Home battery calculator 2026: what storage adds to a rooftop system
An 8 kWh battery on an 8 kWp system in Dresden raises a 4,500 kWh household's self-sufficiency from 37 % to 74 % and adds €468 a year. At €6,000 it pays for itself in 12.8 years.
Checked by Radif Partners · Editorial policy · How we calculate
Extra benefit from the battery
€468 a year
payback 12.8 years at €6,000
| Self-consumption without battery | 20 % · 1,675 kWh |
|---|---|
| Self-consumption with 8.0 kWh | 39 % · 3,314 kWh |
| Self-sufficiency without / with | 37 % / 74 % |
| less fed into the grid | 1,634 kWh |
| Extra benefit per year | €468 |
| Battery payback | 12.8 years |
Step-by-step calculation
- Same system, 8.0 kWp facing south, 30°, 8,505 kWh a year.
- Every quarter-hour: supply the home first, surplus into the battery (about 90% round trip), the rest to the grid; the battery discharges in the evening.
- Each stored kilowatt-hour saves 36.2 ct of bought power but gives up 7.70 ct of feed-in tariff.
Estimate based on average weather 2005 to 2023 (PVGIS) and metered household loads. A single year can differ by a few per cent. How we calculate
Whether a home battery (Stromspeicher) pays off is decided not by self-sufficiency but by a simple margin: every kilowatt-hour the battery shifts from midday to the evening saves the purchase price of 36.2 ct, but gives up the feed-in tariff of 7.70 ct/kWh. That leaves about 28.5 ct, minus the losses from charging and discharging. The calculator simulates every quarter-hour of a year using PVGIS yields for your postcode and 74 metered household profiles from HTW Berlin. In the Dresden example, with 8 kWp facing south and 4,500 kWh of consumption, self-consumption rises from 20 % to 39 % of output with 8 kWh. The system's benefit grows from €1,116 to €1,584 a year. Those €468 are what the battery has to earn over its life; whether it does depends on the price you enter in the calculator.
What the calculator compares
The calculator runs the same rooftop system through a whole year twice, once without and once with a battery. In each quarter-hour, solar power first supplies the home, then the surplus charges the battery, and only then does the rest go to the grid. In the evening and at night the battery discharges until it is empty. On the simulation's assumption, about 90 % of the stored energy is left after the round trip. Yield and consumption come from PVGIS and the load profiles of HTW Berlin; south-facing roof at 30°, surplus feed-in, no smart meter.
The extra benefit is the difference between two sums: saved grid purchases plus feed-in payment with the battery, minus the same without it. Because each stored kilowatt-hour is no longer exported, it costs the rate for the commissioning period, here 7.70 ct/kWh.
Batteries from 0 to 12 kWh in Dresden
| Battery | Self-use | Self-sufficiency | Bought from grid | Benefit/year |
|---|---|---|---|---|
| none | 20 % | 37 % | 2,825 kWh | €1,116 |
| 2 kWh | 26 % | 50 % | 2,258 kWh | €1,278 |
| 4 kWh | 33 % | 62 % | 1,690 kWh | €1,440 |
| 6 kWh | 37 % | 69 % | 1,393 kWh | €1,525 |
| 8 kWh | 39 % | 74 % | 1,186 kWh | €1,584 |
| 10 kWh | 41 % | 77 % | 1,049 kWh | €1,623 |
| 12 kWh | 41 % | 78 % | 982 kWh | €1,642 |
The curve flattens early. The first kilowatt-hours of capacity are used fully on most days of the summer half-year, the last ones only on long summer days with high evening use. How to find the right capacity is explained in the guide to home battery size.
Battery and the 60 per cent cap
Since 25 February 2025, new systems without a smart meter may export at most 60 % of their rated output (section 9(2) EEG). A battery can take part of the midday peak that would otherwise be lost. In the example, though, clipped power only falls from 204 to 199 kWh, because on sunny days the battery is often full by late morning. Using the battery deliberately for the midday peak needs charge control that waits until midday; the calculator does not model that.
Price, tax and payback
No VAT is charged on buying and installing a battery for a photovoltaic system (section 12(3) VAT Act). Payback is the battery price divided by the extra benefit: at €6,000 and €468 a year that is 12.8 years, with no rise in electricity prices. Set this against the warranty period of the cells and enter your own quote. How the whole system with a battery works out over the years is shown under solar PV payback.