Retrofitting AC storage in Austria: when AC coupling makes sense

Many existing systems in Austria have a pure PV inverter without a battery connection. If a battery is to be added, there are two ways: replace the inverter with a hybrid inverter (DC coupling) or keep the existing system and add a battery with its own inverter on the AC side (AC coupling). Which way fits better depends on the existing system. This article shows when AC storage makes sense and what you should clarify before the quotation.

Note: Involtis supplies specialist businesses only. Homeowners receive planning, installation and registration through their electrical or PV installer.

AC and DC coupling in two sentences

• DC-coupled: The battery sits on the direct current side of a hybrid inverter. This controls PV, battery and grid in one unit. When retrofitting, the existing PV inverter is usually replaced.
• AC-coupled: The existing PV inverter remains unchanged. An additional storage-capable inverter with battery is connected on the alternating current side. It charges the battery with PV surplus and discharges it when required.

The Deye hybrid units of the SG05LP3-EU-SM2 series are designed for both roles. According to the datasheet, they support “AC coupling for retrofitting existing solar systems” and also have their own PV inputs (2 MPP trackers).

When AC coupling makes sense

An AC retrofit typically fits when:

• the existing inverter works well and still has service life or warranty left. Replacing it just for the battery would replace working technology.
• the DC side is to remain unchanged. No work is done on strings, modules or roof.
• several or different inverters or micro-inverters are installed. These cannot sensibly be transferred to one hybrid inverter.
• additional modules are to be connected to the new hybrid inverter later. A Deye SG05LP3 can then take both the battery and new PV strings.

When replacing with a hybrid (DC) fits better

Switching to a hybrid inverter is often the better solution when:

• the old inverter is at the end of its life anyway or shows defects.
• the existing strings fit the input data of the hybrid unit. For the Deye SG05LP3-EU-SM2, according to the datasheet, this means max. 800 V DC, an MPPT range of 200–650 V, 2 MPP trackers and the current limits per tracker.
• as few components as possible are wanted in the plant room.

You can find the comparison with subsidies and storage size in the article Retrofitting battery storage.

What AC coupling with a Deye hybrid looks like

The Deye manual (SUN-3…12K-SG05LP3-EU-SM2) describes AC coupling as “Mode IV: AC Couple” (chap. 6, p. 45). The existing grid-tied inverter is connected to the GEN port of the Deye. In the “Gen Port Use” menu (chap. 5.9, p. 42), “Micro Inv Input” is selected for this. According to the manual, this also works with classic grid-tied inverters (“Grid-Tied”).

The associated settings according to the manual:

• Micro Inv Input OFF/ON: If the battery SOC is above the OFF threshold, the connected inverter is switched off. If it falls below the ON threshold, it may operate again.
• AC Couple Frz High: As the SOC approaches the OFF threshold, the Deye shifts the frequency. The output of the connected inverter thereby falls linearly until it switches off at the set frequency.
• MI export to Grid cutoff: This can be used to prevent the generation connected to the GEN port from being fed into the grid.

Important: According to the manual, the “Micro Inv Input OFF/ON” thresholds only work with certain firmware versions. Therefore check the Deye firmware in advance. Also check in the datasheet of the existing inverter whether it responds to frequency changes with a power reduction. If in doubt, the Involtis service will help.

Checklist before the quotation

1. Survey the existing system: inverter type, power, phases and age, module output, meter location and space in the distribution board.
2. Clarify the goal: Is only self-consumption to increase, or is backup power also required? Backup supply requires the load output of the hybrid inverter and proper grid disconnection.
3. Calculate the power balance: rated power of the Deye (e.g. 10 kW or 12 kW AC for the SUN-10K/12K-SG05LP3-EU-SM2), power of the existing inverter and loads on the backup output. According to the datasheet, the max. continuous AC passthrough current is 45 A (3–12 kW) or 70 A (14–20 kW).
4. Choose the battery: The Deye SG05LP3 is a 48 V unit. Low-voltage batteries such as the Easyway UNIV 16 kWh are therefore suitable. Involtis confirms compatibility on a project basis, details in Combining Deye with Easyway storage.
5. Check the installation location: According to the datasheet, UNIV low-voltage batteries are IP20, designed for indoor use and charge at 10–55 °C. The Deye inverter is IP65.
6. Involve the grid operator (see next section).
7. Assess subsidies realistically (see below).

Grid operator: AC storage is a change to the system

• Notification obligation: According to § 98 of the Electricity Industry Act (ElWG), grid users must notify the grid operator of the connection of new energy storage systems unless they have their own metering point. Irrespective of this, the grid operator’s requirements for changes to existing generation systems apply.
• Styria (Energienetze Steiermark): An extension of existing systems is initiated in the feed-in portal at the existing feed-in metering point via “Edit”. As examples of significant changes according to TOR, the grid operator expressly names the “extension by a storage system”, both DC and AC storage systems. For PV systems from 3.68 to 250 kW, the active power specification must be implemented in the course of such changes, i.e. controllability via the smart meter. Instead of a cable, a radio connection can also be used for existing systems with the grid operator’s consent. Completion is reported via the installation document of a licensed electrical contractor.
• Permitted inverters: Energienetze Steiermark refers to the inverter list of Oesterreichs Energie.
• Technical rules: The basis is the TOR power generating plants type A (currently version 1.4) of E-Control. These apply to systems below 250 kW.

In other grid areas, the requirements of the respective grid operator apply. Clarify the procedure before installation.

Subsidies: AC storage alone usually not via EAG

According to oesterreich.gv.at and the EAG processing agency, the EAG investment grant for electricity storage is only available together with a new or extended PV system. A pure battery retrofit on an unchanged existing system is not eligible under EAG. If the PV is extended at the same time, for example by additional modules on the Deye hybrid, this may be different. Details and current calls can be found in the article EAG subsidy 2026.

For homeowners

Whether AC or DC retrofit is decided by your installer after an on-site survey. They also handle the notification to the grid operator and the completion report. Installers obtain Easyway batteries and Deye inverters through Involtis.

Frequently asked questions (FAQ)

Sources

1. Deye: User Manual Hybrid Inverter SUN-3K…12K-SG05LP3-EU-SM2, V2.0.0 (PDF dated 19.08.2025), chap. 5.9 (p. 42), chap. 6 Mode IV (p. 45), datasheet (pp. 49–50) – retrieved on 15.10.2026
2. Deye: Datasheet SUN-4/5/6/8/10/12K-SG05LP3-EU-SM2 (German) – retrieved on 15.10.2026
3. Deye: Datasheet SUN-14–20K-SG05LP3-EU-SM2 (German) – retrieved on 15.10.2026
4. Energienetze Steiermark: Generation systems (feed-in portal, extension, active power specification) – retrieved on 15.10.2026
5. RIS: Electricity Industry Act (ElWG), § 98 notification of new equipment, Federal Law Gazette I No. 91/2025 – retrieved on 15.10.2026
6. E-Control: Technical and Organisational Rules (TOR), incl. TOR power generating plants type A V1.4 – retrieved on 15.10.2026
7. Oesterreichs Energie: Inverter list – retrieved on 15.10.2026
8. oesterreich.gv.at: Photovoltaic subsidy schemes – retrieved on 15.10.2026
9. EAG processing agency: Investment grant photovoltaics and storage – retrieved on 15.10.2026