2. Grid Standard: Select "Belgium IT 3×220V" or "VDE0126 LL230"
3. Voltage Range: Adjust voltage ranges 180-264V between phases
4. Frequency Range: Configure 47.5-51.5Hz according to local standards
5. Protection Type B: Enable DC insulation fault detection
6. Functional test: Check synchronisation 10 minutes before validation

"The IT configuration on Deye requires the latest firmware. We systematically update the inverters before shipment to ensure compatibility with the Belgian 3×230V network." – Wattuneed Technical Service

Essential IT network precautions

• Mandatory insulation measurement: Permanent insulation monitor (PIM) required by standards
• Type B protection: Differential suitable for DC faults
• Adjusted thresholds: Specific IT voltage/frequency ranges different from TN/TT
• Professional testing: Validation by an approved installer before commissioning

In summary, 3×230V AC coupling without neutral is entirely feasible with compatible equipment and specific configuration. Once correctly configured, your system will operate stably on this type of network, as demonstrated by the numerous successful Deye and SMA installations on the Belgian IT network.

Configuration 4: Management of AC load currents and risk of backfeeding

AC coupling means that the battery is charged via a double AC→DC conversion: the PV inverter sends alternating current, which the hybrid inverter converts back into direct current for storage in the batteries. This process is efficient (92-95% efficiency), but there are limits that must be respected to ensure the safety and reliability of the system.

The critical rule of factor 1.0

The key parameter is the maximum AC charging current (or power) that the hybrid inverter can absorb. A recognised best practice (notably by Victron and SMA) is to ensure that the total PV power does not exceed the VA power of the hybrid inverter.

Critical example of oversizing:

• Installation: Sofar HYD5000-EP hybrid inverter (5kVA nominal)
• AC-coupled PV: 8kW micro-inverters (factor 1.6 ❌)
• Dangerous scenario: In full sunlight, 8kW produced supplies 2kW loads
• Surplus: 6kW must go to the battery via the 5kVA hybrid
• Consequence: AC load current exceeds 26A → overheating, oscillations, safety cut-off

Understanding the phenomenon of looping

"Feedback" refers to a scenario where excess power loops uncontrollably between inverters. Imagine this situation:

1. Initial state: Battery 100% full, PV production 7kW, house consumption 1kW
2. Surplus 6kW: Cannot go to battery (full) or grid (zero export activated)
3. Hybrid reaction: Increases frequency to 50.5Hz to lower PV
4. PV reduced to 4kW: Battery sees possible charge, frequency drops back to 50Hz
5. PV rises back to 7kW: Cycle repeats → oscillations 10-20 times/minute
6. Consequence: Premature component wear, 15-25% production loss, risk of power cut

Professional anti-feedback solutions

• Correct sizing: PV AC ≤ hybrid power (factor 1.0)
• Sufficient battery capacity: Minimum 2kWh per kW of coupled PV AC
• Controlled grid injection: Allow limited injection when battery is full (e.g. 3kW max)
• Smart load shedding: Activate SmartLoad to consume surplus (water heater, swimming pool)
• Graduated frequency: Configure a soft ramp from 50.0 to 51.5Hz over 60 seconds

Configuration 5: Prioritise the battery before grid export

One of the main objectives of an AC-coupled installation is to maximise self-consumption, i.e. to consume or store solar energy locally rather than feeding it back into the grid. The system must therefore be configured to prioritise battery charging before any grid injection.

