Battery Bank Sizing Calculator: Off-Grid Guide (2026)
The short answer: size usable kilowatt-hours first, then convert to amp-hours at your voltage, then derate for chemistry. A 10 kWh/day home often tests 12–20 kWh usable (250–420 Ah at 48 V) for Tier B coverage. Educational estimates only — verify with a licensed professional.
Usable vs nominal: DoD decides what you get
- LFP lithium: plan ~90% usable.
- Sodium-ion: similar usable window.
- Lead-acid / AGM: plan ~50% usable.
Nominal needed = usable ÷ DoD. Example: 15 kWh usable needs 16–18 kWh nominal LFP, but 28–32 kWh nominal lead-acid.
Series vs parallel math
- Series adds voltage, Ah unchanged.
- Parallel adds amp-hours, voltage unchanged.
- Energy: kWh = Volts × Ah ÷ 1000.
Example: four 12 V 280–320 Ah LFP in series = 48 V at 280–320 Ah (13–15 kWh nominal, 12–14 kWh usable). Two such strings in parallel = 48 V at 560–640 Ah (26–30 kWh nominal).
12 V vs 24 V vs 48 V
- 12 V: cabins 0.5–1.5 kWh/day, peaks under 1 kW.
- 24 V: 1–4 kWh/day, peaks 1–2 kW.
- 48 V: homes above 3–4 kWh/day or peaks above 2 kW. Lower current, easier expansion.
Current check: 3,000 W ÷ 12 V = ~250 A. 3,000 W ÷ 48 V = ~60–65 A. That division is why nearly every 5–10 kWh/day home ends up 48 V.
Inverter class comes from peak kW
- Peak 0–2 kW: test 2–3 kW inverter class.
- Peak 2–4 kW: test 4–6 kW class.
- Peak 4–8 kW: test 8–10 kW class.
- Motors: allow 2–3x surge for seconds.
Worked 10 kWh/day example at 48 V
- Daily load: ~10 kWh/day.
- Autonomy 1.0–1.5 days → 10–15 kWh baseline.
- Dark-week margin x1.5 → test 15–22 kWh usable Tier B.
- Ah usable = kWh × 1000 ÷ 48: 12 kWh ≈ 250 Ah; 20 kWh ≈ 415 Ah.
- Nominal: LFP ÷ 0.9, lead ÷ 0.5 (double).
Bank config table
| Home | Usable range | Ah at 48 V | Example layout |
|---|---|---|---|
| Cabin 2–3 kWh/day | 4–7 kWh | 85–150 Ah | 4x 12 V 150–170 Ah series |
| Flat 4–6 kWh/day | 7–12 kWh | 145–250 Ah | 48 V 280–320 Ah single string |
| Family 8–12 kWh/day B | 12–20 kWh | 250–420 Ah | 1–2x 48 V 300 Ah strings |
| Same home Tier A | 25–40 kWh | 520–835 Ah | 2–3x 48 V strings parallel |
Safety: fuse, breaker, wire, licensed pro
- Fuse every parallel string plus main DC disconnect.
- Class-T high-interrupt fusing on large LFP banks.
- Torque, gauge, and lug quality matter as much as the diagram.
- No article substitutes for licensed review before energizing.
What banks cost (Aug 2026 ranges)
| Scope | LFP per usable kWh | Includes |
|---|---|---|
| Ex-factory cells | $45–70 | Cells before freight/BMS |
| Landed DIY | $80–125 | Cells + BMS + enclosure |
| Budget retail | $110–165 | Finished pack |
Know your Ah before you wire anything. Run a free simulation for your load and compare bank sizes side by side.
⚡ Run free simulationRelated guides
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- DIY battery: LFP vs sodium-ion
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Frequently asked
How do I calculate battery bank size in Ah?
Usable kWh × 1000 ÷ volts. 15 kWh at 48 V ≈ 310 Ah usable. Then ÷ 0.9 LFP (≈345 Ah nominal) or ÷ 0.5 lead (≈620 Ah).
How many batteries for 10 kWh/day off-grid?
12–20 kWh usable Tier B (250–420 Ah at 48 V); 25–40 kWh Tier A. Typically 1–2 matched 48 V 300 Ah strings Tier B.
Is 12 V, 24 V, or 48 V better?
48 V for full-time homes above 3–4 kWh/day. 12 V only for small cabins.
Do series or parallel increase capacity?
Series raises voltage; parallel raises Ah. kWh = V × Ah ÷ 1000 either way.