Quick Answer: Size your home battery to cover your nighttime electricity use — roughly 2–4 kWh for an average European household. The simplest calculation is: look at your evening and overnight electricity consumption (6 PM to 8 AM), then add 20% margin. A 5 kWh battery works for most small households; 10 kWh covers larger homes with electric heating or cooking.
The calculation in three steps
Step 1: Read your overnight consumption. Check your last utility bill or smart meter for nighttime use (roughly 18:00 to 08:00). In a typical 3-bedroom home, that might be 3–5 kWh. If you use electricity for heating or cooking, it can reach 6–10 kWh.
Step 2: Apply usable capacity. Most home batteries use 90% depth of discharge. So a 5 kWh battery actually gives you about 4.5 kWh overnight.
Step 3: Add 20% margin. Account for cloudy days, extra guest nights, and future electric loads.
Worked example:
- Nighttime consumption: 4 kWh
- Divide by 0.90 usable: 4 ÷ 0.90 = 4.4 kWh
- Add 20% margin: 4.4 × 1.2 = 5.3 kWh
A standard 5 kWh wall-mounted battery (like the Dawnice HZEB-LCT-5) covers this household comfortably.
Matching battery sizes to household type
| Household | Typical nighttime use | Recommended battery |
|---|---|---|
| 1–2 people, small apartment | 2–3 kWh | 5 kWh |
| 3–4 people, average home | 4–6 kWh | 10 kWh |
| Large home, electric heating/cooking | 6–10 kWh | 10–15 kWh (stacked) |
| Off-grid or near-off-grid | 10+ kWh | 15–20 kWh stackable |
Why not just buy the biggest battery?
Bigger is not always better. Three reasons to size carefully:
- Cost per kWh drops with size, but unnecessary capacity adds up. A 15 kWh battery costs €6,000–€9,000. If you only use 4 kWh overnight, the extra 11 kWh sits idle most nights.
- Solar charging limits your useful storage. Your rooftop solar array can only add so much during the day. A 10 kWh battery with a 4 kW solar array may not fully charge on winter days, making the extra capacity wasted.
- Cycle life. A battery that charges and discharges shallowly lasts longer. Over-sizing leads to shallower cycling but lower utilization.
The solar array connection
The battery size also depends on your solar array. Roughly:
- 3 kW solar array → 3–5 kWh battery
- 5 kW solar array → 5–10 kWh battery
- 8 kW solar array → 10–15 kWh battery
If your solar array is too small to fully charge the battery in a day, the battery will run partly on grid power overnight — defeating the self-consumption goal.
A note on cloudy days
One sunny day charges the battery. But what about three cloudy days in a row? If you want full energy independence for extended cloudy periods, you need either:
- A larger battery (15 kWh+)
- A backup generator
- Grid import as a safety net
Most homeowners are comfortable with a battery that covers one night of self-consumption and falls back to the grid on extended cloudy spells.
Don't forget round-trip efficiency
A 92% efficient battery loses 8% every charge-discharge cycle. A 10 kWh battery delivers about 9.2 kWh after accounting for losses. Factor this into your calculation — it is already included in the 90% usable DoD estimate above.
What about daytime self-consumption?
This calculation covers overnight use only. If you also want to avoid buying grid electricity during the day (when you are at home), you need more battery or a larger solar array. For most homes, overnight self-consumption is the main economic benefit, because daytime solar is already self-consumed while you use appliances.
Ruibit recommends the HZEB-LCT-5 for small households and the HZEB-LCT-10 for average families, both wall-mounted units that pair with standard hybrid inverters. If you send us your monthly electricity bill, we can size the battery more precisely than this general guide.
FAQs
Q: Can I use a 5 kWh battery for a family of four? It depends on your nighttime use. If your household uses 4–5 kWh overnight, yes. If you use 7 kWh or more (electric heating, cooking), you need 10 kWh.
Q: What happens if my battery is undersized? The battery runs out before morning and the home draws grid power for the last 1–2 hours. It is not a safety issue, but it reduces the savings you expected.
Q: How long does a 10 kWh battery last overnight? It depends on your load. For a typical family using 5 kWh overnight, a 10 kWh battery covers two nights without charging. For one night, it delivers about 9 kWh after efficiency losses.
Q: Do I need to match the battery size to my solar inverter? Yes. Check that your inverter's battery input voltage and current rating match the battery. Most 4–6 kW hybrid inverters work with 5–10 kWh wall batteries; larger systems need a higher-current inverter.
Q: Is it worth buying a bigger battery for future electric heating? If you plan to switch to a heat pump or electric car in the next 3–5 years, buying 10 kWh now instead of 5 kWh saves the cost of a second installation. If you have no plans to electrify further, 5 kWh is sufficient.
