PLANNING
Battery sizing: how many kWh does my house need?
Updated October 24, 2026 · SodiumStorage.ch
In short: Swissolar recommends two rules of thumb and taking the smaller value: one thousandth of the annual PV production or one thousandth of the annual consumption. A house with a heat pump, 9,000 kWh consumption and 12 kWp of PV thus needs about 9 kWh, exactly the usable capacity of one 10 kWh Heiwit module.
The two rules of thumb
Swissolar's factsheet no. 13 proposes two rules of thumb. The smaller of the two gives a sensible battery size for self-consumption:
- Based on production: 1 to 2 hours of the PV nominal power or one thousandth of the annual production.
- Based on demand: half a day's consumption or one thousandth of the annual consumption.
As an upper limit, the HTW Berlin analysis shown in the factsheet gives about 1.5 kWh of usable capacity per kW of PV power. A larger battery is rarely full over the year and pays off less well.
Worked examples
Assumed PV production: about 1,000 kWh per kWp per year. Values rounded; your own consumption profile is what counts.
| Household | Consumption per year | PV system | Smaller rule-of-thumb value | Heiwit solution |
|---|---|---|---|---|
| Small house | 3,500 kWh | 5 kWp | about 3.5 kWh | rather no battery, or 7.78 kWh (from 2027) |
| Detached house | 4,500 kWh | 8 kWp | about 4.5 kWh | 7.78 kWh (from 2027) or 10 kWh |
| Detached house with heat pump | 9,000 kWh | 12 kWp | about 9 kWh | 1 × 10 kWh (9 kWh usable) |
| With heat pump and electric car | 13,000 kWh | 18 kWp | about 13 kWh | 1 to 2 × 10 kWh (9 to 18 kWh usable) |
In practice, according to Swissolar, about 12 kWh were installed on average in Swiss detached houses in 2024, a little more than the rules of thumb suggest. With a modular system you can start small: the HEIWIT-NA-48V210Ah can be run with up to three modules in parallel.
Nominal and usable capacity
The rules of thumb refer to usable capacity. For the HEIWIT-NA-48V210Ah the recommended depth of discharge is 90%: of 10 kWh nominal capacity you use 9 kWh, with two modules 18 kWh, with three modules 27 kWh.
One module costs CHF 3’896.– incl. VAT, about CHF 433.– per usable kWh; three modules with 30 kWh cost CHF 11’688.–.
Power: choosing the right inverter
Capacity (kWh) says how much energy is stored; the inverter power (kW) says how much can flow at the same time. For evening peaks with cooker, heat pump or charging station, power is what matters.
The single-phase VIRGO has 3, 4.6 or 6 kW but may only be connected on one phase up to 3.7 kVA in Switzerland. The three-phase LYBRA with 6, 10 or 12 kW is the right choice for most detached houses. More in the guide Home batteries in Switzerland.
And in winter?
A battery shifts solar power from the day to the evening and night, not from summer to winter. In the Swiss winter the PV system produces little and the battery is rarely full. For seasonal storage, according to Swissolar, the hydro reservoirs remain important, because batteries are not suited to storage over several months.
Do the maths yourself
HTW Berlin's independence calculator estimates self-sufficiency and self-consumption for your annual consumption, PV power and usable battery capacity. For an exact design, especially with a heat pump or electric car, a simulation with load profile data by your installer is worthwhile.
Sources
This guide does not replace advice from a qualified electrician or the requirements of your grid operator, canton or municipality. Information without guarantee; see the date above.
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