Decisive is your daily evening and night consumption – the electricity that the battery can actually shift. For many single-family homes, you will end up with 8 to 12 kWh of usable capacity. Too small is the more harmless mistake here: a tight battery simply works flat out every day, one that is too large stands around expensive and half empty. (As of: July 2026)
Key points in brief
- The size is not determined by the system on the roof, but by your consumption after sunset – the battery can only shift this electricity.
- The best route to the right number is real consumption data: smart meters and monitoring show evening consumption in black and white.
- The calculation is based on the usable capacity – it is smaller than the gross figure in the brochure.
- Heat pump and electric car do not automatically make the battery larger: the heat pump stores better thermally, the electric car is better charged directly during the day.
- The most common and most expensive mistake is a battery that is too large – paid per kilowatt-hour, only half used.
Why the size determines the economic viability
With the battery, you pay for capacity – currently installed roughly CHF 800 to 1,200 per usable kilowatt-hour. Every kilowatt-hour you buy must therefore earn its money, and it can only do that in one way: by being charged and discharged as often as possible. A battery that runs a full cycle every day works; one that is chronically half-full is dead capital on the basement wall.
This is exactly where the widespread intuition «rather a bit bigger, then I'm safe» fails. An oversized battery runs empty at both ends of the year – in winter it never gets full due to a lack of surplus, in summer it never gets empty due to a lack of evening consumption. The battery that is too small, on the other hand, has a mild failure mode: it is full early on good days, the rest of the surplus is fed in and remunerated normally. Therefore, an asymmetrical rule applies to sizing: If in doubt, one size smaller.
The rule of thumb: your evening and night consumption
The battery has exactly one task: to carry the midday surplus into the hours without sun. Its sensible size is therefore the electricity you actually consume between sunset and sunrise – no more. Anything above that finds no taker; anything below that the battery simply only partially covers.
For many single-family homes, this evening and night consumption is 8 to 12 kWh – this is the range in which most suitable home batteries end up. Where you lie in this range (or outside it) depends on your profile: a family that cooks, washes and heats in the evening is at the top; an economical household with daytime consumption in a home office is significantly lower.
Added to this is the second side of the calculation, which is easily forgotten: the battery can only charge what the system leaves over. A small system on a half-covered roof simply does not produce enough surplus after deducting daytime consumption to fill a large battery daily – then the roof limits the sensible battery size, not the consumption. Both limits apply simultaneously; the smaller of the two decides.
Measuring instead of guessing: the path to the real number
The good news: you do not have to guess your evening consumption. Three data sources provide it, in descending accuracy:
1. Smart meter data. With the intelligent measuring system, you have a right to your consumption data in quarter-hourly resolution – from this, the consumption can be read directly by time of day. A few typical weeks (summer and winter) are enough for a robust picture. 2. Monitoring the existing PV system. Anyone who already has the system and wants to retrofit the battery is sitting on a treasure trove of data: the app shows production, consumption and feed-in – and thus exactly the surplus that a battery would capture. A year of real data beats any brochure calculation. 3. The rough estimate. Without data, the annual calculation helps: annual consumption divided by 365 gives the daily consumption, of which the largest part falls on evening and night in evening-heavy households. This is rough – but it prevents the worst mistakes in both directions.
In a consultation, by the way, you can recognise serious sizing by exactly this sequence: anyone who asks about your consumption profile first calculates for you. Anyone who derives the battery size from the system size («10 kWp, so 10 kWh») calculates for the catalogue – the two numbers have nothing to do with each other, even if they often turn out similarly by chance.
Usable or gross: Which capacity counts?
There are often two numbers in the brochure – and the wrong one is used for calculation. Batteries are never completely discharged; a part of the gross capacity remains locked as a protective reserve. Decisive for your sizing (and for the price comparison per kilowatt-hour) is the usable capacity. The difference varies depending on the system; the usable capacity should be stated in quotes, otherwise you are comparing apples and oranges.
