The short answer is that most German households of two to three people get the sweet spot with a 1.8–2.5 kW photovoltaic (PV) array paired with a 2–4 kWh lithium‑ion battery. That combination usually covers 70–85 % of the yearly electricity demand, keeps the payback period under six years, and still leaves enough headroom for a weekend of cloudy weather.
Why that range? Let’s walk through the numbers and practical considerations that drive the decision.
1. How much electricity do you actually use?
Before picking a system size you need a realistic consumption profile. The table below shows typical annual electricity usage for different household sizes in Germany, together with the average daily demand in kWh.
| Household size | Annual consumption (kWh) | Daily average (kWh) | Typical peak load (W) |
|---|---|---|---|
| 1 person | 1,400–1,700 | 3.8–4.7 | 1,200–1,500 |
| 2 persons | 2,400–2,800 | 6.6–7.7 | 2,000–2,400 |
| 3 persons | 3,300–3,800 | 9.0–10.4 | 2,800–3,200 |
| 4 persons | 4,200–4,800 | 11.5–13.2 | 3,600–4,000 |
These numbers assume a mix of lighting, refrigeration, washing machine, dishwasher, and a few hours of TV/computer use per day. If you run an electric car or an electric heating system, the demand jumps significantly and you’ll need a larger PV array or a second battery string.
2. How much roof or balcony space can you spare?
A typical monocrystalline 400 W panel measures about 1.7 m². Rough guidelines for different orientations in central Europe are:
- South‑facing (≈ 180 °): 1 kW per 10 m² of usable area.
- East‑ or West‑facing (≈ 90° or 270°): 1 kW per 13 m² because of lower sun angles.
- Shaded or flat roof: up to 30 % capacity loss, meaning you may need 15–20 % more modules to hit the same output.
The table below translates roof area into PV capacity for a south‑facing installation.
| Usable area (m²) | Typical PV capacity (kW) | Number of 400 W panels |
|---|---|---|
| 12 | 1.2 | 3 |
| 20 | 2.0 | 5 |
| 30 | 3.0 | 7–8 |
| 40 | 4.0 | 10 |
For a “Balkonkraftwerk” you’re often limited to the balcony railing or a small flat‑roof footprint, so a 1.5–2.5 kW system is the realistic ceiling for most urban apartments.
3. Battery capacity – how many hours of backup?
A battery’s usefulness is measured in depth‑of‑discharge (DoD) and the number of cycles it can endure. Modern lithium‑ion (LFP) packs deliver roughly 4,000 cycles at 80 % DoD before falling below 80 % of original capacity. Here’s a quick guide to how long a given battery will keep essential loads running:
| Battery size (kWh) | Usable capacity (80 % DoD) | Estimated night‑time backup (hours) for average base load of 250 W | Estimated backup for a 500 W base load |
|---|---|---|---|
| 1.5 | 1.2 | 4.8 | 2.4 |
| 2.0 | 1.6 | 6.4 | 3.2 |
| 3.0 | 2.4 | 9.6 | 4.8 |
| 4.0 | 3.2 | 12.8 | 6.4 |
For a two‑person apartment that uses about 250 W during the night (lights, fridge, standby), a 2 kWh pack covers roughly a full night. If you