Battery priority strategies

Zero export configuration on Deye SUN-SG04LP3

Here is the complete procedure for activating zero injection with battery priority on a Deye hybrid inverter:

1. CT clamp connection: Connect the current sensor to terminals CT1/CT2 on the inverter
2. CT direction: Arrow on CT pointing towards the grid (table output), check in monitoring
3. EMS menu → Feed-in Control: Select "Zero Export" mode
4. Feed-in Power Limit: Set to 0W (or 50W technical margin)
5. Priority Settings: Configure "Battery First" before "Grid"
6. SOC Trigger: Feed-in only permitted if SOC > 98%
7. Time Control: Disable night-time export (avoids unnecessary battery discharge)
8. Functional test: Check grid meter = 0W ±50W in solar production

"With zero injection and battery priority correctly configured, our customers achieve 75-85% self-consumption compared to 30-40% without storage. The payback period for the battery system is reduced from 8-9 years to 5-6 years." – Wattuneed 2024 study

Advanced tariff optimisation

In Belgium with prosumer tariffs and in France with dynamic tariffs, time scheduling allows for financial optimisation of the system:

• 00:00-06:00 (off-peak hours 0.18 euros/kWh): Charge battery to 50% SOC if a cloudy day is forecast
• 06:00-09:00 (peak hours 0.28 euros/kWh): Priority battery discharge, full PV if available
• 09:00-16:00 (normal hours): 100% battery charge via PV surplus, zero injection
• 4:00 p.m.-10:00 p.m. (peak rate £0.32/kWh): Maximum battery discharge, direct PV to home
• 10:00 p.m.-12:00 a.m.: Automatic mode based on SOC and AI weather forecasts

This Time-of-Use strategy generates €400 to €700 in additional annual savings compared to a simple battery priority without time programming. 🔗 Complete anti-stalling guide

Configuration 6: Preventing inverter disconnections in AC coupling

A PV inverter disconnects when it suddenly disconnects from the grid, usually due to a voltage or frequency anomaly. In AC coupling, we want to avoid two types of disconnections: those related to the public grid (power surges, instability) and those related to internal system management (out-of-range frequency, overloads).

Main causes of disconnection in AC coupling

Professional anti-disconnection solutions

1. Micro-grid mode configuration on PV inverters

Activate extended island mode, which allows operation with variable frequency/voltage:

• Fronius: Setup menu → Country → MicroGrid → Ranges 48-52Hz / 200-264V
• SMA: Grid Guard Code → Extended settings → Frequency 47.5-52Hz
• Huawei: Advanced → Anti-islanding → Extended range enabled

2. Local grid voltage stabilisation via battery

The surplus solar energy that would have caused the voltage to rise is absorbed into the battery instead of being fed into the grid, keeping the local voltage lower. According to field feedback, a hybrid inverter with storage reduces the risk of grid disconnection to almost zero, compared to 15-30% possible losses without storage in areas with weak grids.

3. Hybrid inverter anti-disconnection settings

• Long reconnection delay: 60 seconds to avoid rapid ON/OFF cycles
• Smooth frequency ramp: 50.0→51.5Hz over 60 seconds (instead of a sudden 30 seconds)
• Extended voltage thresholds: 198-264V to tolerate transient fluctuations
• Disable unnecessary LOM: Loss of Mains detection on backup input if not used

4. Appropriate battery sizing

A sufficiently large battery prevents reaching 100% too quickly during full production, a common cause of floating and PV outages. Rule: battery capacity ≥ 2kWh per kW of coupled PV AC.

"90% of customer disconnections are resolved by three actions: activating micro-grid mode on PV, zero injection with battery priority, and increasing battery capacity from 5 to 10kWh. The ROI on the additional battery is 3-4 years thanks to the elimination of losses." – Wattuneed Technical Support

Monitoring and diagnosis of disconnections

Modern monitoring platforms make it possible to precisely identify the causes of outages:

• Solarman (Deye): Event history with error codes, voltage/frequency curves
• Solar.web (Fronius): Disconnection logs with timestamp and network parameters
• Sunny Portal (SMA): Network quality analysis, threshold exceedance alerts

By analysing this data, an installer can fine-tune the parameters to permanently eliminate recurring outages.

