Step‑by‑Step Calculation
The first thing you need to answer is how many kilowatt‑hours you actually store to cover your consumption when the sun isn’t shining. The quick rule of thumb is:
Usable storage (kWh) = (Daily consumption kWh ÷ System efficiency) ÷ Depth‑of‑discharge (DoD) %
Once you have that number, you convert it to the battery voltage & ampere‑hour rating you must purchase.
1. Gather Your Daily Energy Use
Start with the appliances you run in the apartment or house that will be powered by the Balkonkraftwerk. Typical values for a one‑person flat in Germany are:
- Lighting: 0.3 kWh/day
- Refrigerator (A++): 0.7 kWh/day
- Laptop & phone charging: 0.5 kWh/day
- Microwave, kettle, occasional cooking: 0.8 kWh/day
- Entertainment (TV, router, etc.): 0.4 kWh/day
Add any other significant loads such as a washing machine (≈ 0.5 kWh per wash) or a small air‑conditioner (≈ 1 kWh per hour of use). Write the total in kWh per day. In the example below we’ll use 5 kWh/day.
2. Estimate Your Solar Yield
A 600 W (0.6 kWp) Balkonkraftwerk in central Europe typically delivers 3.5–4.5 kWh per kWp per day. For a 0.6 kWp unit:
- Conservative estimate: 0.6 kWp × 3.5 kWh/kWp = 2.1 kWh/day
- Optimistic estimate: 0.6 kWp × 4.5 kWh/kWp = 2.7 kWh/day
Because the balcony orientation, shading, and seasonal tilt affect output, it’s safest to use the lower figure for sizing storage.
3. Choose Desired Autonomy
Do you want a full day of backup or just a few hours to bridge a cloudy afternoon? Most owners target one full day of autonomy (24 h) and some add a second day for peace of mind. Let’s assume you want 1.5 days as a practical middle ground.
4. Determine Required Battery Capacity
Using the numbers from steps 1‑3, the raw energy that must be stored is:
5 kWh × 1.5 days = 7.5 kWh
Now apply system losses. An inverter typically runs at 92–96 % efficiency, so let’s use 94 % (0.94). Also, battery DoD varies by chemistry:
- LiFePO₄ (Lithium‑Iron‑Phosphate): usable DoD ≈ 80 %
- Lead‑acid (AGM/gel): usable DoD ≈ 50 %
Plug those into the formula:
Usable storage = (Daily consumption ÷ Inverter efficiency) ÷ DoD % = (5 kWh ÷ 0.94) ÷ 0.80 = 6.64 kWh (LiFePO₄)
For a lead‑acid pack, you’d need (5 kWh ÷ 0.94) ÷ 0.50 = 10.64 kWh, which quickly becomes bulky and costly.
5. Convert kWh to Ampere‑Hours at Your Voltage
Most Balkonkraftwerk battery packs run at 12 V, 24 V, or 48 V. Using the LiFePO₄ result of 6.64 kWh:
| Voltage | Required Ah (to get 6.64 kWh) | Typical module size (Ah) | Number of modules needed |
|---|---|---|---|
| 12 V | 6.64 kWh ÷ 12 V = 553 Ah | 100 Ah | 6 × 100 Ah (parallel) |
| 24 V | 6.64 kWh ÷ 24 V = 277 Ah | 100 Ah | 3 × 100 Ah (2S2P) |
| 48 V | 6.64 kWh ÷ 48 V = 138 Ah | 100 Ah | 2 × 100 Ah (2S1P) |
The 48 V option is the most efficient for wiring and reduces current, meaning thinner cables and lower losses over long runs to the inverter.
Example Calculation: A Typical Berlin Apartment
Suppose you have a south‑facing balcony, a 600 W panel, a daily consumption of 5 kWh, and you desire 1.5 days of autonomy.
- Raw storage needed: 7.5 kWh
- System efficiency factor: 0.94
- LiFePO₄ usable DoD: 80 %
- Usable capacity required: 6.64 kWh
- Select a 24 V 100 Ah battery module (2.4 kWh per module)
With two such modules in series you get 48 V × 100 Ah = 4.8 kWh, still short of 6.64 kWh. Adding a third module (parallel) raises capacity to 7.2 kWh, which comfortably exceeds the target.
Result: 3 × 24 V 100 Ah LiFePO₄ modules (≈ 7.2 kWh) will satisfy the 5 kWh daily need for 1.5 days.
Battery Chemistry and Voltage Considerations
LiFePO₄ dominates the market for Balkonkraftwerk storage because of:
- High cycle life: 3 000–5 000 cycles at 80 % DoD
- Excellent thermal stability (no fire risk unlike NMC)
- Low self‑discharge rate: ≤ 3 % per month
For a lead‑acid solution you’d need roughly twice the capacity (because of the 50 % DoD limit) and you’d also suffer a shorter lifespan (≈ 500 cycles). The extra weight and space often make lead‑acid impractical for balcony installations.
When choosing voltage, remember that higher voltages reduce the current for the same power, meaning:
- Less heat loss in wiring
- Thinner cable gauge needed (cost saving)
- Better compatibility with modern hybrid inverters that accept 48 V input
Key Factors That Influence Real‑World Capacity
Temperature: LiFePO₄ capacity drops about 2 % per °C below 20 °C. In a cold Berlin winter, a 10 % loss can be expected if the battery is stored outdoors.
Age: After 10 years, LiFePO₄ typically retains 80 % of its original capacity. This should be factored into long‑term planning.
Inverter efficiency curve: Most inverters are most efficient between 20 %–80 % load. Running a small battery at very low charge can waste energy.
Charging limits: A 600 W panel can only deliver about 25 A at 24 V. If the battery bank is too large, the panel may never fully charge on cloudy days, reducing effective usable capacity.
Budget and Space Constraints
Typical costs for LiFePO₄ modules in the 100 Ah range:
- 12 V 100 Ah: €350–€450 per module
- 24 V 100 Ah (2 × 12 V in series): €650–€850 per set
- 48 V 100 Ah (4 × 12 V in series): €1 200–€1 400 per set
If budget is tight, a 24 V system often provides the best cost‑to‑performance ratio for a 600 W Balkonkraftwerk. Keep in