AC Coupled Solar System Wiring Guide: Sunsynk & Deye Inverters

Many commercial and residential properties across South Africa already operate mature grid-tied solar systems powered by standard string inverters or microinverters. When these property owners seek to add battery storage to survive severe load shedding, replacing functional grid-tied inverters represents an unnecessary capital expense.

Instead, installers can keep the current string inverter and use a Sunsynk inverter with batteries to utilise the excess power from the string inverter to charge these batteries. By routing the alternating current (AC) output through the system, excess daytime generation can be captured to charge battery banks while maintaining full off-grid backup capabilities during utility outages.

This technical engineering guide outlines the two primary AC-coupling architectures, frequency-watt droop curtailment mechanics, and meter telemetry integration on Sunsynk and Deye platforms.

1. Architectural Methodologies: GEN Port vs. Grid/Load Side

Depending on array capacity and physical distribution board layout, installers can implement AC coupling using two distinct configurations:

```
[ METHOD 1: GEN / AUX PORT COUPLING ]
Existing String Inverter AC Output ───► Inverter GEN Terminals ───► Battery Charge / Load

[ METHOD 2: GRID / LOAD SIDE COUPLING ]
Existing String Inverter AC Output ───► Distribution Board Busbar ───► Inverter CT Senses Flow
```

Method 1: Connecting the String Inverter to the GEN terminals of your Sunsynk Inverter

As detailed in official manufacturer documentation:

GEN terminals on the inverter can be used as an input or as an output

• Capacity Matching Rules: When utilizing the GEN terminals, the string inverter size must NOT exceed the size of the Sunsynk inverter.
• Combined DC MPPT Operation: If the hybrid inverter is also running DC strings directly into its own MPPT trackers while AC coupled via the GEN port, manufacturers recommend the string inverter connected has a power rating of no more than half of the Sunsynk Inverter.

Method 2: AC Coupling the String Inverter and the Sunsynk Inverter on the GRID/LOAD Side

When the existing third-party solar array significantly exceeds the capacity of the hybrid inverter, grid-side coupling is required:

• Under these circumstances, it is best if the string inverter has a larger capacity than the Sunsynk inverter.
• In this arrangement, the string inverter will cover the household consumption first and then the Sunsynk CT will detect any export and reroute this into the battery.
• Conversely, if the string inverter does not cover the full household consumption, the Sunsynk inverter will be able to detect this load and discharge the battery to cover this.

2. Curtailment Settings & Linear Frequency Droop Control

When the utility grid fails and the hybrid inverter establishes an islanded microgrid, maintaining power balance is critical:

```
+──────────────────────────────────────────────────────────────────────────+

+──────────────────────────────────────────────────────────────────────────+

+───────────────────────────────+──────────────────────────────────────────+

+───────────────────────────────+──────────────────────────────────────────+
```

Inverter Configuration Parameters

To set up AC coupling safely, navigate on the touchscreen to:
Settings -> Aux Load -> For micro inverter input

Key configuration parameters include:

• MI Zero Export – When using the GEN terminals of the inverter when this setting is selected, the micro inverter or string inverter will not export excess power to the GRID.
• AC Couple OFF Batt - When the battery SOC exceeds setting value, Micro inverter or the string inverter will shut down.
• During throttling, microinverter output power will decrease linearly. When the battery SOC equals the setting value (OFF) the Microinverter will stop working and stop exporting power produced by the microinverter to the grid.
• AC Couple on GRID Side – This should be selected if the string inverter is coupled to the distribution board on the GRID side.

3. Telemetry Integration: Using and Eastron Meter to Read String Inverter PV Data

In retrofitted AC-coupled installations connected on the grid side without auxiliary metering, the system telemetry will see the battery charge coming from the GRID, and the household consumption will only display if not covered by the string inverter.

```
[ Third-Party String Inverter ] ──► [ Dedicated Eastron Meter (Modbus Serial) ]
│
▼
[ Inverter Meter Port ]
│
▼
[ Real-Time PV Display in App ]
```

Dedicated Inverter Metering

To achieve transparent monitoring, the manufacturer outlines:

• Using and Eastron Meter to Read String Inverter PV Data by installing a dedicated Eastron energy meter directly on the AC output conductors of the string inverter.
• Connect the RS485 Modbus serial communication bus into the inverter auxiliary meter port.
• This configuration enables the hybrid inverter to separate third-party solar production from grid import on mobile and web dashboards.

4. Parameter Configuration Summary

Summary Checklist

• Evaluated string inverter size to select between GEN port coupling (1:1 ratio) and grid-side coupling.
  
• AUX load operational mode configured to Micro Inv on inverter touchscreen.
  
• Linear frequency-watt droop control tested under off-grid battery float conditions.
  
• Dedicated Eastron energy meter installed on string inverter output to provide unified telemetry.