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.
Use an incremental calculation
Consider an example where storage accepts 1,000 kWh of solar surplus and later delivers 900 kWh. At €0.30/kWh for purchases and €0.08/kWh for exports, avoided purchases are €270 and forgone export income is €80. The difference is €190 before storage costs. This calculation isolates what the battery changes. It does not count the panels’ existing direct-use saving a second time.
Make replacement and ongoing charges visible
List the extra installation price, any service subscription and a scenario for replacement or reduced usable capacity. Ask whether the warranty depends on energy throughput, operating settings or online connectivity. Separate a contractual guarantee from a forecast. Compare a range of annual benefits with the money committed so the decision does not rest on one favourable year being repeated indefinitely.
Ask how competing uses share the same battery
Solar storage, cheap-rate grid charging and backup reserve all use the same capacity. Ask the supplier how its controls prioritise them and whether one calculation assumes capacity that another has already reserved. Include network and tariff restrictions relevant to the offer. A combined model should show energy flows and fees together, rather than adding the maximum saving from several separate sales examples.
Your planning checklist
- A solar-only baseline and the additional battery cash flow.
- Energy entering and leaving storage, with losses explicit.
- Subscriptions, replacement assumptions and warranty conditions.
- One combined model for solar, tariff charging and reserve.
Common questions
Should the solar system’s entire saving pay back the battery?
No. Compare the battery’s extra cost with the extra benefit it creates. Solar generation already used directly at home belongs in the solar-only baseline.
Does a dynamic tariff make a large battery worthwhile?
Not by itself. The result depends on the price difference after charges, usable capacity, losses, cycling and actual operation. Request a separate tariff model and then check how it interacts with solar storage.