Case studies: successful complete AC Coupling configurations

Case 1: Single-family home in Belgium with existing Fronius

• Initial installation: Fronius Primo 5.0-1 + 12 415Wp panels (5kWp South, since 2020)
• Problem: 65% production feed-in, prosumer €372/year, frequent dropouts >253V
• AC Coupling solution: Addition of Deye SUN-5K-SG04LP3 + 2× Pylontech US5000 (9.6kWh)
• Configuration:
  • Fronius on GRID, Deye on GRID, battery on Deye
  • Zero export enabled with CT clamp 100A
  • Fronius micro-grid mode 48-52Hz
  • 100% battery priority before injection
• 12-month results:
  • Self-consumption 38% → 78% (+40 points)
  • Prosumer avoided: £372/year saved
  • Outages: 22 incidents/month → 0 incidents
  • Battery system ROI: 5.8 years

Case 2: Three-phase 3×230V IT business with multiple inverters

• Initial installation: 2× SMA Sunny Boy 5.0 + 18 Enphase IQ7+ micro-inverters (total 14.4kWp)
• Grid: Three-phase 3×230V without neutral (old Belgian IT type)
• Problem: Massive 12kW injection at midday, under-consumption at night, high grid costs
• AC Coupling solution: Deye SUN-10K-SG04LP3 (IT firmware) + 4× Pylontech Force-L2 (14.2kWh)
• Specific IT configuration:
  • Grid Standard = Belgium IT 3×220V
  • Voltage Range 180-264V phase-to-phase
  • Type B protection enabled
  • 3 SMA/Enphase inverters distributed over 3 phases
  • Limited injection 3kW (prosumer threshold)
• 12-month results:
  • Self-consumption 32% → 81% (+49 points)
  • Bill -1,840 euros/year (prosumer + peak/off-peak hours)
  • Peak shaving 15kW → 9kW peak (avoids meter upgrade)
  • System ROI: 4.2 years

Case 3: Villa in France with Sofar + SmartLoad load shedding

• Initial installation: Sofar Solar 8.8KTL-X + 24 375Wp panels (9kWp south-east/south-west)
• Energy-intensive equipment: 2.5kW swimming pool, 3kW water heater, 7kW EV charging station
• Problem: Unused midday surplus (buyback 0.10 euros/kWh), expensive evening peaks (0.32 euros/kWh)
• Advanced AC Coupling solution: Sofar HYD6000-EP + BTS 10.24kWh battery + SmartLoad
• Smart configuration:
  • Mixed AC coupling: PV string on GRID, swimming pool micro-inverters on GEN port
  • SmartLoad: Pool activated if SOC>70% + surplus >1.5kW
  • Water heater controlled via contactor if SOC>60% 11am-3pm
  • Time-of-Use: HC battery charge at night, HP discharge in the evening
  • Strict zero export to maximise self-consumption
• 12-month results:
  • Self-consumption 41% → 89% (+48 points)
  • Bill -€2,150/year (tariff optimisation + controlled equipment)
  • 100% solar-powered swimming pool (€0 electricity cost compared to €380/year previously)
  • EV charging station 68% charged by solar power + battery
  • System ROI: 4.9 years

AC coupling vs DC coupling comparison

The choice between AC and DC coupling depends on your specific situation. Here is an objective comparison to help you decide:

Final score: AC coupling 8 points | DC coupling 5 points

AC coupling is clearly the best choice for existing installations and scalability needs. DC coupling remains preferable for new installations that are optimised from the outset.

Frequently asked questions (FAQ) AC Coupling

Can I use AC coupling with any brand of PV inverter?

Yes, AC Coupling is universal because communication is via AC grid voltage/frequency. All modern grid-tie inverters (Fronius, SMA, Huawei, Solaredge, Enphase/APSystems micro-inverters, etc.) work with AC Coupling with Deye, Sofar, Victron hybrid inverters, etc. The only requirement is to activate micro-grid mode if you want a functional backup.

What is the efficiency loss due to double AC/DC conversion?

