The right way to size a solar battery bank
Battery sizing is the step where most home solar projects either lock in real cost savings or silently over-spend for years. The math is not hard — but the gotchas around depth of discharge (DoD), round-trip efficiency, battery chemistry, and parallel-string imbalance are where buyers get hurt. This calculator gives you a conservative, production-ready sizing plan in seconds.
The core equation is: Total Ah = (Daily kWh × days of autonomy × 1000) ÷ (System voltage × DoD × round-trip efficiency). For a 5 kWh daily load, 1 day autonomy, 48V lithium bank (DoD 0.9, η 0.95): 5000 ÷ (48 × 0.9 × 0.95) ≈ 122 Ah. On a 100 Ah per battery preset, that rounds up to 2 × 100Ah lithium batteries in a 48V 200Ah bank. Straightforward — until you add lead-acid or tubular, where DoD drops to 50-60% and your bank doubles in size for the same usable energy.
Chemistry cheat-sheet (2026 prices)
| Chemistry | DoD | Cycles | ₹ / Usable Wh | Best For |
|---|---|---|---|---|
| LiFePO₄ (Lithium) | 90% | 3,000-6,000 | ₹12-18 | Long-life, daily cycling |
| Tubular | 60% | 1,500-2,000 | ₹7-10 | Budget UPS cycling |
| Lead-Acid (Flooded) | 50% | 500-800 | ₹6-9 | Occasional backup only |
Once you've picked the bank, verify the rest of the off-grid chain: the inverter size matches the bank voltage, the backup hours align with your real load, and the DC cable size handles the full discharge current without voltage drop.
Series-parallel wiring basics
Batteries connect in series to multiply voltage (4 × 12V batteries = 48V bank) and in parallel to multiply capacity (2 strings of 100 Ah = 200 Ah bank). Our calculator shows the exact combination. A few rules:
- Use identical batteries — same model, same age, same manufacture date.
- Each parallel string gets its own fuse and balanced-length busbar runs.
- Don't exceed 3-4 parallel strings without a monitor; imbalance shortens the bank's life.
- Add a battery management system (BMS) for lithium — cell balancing and over-discharge protection are critical.
- Keep batteries in a ventilated, temperature-controlled area (20-30°C is ideal).