How to Size a Battery Bank for Off-Grid Solar
- Formula: (Daily load × days of autonomy) ÷ (system voltage × depth of discharge) = Ah needed.
- LiFePO4 batteries can discharge to 80–90% DoD. AGM should stay above 50% DoD.
- Most off-grid systems are designed for 2–3 days of autonomy without solar input.
- Always size in usable capacity, not total capacity — a 100Ah AGM only gives you 50Ah usably.
- LiFePO4 is almost always the better long-term choice despite higher upfront cost.
The formula
Ah = (Daily load Wh × Days autonomy) ÷ (System voltage × Depth of discharge)
Example: 2,000 Wh/day × 2 days ÷ (24V × 0.8 DoD) = 208 Ah at 24V for a LiFePO4 bank.
Battery bank calculator
Depth of discharge explained
Depth of discharge (DoD) is the percentage of a battery's capacity you can safely use before recharging. Using a battery beyond its recommended DoD dramatically shortens its life.
| Battery type | Recommended DoD | Usable from 100Ah bank | Cycle life at recommended DoD |
|---|---|---|---|
| LiFePO4 | 80% | 80Ah | 3,000–6,000 cycles |
| AGM | 50% | 50Ah | 500–800 cycles |
| Flooded lead-acid | 40–50% | 40–50Ah | 1,000–1,500 cycles |
| Gel | 50% | 50Ah | 500–1,000 cycles |
Days of autonomy: how many do you need?
| Situation | Recommended autonomy | Reasoning |
|---|---|---|
| Sunny climate (Southwest US) | 1–2 days | Rare cloudy periods; panels recharge quickly |
| Moderate climate (national average) | 2–3 days | Occasional 2–3 day cloudy spells common |
| Cloudy/rainy climate (Pacific NW) | 3–4 days | Extended low-sun periods in winter |
| Critical loads / no backup | 3–5 days | Medical equipment, remote cabin, no generator |
LiFePO4 vs AGM: the real cost comparison
AGM batteries are cheaper upfront but need replacing every 3–5 years. LiFePO4 costs more but lasts 10–15 years. Over a 15-year system life:
| LiFePO4 (once) | AGM (3× replacements) | |
|---|---|---|
| 10kWh bank purchase cost | $5,500 | $2,000 × 3 = $6,000 |
| Weight | ~100 kg | ~270 kg |
| Usable capacity | 8kWh (80% DoD) | 5kWh (50% DoD) |
| Cycle life | 4,000+ cycles | 600 cycles each |
| 15-year total cost | $5,500 | $6,000+ |
See our best LiFePO4 battery guide and full chemistry comparison for more detail.
Wiring batteries in series vs parallel
- Series: Connects + to − between batteries. Doubles voltage, keeps Ah the same. Use to reach your system voltage (e.g. two 12V 100Ah → 24V 100Ah).
- Parallel: Connects + to + and − to −. Doubles Ah, keeps voltage the same. Use to increase capacity (e.g. two 12V 100Ah → 12V 200Ah).
- Series-parallel: Combination for larger banks. Always use identical batteries (same brand, age, capacity).
Frequently asked questions
Can I mix different battery brands or capacities?
No. Always use identical batteries in a bank — same brand, same model, same age, same capacity. Mixing batteries causes imbalanced charging, accelerated degradation and premature failure of the entire bank.
How do I know when my battery bank is full?
A battery monitor (like the Victron BMV-712) tracks state of charge accurately. Voltage alone is unreliable, especially under load. A monitor is a worthwhile investment for any system over $1,000.
What's the minimum battery bank worth buying?
For a functional off-grid system, aim for at least 1 day of autonomy. Smaller banks are possible but you'll run out of power on any cloudy day and constantly stress the batteries with deep discharges.