Anyone planning a backup power or emergency power function calculates additionally: a part of the capacity can be held back as a reserve for a grid failure – this part is then not available for daily shifting. Backup power therefore «costs» not only for the device, but also for the effective size.
Heat pump and electric car: calculate larger – or differently?
Intuition says: large consumers, large battery. Practice is more nuanced – and cheaper:
The heat pump stores its solar energy best itself – as heat in the boiler and in the building. A solar-controlled management system heats the hot water cylinder at midday, thereby relieving precisely the evening hours for which battery capacity would otherwise be necessary. Only then is it worth looking at what electrical evening consumption remains.
The electric car is itself the largest battery in the household – charging it via the home battery would mean filling one battery from another, with losses at every step. More economical is direct charging during the day, wherever everyday life allows it. Those who regularly have to charge at night can consciously plan some battery for this – but should calculate the trade-off, not round up across the board «because of the electric car».
In short: the large consumers enlarge the sensible battery less than the brochure suggests – if the management system is right. The leverage sequence also applies here: first control, then store.
How to recognise a wrong size in operation
Even after the purchase, it is worth taking a look – at the latest during the annual check:
| Observation in monitoring | Diagnosis |
|---|---|
| Battery in summer almost daily full AND empty | Size fits – it is working |
| Summer: full at midday, never empty in the evening | too large for evening consumption |
| Summer: already full in the morning, a lot of feed-in | rather too small – or chosen exactly right economically |
| Winter: barely charged for weeks | normal – no sizing error (why) |
Rules of thumb for typical single-family home systems, As of: July 2026 – short special phases (holidays, extreme weather) distort the picture and do not count.
The last line is important: the half-empty battery in December is physics, not a planning error. The size can only be assessed in the summer half-year – the daily cycle must be visible there.
The battery size is the point where we most frequently correct downwards in consultations – against the first wish of the customers and against some quotes that are already on the table. The pattern: the interested party has planned a large system and wants a large battery «to match»; to the question about evening consumption there is usually only a shrug. When we then look into the consumption data together, the battery regularly shrinks by several kilowatt-hours – and nobody misses them later. Conversely, we have rarely met anyone who regretted their consciously tightly chosen battery: on good days it is full early, the rest is fed in, and the calculation is right. Our short formula for this in the team: the battery should match the evening, not the roof.
Frequently asked questions
Can I expand the battery later?
Depending on the system, yes – many batteries are modular and can be expanded with battery modules, but often only within the same product generation and within a few years. Anyone starting consciously tight clarifies the expandability before buying – then «start small for now» is a strategy instead of a risk.
Does the gross or the usable capacity count?
For sizing and price comparison, always the usable one – it is what the battery can actually shift. If there is only a gross figure in the quote, ask; reputable providers state both values.
What happens if my battery is too small?
Nothing dramatic: it is full earlier on sunny days, and the remaining surplus is fed in and remunerated as usual. You miss out on a part of the possible self-consumption, but you have no dead capital – which is why too small is a significantly milder mistake than too large.
Do I need a larger battery for backup power?
If you want to bridge more than briefly during an outage, yes – the backup power reserve is deducted from the daily use. Determine which consumers should run for how long in an emergency; this results in the reserve that is added to the normal sizing.
Are 20 kWh and more worthwhile in a single-family home?
Rarely – this requires a correspondingly high evening and night consumption and a system that also fills this battery. Such profiles exist (large households with heat pump and night charging), but they are the exception. Anyone who ends up at this size should want to see the derivation with real consumption data.
How much battery storage makes sense?
As much as you actually consume in the evening and at night – this is the electricity that the battery can shift. For most single-family homes, this is 8 to 12 kWh of usable capacity; with a heat pump or electric car that can charge during the day, the battery does not automatically get larger. A battery that is too small works flat out every day, one that is too large stands half empty and costs more per stored kilowatt-hour. Calculate with your evening consumption, not with the system size.
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Sources: EnergieSchweiz (basics of battery storage); empirical values from the planning and consultation practice of ecoEn GmbH, Zurich region.
Last updated: 9 July 2026 · Author: ecoEn editorial team
