Retrofitting PV storage: which option suits your system?

A PV system without a storage unit can be retrofitted with a battery at a later date. However, the storage unit never operates on its own: an inverter must charge it, discharge it and connect it to the domestic mains supply. That is why the first question is not about the battery, but about the inverter, which is already mounted on the wall.

The type plate and data sheet specify the type of device in question. A hybrid inverter has its own battery input and lists the storage systems it supports in the data sheet. A grid-tied inverter only converts the current from the modules and generally has no battery connection.

If the system already has a hybrid inverter, retrofitting involves adding only battery modules and, depending on the system, a BMS unit. The inverter’s storage list is decisive; only storage units listed there are approved. For some devices in the range, the manufacturers’ lists specify:

• Growatt MOD HU: exclusively the APX storage unit
• Growatt MIN XH: ARK and APX
• Growatt MOD XH: ARK; MOD XH BP: APX
• Solplanet ASW H-T2 and H-T3: Ai-HB (compatibility confirmed by the manufacturer)

For all other devices, the relevant data sheet applies. The inverter also determines whether a high-voltage or low-voltage storage unit is suitable. The battery modules and ready-made storage sets can be found under ‘Storage for inverters’.

An AC-coupled storage unit comes with its own inverter and is connected on the AC side, i.e. downstream of the existing inverter. The existing system remains unchanged; the storage unit charges using any surplus electricity currently available in the home and discharges it later.

The model in the range is the Growatt Aura 5000; availability is listed on the product page. According to the manufacturer’s English data sheet, it stores 5.02 kWh in LiFePO4 cells and delivers 800 W when connected via a plug, or up to 2,500 W when permanently connected to the sub-distribution board. It works in conjunction with a smart meter that measures consumption and has its own 2,500 W output for power cuts.

If an older grid-connected inverter is due to be replaced anyway, hybrid inverters and storage systems can be combined. The storage-inverter sets bundle both from a single manufacturer:

Check in advance whether the existing modules are compatible with the new unit. According to the data sheet, the Solplanet ASW 10kH-T3 accommodates up to 15 kWp of module power, whilst the Trina TRH-6K-T2 accommodates up to 9 kWp. Further units can be found under ‘hybrid inverters’.

The key factor is consumption after sunset and in the early morning, as this is the only period the storage system can cover. A storage system that is significantly larger will not be fully charged on many days. You can find out how to estimate the size using three rules of thumb under ‘Electricity storage: How big should it be, and how much does it cost?’.

As modular storage systems scale up in fixed increments, it is possible to start small and expand later: for Growatt ARK, Solplanet Ai-HB and Trina in increments of 2.56 kWh; for Fronius Reserva in increments of around 3.15 kWh of usable capacity; and for Growatt APX in increments of around 5 kWh. The upper limit per system is specified in the data sheet; for example, 20.48 kWh per unit for the Solplanet Ai-HB G2 Pro, 25.6 kWh for the Growatt ARK and 30 kWh for the Growatt APX. The usable capacity is slightly below the rated capacity; for the Growatt ARK module, for example, it is 2.3 kWh out of 2.56 kWh.

Whether electricity continues to flow in the home during a mains power cut is determined not by the storage system, but by the inverter. According to the data sheet, the hybrid inverters in the complete PV kits with storage – Growatt SPH, Solplanet ASW and Trina TRH – automatically switch to the storage system in under 10 milliseconds in the event of a mains power cut, whilst the off-grid Growatt SPF typically does so in 10 milliseconds. The Growatt Aura 5000 supplies power to devices via its own output in the event of a grid failure; according to the data sheet, the switchover takes less than 20 milliseconds. For information on other options, see ‘Emergency power, backup power, off-grid system’.

Ready-made storage sets for hybrid inverters in the range cost about 190 to 700 €/kWh per kWh of capacity; most cost between 210 and 340 € per kWh (as at October 2026). Larger sets are usually cheaper per kWh. The storage-only sets do not include an inverter; in Option 3, this is added, along with installation. The price on the product page is decisive. No VAT is payable on photovoltaic systems and their essential components under the conditions set out in Section 12(3) of the German Value Added Tax Act (UStG). The relevant information can be found under ‘VAT on photovoltaic products’. Please specify during the ordering process whether these conditions apply.

A hybrid inverter and a permanently connected storage system are integrated into the domestic electrical installation; this is carried out by a qualified electrical contractor. The grid operator and the Federal Network Agency provide information on registration with the grid operator and in the market master data register.

Those planning a new installation rather than retrofitting one will find complete systems under ‘Complete PV Sets with Storage’ and step-by-step planning guidance under ‘Planning a PV System’. There are dedicated, plug-and-play storage units for balcony power plants; whether one is worthwhile is explained under ‘Balcony Power Plant with Storage: When Does It Make Sense?’.