Home battery size 2026: how many kWh are really worth it
A household using 4,500 kWh with 8 kWp covers 37 % of its needs itself without a battery, 69 % with 6 kWh and 80 % with 15 kWh. Every kWh above about 6 kWh adds little.
Checked by Radif Partners · Editorial policy · How we calculate
How big the battery should be
Self-sufficiency with 6 kWh
69 %
| Self-sufficiency 0 / 3 / 6 / 10 kWh | 37 % / 56 % / 69 % / 77 % |
| Extra benefit with 6 kWh | €408 |
| extra 6 → 10 kWh | €98 |
The right size for a home battery (Stromspeicher) follows from consumption between sunset and sunrise, not from the size of the solar system. The quarter-hour simulation with 74 metered German households shows this clearly: a household using 4,500 kWh a year with an 8 kWp system on the national average reaches 37 % self-sufficiency without a battery. The first 2 kWh of storage add €81 of benefit per kWh a year, because they are charged and emptied almost every evening. From about 6 kWh this value halves, and going from 10 to 15 kWh each extra kWh adds only €7, because a large battery is not filled in winter and not emptied on summer nights. With 6 kWh, the system’s benefit rises from €1,109 to €1,517 a year. As a rule of thumb for homes without a heat pump or electric car, the simulation supports a little over one kWh of usable capacity per 1,000 kWh of annual consumption. Since 2023, no VAT is charged on batteries for systems up to 30 kWp.
The battery follows your evenings, not your roof
A battery shifts solar power from midday to the evening and night. It is only worth as much as it can actually deliver in that time, and evening consumption sets the limit: fridge, lighting, cooking, television, router, standby. In a typical home without electric heating, that is a few kilowatt-hours a night. A battery larger than that need is not emptied on most days, and a kWh that is never discharged saves nothing. So the first question is not how big the solar system is, but how much power you use at night. The system only has to be large enough to fill the battery on sunny days on top of daytime use, which typical rooftop systems manage almost every day from spring to autumn.
What each extra kWh brings
The table shows the base case of the calculator above: 8 kWp facing south on the national average according to PVGIS, 4,500 kWh consumption, simulated with the HTW Berlin load profiles and an electricity price of 36.2 ct/kWh according to Eurostat.
| Battery | Self-sufficiency | Self-consumption share | Benefit per year | Extra benefit per added kWh |
|---|---|---|---|---|
| 0 kWh | 37.2 % | 19.9 % | €1,109 | – |
| 2 kWh | 49.8 % | 26.6 % | €1,271 | €81 |
| 4 kWh | 62.4 % | 33.3 % | €1,433 | €81 |
| 6 kWh | 68.9 % | 36.9 % | €1,517 | €42 |
| 8 kWh | 73.5 % | 39.3 % | €1,576 | €30 |
| 10 kWh | 76.6 % | 40.9 % | €1,615 | €20 |
| 12 kWh | 78.0 % | 41.7 % | €1,634 | €9 |
| 15 kWh | 79.7 % | 42.6 % | €1,655 | €7 |
The curve has a clear bend. Up to about 6 kWh, each kWh brings a lot, because it cycles fully on almost every day from March to October. After that, the return per kWh drops markedly, and above 10 kWh it is so small that it barely pays for a battery. Self-sufficiency keeps rising, but ever more slowly: chasing high percentages mostly buys expensive winter reserve that cannot be charged in winter for lack of surplus.
Three households compared
| Household | no battery | medium battery | large battery |
|---|---|---|---|
| 5 kWp, 2,500 kWh | 38.0 % / €665 | 3 kWh: 69.1 % / €887 | 6 kWh: 79.0 % / €958 |
| 8 kWp, 4,500 kWh | 37.2 % / €1,109 | 6 kWh: 68.9 % / €1,517 | 10 kWh: 76.6 % / €1,615 |
| 12 kWp, 6,000 kWh | 38.0 % / €1,580 | 6 kWh: 66.9 % / €2,078 | 10 kWh: 74.1 % / €2,201 |
The comparison confirms the rule of a little over one kWh per 1,000 kWh of annual consumption. The small household gains most per euro with 3 kWh, the large one with 6 to 10 kWh. A heat pump or an electric car charged in the evening pushes the figure up, because power is then needed at night as well; in that case, enter the higher consumption in the home battery calculator.
What the simulation assumes
- Usable capacity, not the gross capacity of the cells.
- Efficiency of 94.9 % when charging and again when discharging, 90.1 % for the round trip.
- Charge and discharge power of 0.5 kW per kWh of capacity, at least 0.8 kW.
- The battery charges only from solar surplus, never from the grid, and discharges only for your own consumption.
- Metered load profiles of 74 households from 2010, so without heat pumps or electric cars.
Ageing is not included. A battery loses capacity over the years; if you are on the borderline, choose the next size up rather than the smaller one. Tax does not matter: supply and installation of a battery for a system up to 30 kWp are zero-rated for VAT (section 12(3) VAT Act).
When a larger battery does make sense
Three situations argue for more capacity. First, high evening and night consumption from a heat pump, electric car or shift work. Second, zero export, where every kWh not stored is lost rather than paid for; see zero export. Third, a stricter export limit: the government draft of the EEG 2027 proposes a permanent cap at 50.0 % for new rooftop systems, and with a smart meter the hours with negative power prices already go unpaid today. A battery only helps against these if an energy manager charges it deliberately at midday instead of filling it in the morning. Without such control, sizing to your evening consumption remains the better choice.