Double conversion (PV→AC by grid-tie inverter, then AC→DC by hybrid inverter to charge the battery) results in a loss of 5-8% compared to direct DC coupling. In concrete terms, out of 1000kWh of PV surplus: AC Coupling stores 920-950kWh vs DC Coupling 960-980kWh. This difference is largely offset by the flexibility and universal compatibility of AC Coupling.

How much does it cost to add an AC coupling system to an existing installation?

Indicative budget for Belgium/France in 2025 for an existing 5-6kWp house:

• 5kVA hybrid inverter: £1,200-1,800 (Deye/Sofar/WKS)
• 10kWh lithium battery: £4,500-6,500 (Pylontech/BYD/Delong)
• Accessories: £200-400 (CT clamp, cables, protection)
• Professional installation: £800-1,500 (including configuration)
• Total: £6,700-10,200 including VAT, depending on brand and capacity
• Average ROI: 5-7 years with tariff optimisation

Does AC coupling work with microinverters?

Absolutely, it's even a very common configuration. Micro-inverters (Enphase, APSystems, Hoymiles) feed their AC output into the domestic grid, just like a conventional string inverter. The hybrid inverter recovers this AC flow to charge the battery. Please note: check that your micro-inverters support frequency-shifting mode (48-52Hz) for backup operation, otherwise they will shut down in the event of a grid failure.

Can I add DC panels to the hybrid inverter in addition to AC coupling?

Yes, this is the high-performance "mixed hybrid" configuration! Inverters such as the Deye SUN-SG04LP3 have two independent DC MPPTs. You can therefore:

• MPPT 1+2: Connect 3-5kWp of new panels in direct DC (optimal efficiency 98%)
• GRID AC: Recover production from existing PV inverters (93% efficiency)
• Advantage: Maximise self-consumption with dual sources, MPPT optimisation by orientation

This configuration combines the advantages of both worlds.

How to programme the hybrid inverter to optimise off-peak/peak rates?

The Time-of-Use function (available on Deye, Sofar, Victron) allows you to programme up to 6 daily time slots. Example of optimal configuration for Belgium:

1. 00:00-06:00 (HC £0.18): Charge grid battery if SOC<40% and cloudy weather forecast
2. 06:00-09:00 (HP £0.28): Priority battery discharge, minimise grid draw
3. 09:00-17:00: Normal self-consumption mode, charge PV surplus battery, zero injection
4. 17:00-22:00 (HP £0.32): Max battery discharge, prohibit grid charging
5. 10 p.m.-midnight: Automatic mode according to SOC

Conclusion: AC Coupling, the versatile solution for 2025

AC coupling is undeniably the most versatile and scalable solution for adding battery storage to a photovoltaic installation in 2025. Whether you have a standard single-phase, three-phase 3×400V, or even three-phase 3×230V without neutral (Belgian IT) system, modern hybrid inverters offer configurations adapted to every situation.

The six configurations explored in this guide cover almost all of the scenarios encountered in the field:

1. Simple AC coupling: Existing PV inverter + hybrid inverter + battery (85% of cases)
2. Multi-inverters: Several PV sources coupled to a centralised battery system
3. 3×230V IT network: Special configuration for Belgium with compatible inverters
4. Current management: Compliance with the 1.0 factor rule to avoid feedback loops
5. Battery priority: Zero injection and tariff optimisation for maximum ROI
6. Anti-stalling: Fine tuning for year-round production stability

"Controlled AC coupling allows you to realise the full potential of your solar installation in 2025, in a flexible and scalable way, while ensuring system stability and maximising self-consumption." – Wattuneed, 15 years of solar expertise

The result is a significant gain in energy autonomy (70-85% self-consumption vs. 30-40% without a battery), a drastic reduction in inverter stalls (almost zero with a battery), and an improved ROI of 5-7 years thanks to intelligent energy flow optimisation.

Your solar installation will be able to evolve with new technologies while protecting your equipment and optimising your electricity bill. AC coupling, properly configured according to the six principles detailed in this guide, offers you the best compromise between universal compatibility, economic performance and flexibility for future upgrades.

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