Sizing and Connecting Battery Storage: AC vs. DC Coupling
A battery storage system raises PV self-consumption from typically 30 % to 60-80 %. But: not every storage system fits every PV. This guide shows how to size and correctly connect a storage system.
AC vs. DC coupling
The connection point determines efficiency, flexibility and cost.
AC coupling
Storage connects via its own inverter to the 230V AC grid. Works with any PV system — also retrofitted.
DC coupling
Storage sits directly at the DC input of the PV inverter. Higher efficiency (95+ % vs. 90 %), but only with hybrid-capable inverter.
When which?
New system with hybrid inverter: DC. Retrofit on existing PV: AC. When expanding PV size: AC is more flexible.
Sizing the storage
Too small brings little effect, too large wastes money. Sizing follows three factors.
Rule of thumb
1 kWh storage per 1 kWp PV — for 10 kWp also 10 kWh storage. With high self-consumption (EV, heat pump): up to 1.5 kWh / kWp.
Annual consumption
A 4-person household with 4,500 kWh annual consumption typically needs 8-10 kWh storage.
| PV capacity | Household | Storage size |
|---|---|---|
| 5 kWp | 2 persons | 5 kWh |
| 8 kWp | 4 persons | 8-10 kWh |
| 10 kWp + heat pump | 4 persons + EV | 10-15 kWh |
Installation and connection
Location
Dry utility room, 10-30 °C, well-ventilated. Lithium storage cannot be in living rooms, boiler rooms, occupied rooms.
Statics
10 kWh storage weighs about 100-150 kg. For wall mounting the wall must support that — drywall unsuitable.
Cabling
AC connection usually 3-phase (5×4 mm² or 5×6 mm²). DC connection per manufacturer spec, often 2×6 mm².
Protection
Own circuit in the distribution board, type B RCD (all-current sensitive) — same requirement as wallbox.
Registration and funding
Grid operator registration
Storage is a generating system per VDE-AR-N 4105. Register before commissioning — otherwise no grid connection.
KfW funding
Storage funding regularly reopens. Check before purchase — funding must be requested before ordering.
Feed-in tariff
Storage reduces feed-in — tariff rates remain the same. With full feed-in, no storage needed.
Backup / island operation
Some storage systems can supply the house during grid outages. Requirement: backup switchover box, dedicated circuit.
Common planning mistakes
Oversized
A 15 kWh storage with 5 kWp PV brings almost no added value — the storage never fills. Better to choose 5-7 kWh and save.
Heat pump not considered
A heat pump needs 3-4 kWh extra daily. Anyone not planning that is at the storage limit quickly.
Location overlooked
Storage in boiler room or living room — not allowed. Check location before ordering.
Conclusion: storage pays off — but only in the right size
A battery storage pays off when sized to match PV and consumption. Anyone planning too big wastes money. Anyone too small leaves potential on the table.
💡 Tip: Plan storage already with the PV connection — a hybrid inverter makes later DC expansion significantly cheaper.
Storage and PV in the plan
myElectricPlan supports PV and storage systems directly in the system schematic — including protection concept and registration templates.
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