Key takeaways
- Bank energy = daily load × autonomy days ÷ depth of discharge.
- Amp-hours = bank watt-hours ÷ system voltage — so 48 V needs ¼ the Ah of 12 V.
- Use ~50% DoD for lead-acid, 80–100% for LiFePO₄.
- 3,000 Wh/day × 2 days at 50% DoD = a 12 kWh (≈500 Ah at 24 V) bank.
How to size a battery bank
A battery bank is sized from the energy it must store, not just a single number. Start with your daily energy use in watt-hours, decide how many days it should run without charging (days of autonomy), then divide by how deeply you can safely discharge the chemistry. Finally convert that energy to amp-hours at your system voltage.
Depth of discharge is the lever most people miss: pulling a lead-acid bank below ~50% shortens its life, so you size it twice as big as the usable energy. LiFePO₄ tolerates 80–100%, so the same job needs a smaller bank.
Worked example: 3,000 Wh/day, 2 days, 24 V
Usable energy = 3,000 × 2 = 6,000 Wh. At 50% DoD, battery energy = 6,000 ÷ 0.5 = 12,000 Wh (12 kWh). At a 24 V bus that's 12,000 ÷ 24 = 500 Ah — five 100 Ah batteries. Switch to LiFePO₄ at 80% DoD and you'd need only 7,500 Wh.
Depth of discharge by chemistry
| Battery chemistry | Recommended DoD | Note |
|---|---|---|
| Flooded lead-acid | ~50% | Deeper cycling shortens life |
| AGM / gel | ~50% | Similar to flooded |
| LiFePO₄ (lithium) | 80–100% | Most usable energy per Ah |
Start with the load, then check the voltage
Every sizing job starts from an honest daily load — total your appliances with the off-grid load calculator first. Then make sure your array can refill the bank each day with the solar array sizing calculator. Choosing a higher system voltage (24 or 48 V) cuts the amp-hours and lets you use thinner, cheaper cable.
Frequently asked questions
How do I size a battery bank for off-grid?
Daily Wh × autonomy days ÷ DoD = battery Wh; ÷ system voltage = amp-hours. 3,000 Wh/day, 2 days, 50% DoD, 24 V ≈ 500 Ah.
How many batteries do I need?
Divide required Ah by one battery's Ah rating and round up — a 500 Ah bank ≈ five 100 Ah batteries.
What depth of discharge should I use?
~50% for flooded/AGM lead-acid, 80–100% for LiFePO₄. Lower DoD means a bigger bank for the same usable energy.
Why does system voltage change the amp-hours?
Ah = Wh ÷ V. The same 12 kWh bank is 1,000 Ah at 12 V, 500 Ah at 24 V, or 250 Ah at 48 V.
What are days of autonomy?
How long the bank runs your loads with no charging — through clouds or an outage. 1–2 days is common; critical systems use 3+.
How big a bank for 3,000 Wh/day?
2 days at 50% DoD = 12,000 Wh ≈ 500 Ah at 24 V; LiFePO₄ at 80% DoD needs only 7,500 Wh.
Depth-of-discharge and usable-capacity guidance follows lithium and lead-acid manufacturer data — see Battle Born Batteries. The Wh, Ah, and DoD relationships here are exact arithmetic.
Last reviewed June 2026