Energy storage: How big should it be, and how much does it cost?
In short: An energy storage system should match consumption and the PV system, not be as large as possible. HTW Berlin recommends at most 1.5 kWh usable capacity per 1,000 kWh annual consumption and per 1 kW PV output; for a household with 4,000 kWh and a 10 kW system, that is at most 6 kWh. Ready-made storage kits in the range usually cost €210 to €340 per kWh.
Contents
What rules of thumb apply to storage capacity?
An electricity storage unit stores the solar power that the house does not need at midday and releases it in the evening and at night. If it is too small, surplus power flows into the grid on sunny days. If it is too large, it will not be fully charged on many days and will cost more than it is worth. The correct size therefore depends on two figures that you know: your annual consumption and the output of your PV system.
The Solar Storage Systems Research Group at HTW Berlin has derived three rules of thumb from simulations (Orth, Weniger, Meissner, Sonnenenergie 02/2022):
- PV output: at least 0.5 kW per 1,000 kWh of annual consumption.
- Storage for the system: a maximum of 1.5 kWh usable capacity per 1 kW of PV output.
- Storage for consumption: a maximum of 1.5 kWh of usable capacity per 1,000 kWh of annual consumption.
The lower of the two maximum values from Equations 2 and 3 is decisive. The HTW example: for a system with 10 kW and an annual consumption of 4,000 kWh, the storage unit should have a maximum usable capacity of 6 kWh.
| Annual consumption | PV output | Maximum (usable) storage capacity |
|---|---|---|
| 3,000 kWh | 6 kW | 4.5 kWh |
| 4,000 kWh | 10 kW | 6 kWh |
| 5,000 kWh | 10 kW | 7.5 kWh |
| 6,000 kWh | 12 kW | 9 kWh |
The values are upper limits; HTW Berlin specifies them for detached houses with a PV system. A storage system is only worthwhile if there is a sufficient surplus, and according to HTW, the benefit of each additional kWh decreases progressively.
What is the difference between usable capacity and nominal capacity?
The rules of thumb are based on the usable capacity. The product often states the nominal capacity, which is slightly higher because the battery is never fully discharged for safety reasons. For the Growatt ARK-2.5H-A2 module, the data sheet specifies a 90 per cent depth of discharge; out of 2.56 kWh, this leaves around 2.3 kWh available for use. You should therefore use the value from the data sheet in your calculations.
What are the steps involved in expanding a storage unit?
Module storage systems consist of a control unit (BMS) and battery modules of equal size. You can start small and add modules later, up to the upper limit of a tower; depending on the system, several towers can be connected in parallel:
| Storage systems in the product range | Step by step per module | Tallest tower according to the data sheet |
|---|---|---|
| Growatt ARK HV | 2.56 kWh | 25.6 kWh |
| Solplanet Ai-HB G2 Pro | 2.56 kWh | 20.48 kWh |
| Fronius Reserva | 3.15 kWh | 15.8 kWh |
| Growatt APX HV | around 5 kWh | 30 kWh |
For around 10 kWh, the range includes the Growatt APX 10 kWh, the Solplanet Ai-HB 075A with 10.24 kWh and the Growatt ARK-2.5H-A2 with 10.2 kWh; all three are among the systems listed in the table and can be expanded in the steps shown there. The rules of thumb above will help you determine whether 10 kWh is suitable for you.
The storage system list determines which storage unit is connected to which inverter; instructions on how to do this can be found under ‘Retrofitting PV storage’.
Do heat pumps and electric cars count?
The rules of thumb are based on domestic electricity consumption. Anyone who operates a heat pump or charges an electric car at home will have a higher annual consumption. If a heat pump or an electric car is added, a higher capacity may be advisable, according to HTW Berlin. You should consult a specialist company to determine how high this capacity should be.
How much does an electricity storage system cost?
Ready-made storage sets for hybrid inverters in the range cost about 190 to 700 €/kWh per kWh of capacity, with most costing between 210 and 340 € per kWh (as of October 2026). Larger sets are usually cheaper per kWh. The storage-only sets do not include an inverter; if you are planning a new installation, you will find inverters and storage systems together under ‘Storage-inverter sets’ and complete systems under ‘Complete PV sets with storage’. The price on the product page is decisive. Under the conditions set out in Section 12(3) of the German Value Added Tax Act (UStG), no VAT is payable on photovoltaic systems and their essential components. Please indicate during the ordering process whether these conditions apply.
Is a storage system worth it for you?
A storage device does not generate electricity; it simply transfers it. You can assess for yourself whether it is cost-effective in four steps:
- How much electricity do you use in the evening and at night? Read the meter in the evening and in the morning on a few days.
- How often does the storage facility fill up? Mainly from spring to autumn; rarely in winter.
- Multiply the additional kilowatt-hours you use yourself per year by your electricity price, minus the feed-in tariff that you forgo as a result, to calculate the saving.
- The price of the storage device divided by these savings gives the number of years until it has paid for itself. Compare this with the service life, i.e. the number of charge cycles as stated in the data sheet.
Who installs the storage unit?
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.
All storage systems are listed under ‘Battery Storage’, whilst the modules for hybrid inverters are listed under ‘Storage for Inverters’. There are separate, smaller storage systems available for balcony power plants; whether one is worthwhile is discussed under ‘Balcony Power Plant with Storage: When Does It Make Sense?’.
