Terminologies You Should Know
Before diving into calculations, How Many Batteries here are a few important terms you should understand:
🔋 Depth of Discharge (Dod)
This is the percentage of a battery’s total capacity that you can safely use.
- Lead-acid batteries: 50% Dod
- Lithium batteries: Up to 90% Dod
🔋 Battery Efficiency
This shows how much of the stored energy is usable.
- Lead-acid batteries: ~65% efficient
- Lithium batteries: Up to 95% efficient
🔋 Days of Autonomy
The number of days your solar system can run without sunlight (cloudy days or nighttime), based solely on battery power.
Tip: If you’re new to these terms, check out our detailed guide on how to choose the best solar battery in Pakistan.
🔍 How to Calculate Battery Size for Your Solar System
Calculating the battery size depends on your daily energy usage, battery type (lead-acid or lithium), and other technical factors. Here’s how to do it:
✅ Step 1: Determine Your Daily Power Usage
Your first step is to calculate how much electricity you use daily.
Here’s how to do it:
- Take your monthly power usage from your electricity bill (in kWh or units).
- Divide by 30 to get the daily usage.
Example:
If you use 300 kWh per month, then:300 ÷ 30 = 10 kWh per day
✅ Step 2: Choose Your Battery Type
Each battery type affects your total battery bank size.
⚙️ For Lead-Acid Batteries:
- Dod: 50%
- Efficiency Factor: 1.2 (to account for energy loss)
- Formula:
Battery Capacity (kWh) = (Daily Usage × Days of Autonomy × Inefficiency Factor) ÷ DoD
⚙️ For Lithium-Ion Batteries:
- Dod: 80%
- Efficiency Factor: 1.05
- Formula:
Battery Capacity (kWh) = (Daily Usage × Days of Autonomy × Inefficiency Factor) ÷ DoD
🔢 Battery Requirements for Common Solar System Sizes in Pakistan
Here’s a quick overview of how many batteries you need based on system size, assuming 1 day of autonomy:
| System Size | Daily Load (kWh) | Lead-Acid Batteries (12V 150Ah) | Lithium Batteries (48V 100Ah) |
|---|---|---|---|
| 3kW | ~12 kWh/day | 8–10 batteries | 3–4 batteries |
| 5kW | ~20 kWh/day | 12–14 batteries | 4–6 batteries |
| 6kW | ~24 kWh/day | 16 batteries | 5–7 batteries |
| 7kW | ~28 kWh/day | 18–20 batteries | 6–8 batteries |
| 10kW | ~40 kWh/day | 25–28 batteries | 8–10 batteries |
| 15kW | ~60 kWh/day | 36–40 batteries | 12–14 batteries |
| 20kW | ~80 kWh/day | 48+ batteries | 16–18 batteries |
Note: These are rough estimates. Your battery bank may vary based on usage patterns, system efficiency, and the number of backup days needed.
🧮 Example Calculation for a 5kW Solar System (Lead-Acid)
- Daily Usage: 20 kWh
- Days of Autonomy: 1
- Inefficiency Factor: 1.2
- Dod: 50%
Required Capacity:(20 × 1 × 1.2) ÷ 0.5 = 48 kWh
Now divide by the capacity of one lead-acid battery (e.g., 12v 150Ah = 1.8 kWh):48 ÷ 1.8 ≈ 27 batteries
You may reduce this by using higher-capacity or lithium batteries.
🏡 Final Thoughts: Choosing the Right Battery for Your Solar Needs
- Lead-acid batteries are cheaper but require maintenance and more space.
- Lithium-ion batteries are more efficient, compact, and have a longer lifespan — ideal for urban homes.
Always consult a qualified solar installer in Pakistan to size your battery bank accurately. If you’re unsure, Ecospark Solar can help you pick the best battery solution tailored to your budget and energy needs.
