Step 1 – Know Your Night‑Time Energy Need
Before you pick any battery, you have to answer one simple question: “How many kilowatt‑hours (kWh) will actually be drawn after the sun goes down?” Write down every device that runs at night, its power draw in watts, and how many hours it stays on. Multiply watts × hours to get watt‑hours (Wh). Add the numbers together. This is the baseline night‑time demand you must cover.
| Typical night‑time appliance | Average power (W) | Hours per night | Energy per night (Wh) |
|---|---|---|---|
| LED ceiling light (bedroom) | 12 | 8 | 96 |
| Bedside lamp | 5 | 7 | 35 |
| Wi‑Fi router | 10 | 10 | 100 |
| Refrigerator (mini‑bar) | 45 | 12 | 540 |
| Laptop + charger | 60 | 4 | 240 |
| Phone charging hub | 15 | 6 | 90 |
For the example above the total night‑time demand is 1,101 Wh (≈ 1.1 kWh). If your actual load is different, adjust the numbers accordingly.
Step 2 – Decide How Many Nights of Autonomy You Want
Do you need the battery to last one cloudy evening or three full days? The industry term is autonomy days (D). Most residential balcony‑solar setups aim for 1–2 days of night‑time cover. Choose D that matches your local weather patterns and how much you can tolerate a power cut.
- One night only (D = 1) – cheapest, good for regions with frequent sunshine.
- Two nights (D = 2) – a safe middle ground for most European climates.
- Three nights (D = 3) – useful for high‑latitude locations where cloudy spells can linger.
Step 3 – Account for System Voltage and Depth‑of‑Discharge (DoD)
Battery capacity is usually quoted in amp‑hours (Ah) at a certain voltage (e.g., 12 V, 24 V, 48 V). To translate the required kWh into Ah you use the simple formula:
Capacity (Ah) = (Night‑time demand (kWh) × 1,000) / System Voltage (V) × DoD factor
- DoD – the percentage of the battery you are allowed to discharge without damaging it. For lithium‑ion packs, a 80 % DoD is common; for lead‑acid it’s usually 50 %.
Insert the numbers: if you need 1.1 kWh, have a 24 V system, and use an 80 % DoD lithium pack:
Ah = (1.1 × 1000) / 24 × (1 / 0.80) ≈ 57.3 Ah
If you prefer a 12 V pack with the same DoD, the result is about 114.6 Ah. The higher the system voltage, the lower the current and the thinner the cable you need.
Step 4 – Add a Safety Margin and Round Up
Manufacturers usually list nominal capacity, not usable capacity. Temperature, aging, and voltage drops can shave off 5–10 % of usable energy. Add a 10 % buffer on top of the calculated Ah. Then round to the nearest standard battery size (e.g., 60 Ah, 100 Ah, 150 Ah).
- Calculated needed Ah (without buffer) = 57.3 Ah
- Add 10 %: 57.3 × 1.10 = 63.03 Ah
- Round to the next common size → 65 Ah (or a 60 Ah pack if you are comfortable with a tighter margin)
Step 5 – Real‑World Validation with a Balcony‑Solar Example
Imagine you have a 400 W balcony solar panel, a 24 V lithium battery, and you want to keep lights, Wi‑Fi, a mini‑fridge, and a laptop running for two cloudy nights.
- Night‑time demand from the table = 1,101 Wh
- Autonomy = 2 nights → total energy = 1,101 × 2 = 2,202 Wh
- DoD = 80 % → required capacity = 2,202 / 0.80 = 2,752.5 Wh
- Convert to Ah at 24 V: 2,752.5 / 24 ≈ 114.7 Ah
- Add 10 % safety margin → 114.7 × 1.10 ≈ 126.2 Ah
- Select a 130 Ah 24 V lithium pack (often sold as a speicher für balkonkraftwerk) that gives you ~2.3 kWh usable energy under normal conditions.
“Always check the battery’s datasheet for the rated cycle life at the chosen DoD. A 80 % DoD on a high‑quality LiFePO4 cell can exceed 3,000 full cycles, while a lead‑acid pack may drop below 500 cycles at 50 % DoD.” – Solar Energy Industry Association, 2023 Best‑Practice Guide
Key Variables to Watch
- Temperature – Cold reduces capacity; a battery rated at 25 °C may lose 5–10 % at 0 °C.
- Load Profile – Intermittent high‑draw devices (e.g., microwave) need higher surge capacity from the inverter; this does not change the energy need but may require a battery that can deliver higher discharge currents.
- Charging Efficiency – Most lithium packs are 95 % efficient; lead‑acid around 85 %. Include this factor when you calculate how much solar energy you need to replenish the night‑time draw.
Tools & Calculators You Can Use
- Online solar‑battery calculators (e.g., PV‑Lib, SolarEdge sizing tool) – input load, autonomy, voltage.
- Spreadsheet template that you fill with your own device list; automatically sum Wh, apply DoD and safety margin.
- Manufacturer‑provided sizing worksheets for the specific brand you intend to buy.
Common Mistakes and How to Avoid Them
- Ignoring surge currents – A 150 W fridge can draw 600 W on startup. Choose a battery with a discharge rating at least 1.5× the peak load.
- Using only the rated Ah without DoD – A 100 Ah 12 V lead‑acid actually delivers 50 Ah usable (50 % DoD). You must factor the usable portion.
- Skipping the safety margin – Real‑world efficiency is never 100 %; a 10 % buffer is the minimum recommended.
- Over‑sizing for a tiny system – If you only need 0.5 kWh a night, a 10 kWh pack is wasteful, heavy, and costly.