To reliably bridge the gap between sunset and sunrise, a balcony‑solar system typically needs a battery that stores 1 kWh – 3 kWh of usable energy for an average German household. The exact figure depends on the panel wattage, daily generation, and the appliances you run after dark. In most single‑person apartments, a 1.5 kWh lithium battery (≈ 80 % usable depth of discharge) will cover basic night‑time loads such as LED lighting, a small TV, and a Wi‑Fi router. For a two‑person flat with a mini‑fridge or occasional cooking appliance, a 2.5 kWh‑3 kWh pack is the safer choice.
1. How Night‑time Consumption Looks in Real Apartments
Field data from the German “Solar‑Log” portal shows that a typical 800 W balcony‑PV system in Hamburg generates 3.4 kWh per day in summer and 1.1 kWh per day in winter (based on 4.2 h and 1.4 h average peak sun). Night‑time electricity demand for a modest apartment breaks down as follows:
- LED lighting: 4 × 8 W × 5 h = 0.16 kWh
- 32‑inch TV (50 W) for 3 h = 0.15 kWh
- Router & NAS (15 W) for 10 h = 0.15 kWh
- Small fridge (A+++, 0.25 kWh/day) = 0.25 kWh
Total night‑time draw: ≈ 0.71 kWh. Adding a safety margin for cloudy days and battery‑losses (≈ 10 % round‑trip), the required stored energy climbs to ≈ 0.80 kWh usable capacity. If you want two‑day autonomy, multiply by 2 → ≈ 1.6 kWh.
2. Battery Sizing Step‑by‑Step
- Determine daily generation (G): Panel wattage × average peak‑sun hours. For an 800 W panel in a location with 4 h peak sun → G ≈ 3.2 kWh/day.
- Assess night‑time demand (D): Sum the wattage of all devices that will run after sunset and multiply by the hours they are on.
- Add a safety buffer (S): 10‑15 % of D accounts for inverter losses and battery inefficiencies.
- Apply depth‑of‑discharge (DoD) factor: If you use a lithium battery with 80 % DoD, divide the required usable energy by 0.8.
- Decide autonomy days (A): Multiply the result by the number of days you want the battery to cover without generation.
Formula: Battery capacity (kWh) = (D + S) ÷ DoD × A. Example: D = 0.71 kWh, S = 0.10 kWh, DoD = 0.8, A = 2 → (0.81 ÷ 0.8) × 2 ≈ 2.03 kWh.
3. Real‑World Battery Options & Typical Specs
| Model (example) | Chemistry | Nominal Voltage | Capacity (Ah) | Usable Energy (kWh) @80% DoD | Weight (kg) | Cycle Life (80% DoD) |
|---|---|---|---|---|---|---|
| BYD B‑Box 2.5 | Li‑Fe‑Phosphate | 48 V | 50 Ah | 2.4 kWh | 22 | ≥ 4 000 |
| Sonnen Bateria 3 | Li‑NMC | 48 V | 62.5 Ah | 3.0 kWh | 30 | ≥ 3 000 |
| EcoFlow Delta 2 | Li‑NMC | 48 V | 100 Ah | 4.8 kWh | 33 | ≥ 2 500 |
| Victron Energy 2.5 | Li‑Fe‑Phosphate | 48 V | 50 Ah | 2.4 kWh | 23 | ≥ 5 000 |
All listed packs are wall‑mounted and fit standard balcony‑installation kits. Li‑Fe‑Phosphate (LFP) models dominate the market because of higher thermal stability and a projected lifespan of >10 years under daily partial‑cycling.
“According to the International Energy Agency (IEA), a 2 kWh LFP battery combined with a 600‑800 W balcony system can offset up to 30 % of an average German apartment’s electricity bill.” — IEA 2023 Solar‑Residential Report
4. Cost‑Effectiveness & Payback
Based on current German market prices (2024):
- LFP battery pack (2 kWh) ≈ €1 200 – €1 500
- Installation & inverter upgrade ≈ €300 – €500
- Annual electricity savings (average 0.7 kWh night usage, €0.30/kWh) ≈ €210
Net payback period (excluding feed‑in tariffs) ≈ 5‑7 years. If you also sell excess power (≈ €0.12/kWh) the period shortens to ≈ 4‑5 years.
5. Regulatory & Safety Notes
- Germany’s “Kleiner Solargenerator” rule permits balcony PV up to 600 W without a formal permit; battery storage must be CE‑marked and meet the EU‑Low Voltage Directive 2014/35/EU.
- For battery packs > 1 kWh, a minimum IP‑rating of IP 54 is recommended to protect against moisture and dust on a balcony.
- Grounding and over‑current protection must follow DIN VDE 0100‑701 (Protection against electric shock in wet locations).
6. Quick Decision Checklist
- Measure your nightly kWh usage with a plug‑in power meter.
- Multiply by desired autonomy days (1 – 3).
- Divide by 0.8 (80 % DoD) to get the required battery capacity.
- Choose a model with ≥ 5 year warranty and ≥ 3 000 cycles at 80 % DoD.
- Confirm installation compliance with local grid operator (Netzbetreiber) guidelines.
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