Judge the battery’s additional cost against its additional benefit, not the savings of the entire solar installation.
Keep a no-battery comparison
Start with the solar-only proposal and its expected direct use and exports. Then calculate what changes when storage is added. Electricity moved into the evening avoids some purchases, but also reduces exports and involves losses. The whole solar saving must not be credited to the battery.
Examine the proposed size
Ask how usable capacity relates to your evening and overnight demand. The Verbraucherzentrale’s battery guidance provides useful questions about sizing and economics. Request separate results for summer and winter, plus warranty conditions and a replacement allowance.
Separate tariff trading from solar storage
If a supplier proposes charging from the grid on a dynamic tariff, ask for that model separately. Include fees, losses, operating restrictions and the risk of different future prices. Do not let hypothetical trading income obscure the ordinary storage calculation.