Is a solar battery worth it? Compare the extra savings
Guide text reviewed: . This date does not certify current tariffs or validated PVGIS results.
A battery can increase self-consumption without improving financial returns. The question is not how much the whole installation saves, but how much additional saving the battery delivers over the same panels without storage.
Compare two installations that differ only in storage
- Fix the country, location, PV capacity, annual consumption and tariffs. Calculate the solar-only case first.
- Add battery capacity and its installed cost without changing the other inputs. If comparing cheap grid charging, keep its price and hours explicit in both scenarios.
- Subtract annual savings:
extra saving = saving with battery − saving without battery. - Divide the extra cost by that saving:
incremental payback = extra cost / extra annual saving. Zero or negative extra saving means no positive payback under those assumptions.
Include the battery, installation and any necessary inverter or protection upgrades in the extra cost. Do not divide the battery cost by the total solar saving: that credits the battery with savings the panels already provide.
Two hypothetical examples, not PVGIS results
A constructed arithmetic exercise, not a forecast or market pricing. Each day has 6 kWh of demand and 4 kWh of generation. All generation occurs in the 12:00–13:00 interval, with the remaining demand in the 20:00–21:00 interval; the other 22 hours are zero. The battery starts empty, has 3 kWh usable capacity, is lossless and can accept and deliver the stated energy within an hour. There is no grid charging, export income or minimum reserve.
Repeat this exact day 365 times. The avoided import price is €0.25/kWh at every hour; panels cost €6,000 and adding the battery costs €3,000. These are illustrative assumptions, not quotations. The battery finishes each day empty again.
| Measure | Daytime household | Evening household |
|---|---|---|
| Demand at 12:00 / 20:00 (kWh/day) | 3 / 3 | 1 / 5 |
| Direct solar use (kWh/day) | 3 | 1 |
| Surplus available to battery (kWh/day) | 1 | 3 |
| Battery energy delivered (kWh/day) | 1 | 3 |
| Annual saving without battery | €273.75 | €91.25 |
| Annual saving with battery | €365.00 | €365.00 |
| Extra annual saving | €91.25 | €273.75 |
| Incremental battery payback | 32.9 years | 11.0 years |
Reproduce each column using solar-only saving = direct use × 365 × 0.25 and saving with storage = (direct use + stored energy used) × 365 × 0.25. Then divide €3,000 by the difference. Whole-system payback is 9,000 / 365 = 24.7 years in both cases, despite very different incremental battery value. A division producing 32.9 years does not establish that the battery will last that long.
When the real comparison changes
- High daytime demand: you already use solar directly; shifting laundry, water heating or vehicle charging may compete with buying storage.
- Evening demand: storage helps only when earlier surplus and spare capacity are available. A household battery does not move summer surplus into the whole winter.
- Export payments: storing a kWh gives up the income from exporting it. HeliosROI does not deduct that opportunity cost.
- Cheap grid charging: net savings depend on the tariff spread, losses and avoiding a full battery before solar arrives. The model deducts the electricity purchased for this charging.
- Service life: losses, degradation, replacement, maintenance and finance are excluded. Backup during outages is not valued either; check whether the equipment actually provides it.
Before accepting a quotation
Ask for usable capacity, charge/discharge power, round-trip efficiency, warranty in years and throughput, replacement cost and inverter compatibility. Repeat the comparison with less surplus, a smaller tariff spread and a higher installed cost. Use bills and, where available, hourly meter readings: the calculator's regional profile does not recreate these two custom households.
For the solar resource see JRC PVGIS; for practical storage considerations see Energy Saving Trust. These are external references, not validation of the hypothetical numbers above.