Battery Bank Calculator
Size a battery bank for off-grid solar systems.
Calculate battery capacity needed based on daily energy usage and backup days.
Power bank capacity and charging cycles planning ensures you have enough backup power to keep your devices running through travel, outages, or outdoor adventures.
Usable capacity formula:
Power banks advertise capacity in mAh at 3.7V (battery cell voltage). Your devices charge at 5V (USB). Due to voltage conversion and heat losses, real usable capacity is lower:
Usable Capacity (mAh at 5V) = Rated mAh × 3.7 / 5 × Efficiency (0.85–0.90)
Charge cycles per device:
Charge Cycles = Usable Capacity (mAh) / Device Battery Capacity (mAh)
Worked example — 20,000 mAh power bank:
- Usable capacity: 20,000 × 3.7 / 5 × 0.87 = 12,876 mAh effective at 5V
- iPhone 15 (3,349 mAh): 12,876 / 3,349 = 3.8 full charges
- iPad Air (28,650 mAh): 12,876 / 28,650 = 0.45 charges (less than half)
- Galaxy S24 (4,000 mAh): 12,876 / 4,000 = 3.2 full charges
Device power draw reference:
| Device | Typical Capacity |
|---|---|
| Earbuds case | 300–700 mAh |
| Smartwatch | 300–500 mAh |
| Smartphone | 3,000–5,000 mAh |
| Tablet | 7,000–30,000 mAh |
| Laptop | 45,000–100,000 Wh |
Charging time formula:
Charge Time (hours) = Power Bank Capacity (mAh) / Charger Output (mA)
Example: 20,000 mAh bank charged via 25W USB-C (5A @ 5V = 5,000 mA): 20,000 / 5,000 = 4 hours to recharge the bank itself
Travel tip: Most airlines allow power banks up to 100 Wh (≈27,000 mAh at 3.7V) in carry-on luggage. Above 100 Wh requires airline approval; above 160 Wh is prohibited on passenger aircraft.