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What size battery bank do I need?
Panels make power during the day; the battery bank is what runs your fridge at 2 a.m. and carries you through a cloudy stretch. Its size depends on how much energy you use per day, how many no-sun days you want to survive, and — critically — what fraction of the battery you can actually use.
Start with usable capacity, not rated capacity
A battery's label capacity is not all usable. Draining a battery fully shortens its life, so you only use a fraction — the depth of discharge (DoD). This is the single biggest difference between chemistries:
| Chemistry | Safe DoD | Notes |
|---|---|---|
| LiFePO4 (lithium) | 80–100% | Light, long cycle life, costs more upfront |
| AGM (sealed lead) | 50% | Cheaper, heavier; deeper draws kill it fast |
| Flooded lead-acid | 50% | Cheapest, needs ventilation + maintenance |
The practical takeaway: a 100 Ah AGM battery gives you ~50 Ah; a 100 Ah LiFePO4 gives you 80–100 Ah. For the same usable energy, lead-acid needs roughly double the rated capacity.
The sizing formula
Convert your daily load to amp-hours at your system voltage, multiply by the days of autonomy you want, then divide by usable DoD:
Battery Ah = (Daily Wh ÷ System V) × Days of autonomy ÷ DoD
Worked example — 660 Wh/day on a 12V system, 2 days of autonomy:
| Chemistry | Math | Rated Ah needed |
|---|---|---|
| LiFePO4 (0.9 DoD) | (660 ÷ 12) × 2 ÷ 0.9 | ~122 Ah |
| AGM (0.5 DoD) | (660 ÷ 12) × 2 ÷ 0.5 | ~220 Ah |
Days of autonomy is how long the bank runs with no solar input. One day is fine for fair-weather weekend use; 2–3 days is sensible for full-time off-grid where a cloudy spell shouldn't leave you dark.
Lead-acid loses usable capacity in the cold, and any 120V AC loads add ~10–15% inverter loss on top. Add margin rather than sizing to the exact minimum.
Watt does this for your exact build
Enter your devices, voltage, battery chemistry, and desired autonomy — Watt returns the rated bank size, the matching solar array, and charge-controller and wire-gauge picks, with every assumption shown. For van, RV, and off-grid systems. Free to download.
Sources
- Battery manufacturer cycle-life vs. depth-of-discharge data (LiFePO4 and lead-acid)
- Standard off-grid system-design practice (autonomy and derating factors)
Cycle life and safe DoD vary by manufacturer — follow your battery's datasheet for its rated limits.