Guide · Switzerland
Solar battery, electric car and heat pump: what size do you need?
Same solar system, same consumption, different battery size. Two worked examples show what changes when your car charges during the day or after work.
Short answerShort answer
Your system needs to fit your life.
The right battery size depends on when you produce and use electricity, not just how much. Charging your car directly from daytime PV surplus avoids routing that energy through the home battery. If you return in the evening, more storage can help. In our example, the economic maximum is 18 kWh for daytime charging and around 23–24 kWh for evening charging. These are scenario results, not a rule for every home.
At midday, your car is at work — not under your solar roof
Charging in the sunshine sounds obvious on paper. But if you drive to work in the morning and return in the evening, your car simply is not home during the most useful solar hours. That is not a flaw in your routine. It is something the system design needs to respect.
We compare two households with the same PV system, heat pump and driving demand. Only charging time changes. The daytime case requires the car to be genuinely available at home in solar hours, perhaps because working hours allow it. The evening case reflects a routine where you drive to work and charge once you return home.
The example: 15 kWp in Interlaken and a 30 km daily commute
At 3800 Interlaken, 15 kWp facing south at 30° produces about 19,001 kWh a year in the PVGIS profile used here. Four people and a heat pump consume 9,000 kWh: the model allocates 4,050 kWh to the household and 4,950 kWh to heating. The car is additional.
We assume 30 km on each of five driving days a week. This is a rounded return journey based on the FSO’s 2022 average car commute of 15.3 km each way. Across 52 weeks that is 7,800 km a year, without extra leisure trips or a separate deduction for holidays.
The chosen 20 kWh/100 km is an assumption, not a claimed Swiss fleet average. It means driving energy before charging losses: 7,800 × 20 ÷ 100 = 1,560 kWh. With 90% charging efficiency, the electricity required is about 1,733 kWh. Both households therefore consume about 10,733 kWh a year.
The same 10 kWh battery, a different result
First we hold the battery constant too: 10 kWh gross, 9 kWh usable, at a modelled cost of CHF 7,000. This isolates the effect of charging time.
| Metric | Daytime PV surplus | Evening / night |
|---|---|---|
| Self-sufficiency | 73.0 % | 58.8 % |
| Annual grid purchases | 2,894 kWh | 4,418 kWh |
| Total annual benefit | CHF 3,002 | CHF 2,635 |
| 20-year surplus · complete PV and battery system | CHF 26,333 | CHF 19,003 |
Daytime charging cuts grid purchases by about 1,524 kWh a year and increases the annual financial benefit by about CHF 367. The car uses surplus directly. Evening charging overlaps more strongly with the home’s other demand.
Which size gives each household its best 20-year result?
Next we let battery size fit each routine, comparing 0–40 kWh in 1 kWh steps for this article. We look for the battery size that delivers the greatest financial gain over 20 years. Each option is compared with the same PV system without storage.
| Metric | Daytime PV surplus | Evening / night |
|---|---|---|
| Gross / usable capacity | 18 / 16.2 kWh | 24 / 21.6 kWh |
| Battery price | CHF 10,760 | CHF 13,580 |
| Total investment after Pronovo | CHF 37,460 | CHF 40,280 |
| Total annual benefit | CHF 3,312 | CHF 3,301 |
| 20-year surplus · complete PV and battery system | CHF 28,784 | CHF 25,731 |
| Additional battery benefit over 20 years | CHF 9,411 | CHF 14,184 |
| Economically near-equivalent range (90% and above) | 14–23 kWh | 18–30 kWh |
For evening charging, 23 and 24 kWh differ by only about CHF 0.55 over the entire 20 years: effectively a tie. The table shows the mathematical maximum at 24 kWh. An actual quotation and usable capacity matter far more than that decimal place.
The near-optimal range achieves at least 90% of the battery’s maximum additional benefit: 14–23 kWh for daytime charging and 18–30 kWh for evening charging. This does not mean every size costs the same or performs equally.
Continue with these calculator examples
The links load the same assumptions and the specified battery size. Then change your tariff, mileage or battery price. Article results reflect calculations dated 22 September 2026; later data or model updates may change them.
A more valuable battery does not necessarily mean a more profitable system
Storage contributes more to the evening household: about CHF 14,184 extra over 20 years, compared with CHF 9,411 for daytime charging. That makes sense: this household has more reason to save electricity for later.
Yet the complete system still comes out ahead with daytime charging: about CHF 28,784 versus CHF 25,731 over 20 years, a difference of CHF 3,053. Direct use needs less home storage and avoids its additional storage losses. A larger battery offsets part of the timing disadvantage; it does not make later charging the cheaper option here.
What sits behind the CHF figures
We use 30 ct/kWh for imports and 6 ct/kWh for exports: comparison assumptions, not verified local tariffs. PV costs CHF 32,100 before incentives, or CHF 26,700 after an assumed CHF 5,400 Pronovo one-off payment. Storage costs CHF 2,300 plus CHF 470 per gross kWh, including installation and VAT. Local incentives, separate guarantee-of-origin revenue and tax effects are excluded.
Annual benefit is avoided electricity purchases plus export income. The 20-year surplus is 20 times that benefit minus the initial investment after incentives. For example: 20 × CHF 3,312.1935 − CHF 37,460 ≈ CHF 28,784, using unrounded values in the calculation. This is not a complete profit forecast: tariffs and output stay constant, with no degradation, replacements, financing or additional running costs.
Guidance, not a promise
PVGIS 5.3/SARAH3 data for 2014–2023 is averaged hour by hour into a typical year, with 14% PV system losses. This smooths out individual dark weeks and can favour daytime charging. It is not a promise that the car will charge entirely from solar throughout a real year.
Surplus mode uses available daytime PV and tops up any missing energy from 18:00 on the same driving day. Evening/night charging spreads demand across 18:00–07:00. Specific departures, vehicle state of charge and wallbox power limits are not simulated. The home battery has 90% usable capacity and 90% round-trip efficiency.
The 20 kWh/100 km is an annual average without a separate temperature-driven winter uplift. The heat pump does use a seasonal demand profile. A home battery shifts energy by hours, not from summer into winter.
Retrofitting storage: use the data your system already provides
If you already have PV and an electric car, monitoring and bills provide a much better starting point: generation, exports and grid purchases. Use a full year in which you were already charging the car regularly if possible. Your commute, habits and heating season will then be reflected in the starting data.
The WattBilanz retrofit calculator uses those measured annual totals, but still models their timing. That is not the same as simulating your measured hourly history. If you have hourly or quarter-hourly records, ask your installer to analyse them as well. This brings the design closer to your actual life than a generic household example.
Official and specialist sources
- FSO: commuting and journey distances (2022)
- European Commission: PVGIS data and method
- TCS: electric cars in cold weather
- WattBilanz: Pronovo, exports, guarantees of origin and tax
Calculated: 22 September 2026 · Edited: 23 September 2026.