Solar Battery Bank Calculator
Size or check a solar / off-grid battery bank (also called a battery bank size calculator) from daily kWh, DoD, and autonomy days—or reverse for days of storage. For PV bank kWh planning, not UPS runtime or UPS string layout.
Quick answer
Required bank kWh ≈ (Daily kWh × Days) ÷ (DoD × η). Example: 10 kWh/day, 2 days, DoD 0.80, η 0.90 → ≈ 27.8 kWh. Or Days = (Bank × DoD × η) ÷ Daily. Need Ah at a pack voltage? Ah ≈ (kWh × 1000) ÷ V—use the Ah to Wh / kWh calculator. UPS series/parallel blocks? UPS battery bank. Hub: Solar calculator.
Quick Solar Battery Bank Calculator
Defaults: 40 kWh bank · 10 kWh/day · DoD 0.80 — see days, or set target days below for required bank.
40 × 0.80 × 0.90 ÷ 10.
Advanced Solar Battery Bank Calculator
Battery Bank & Autonomy Results
Engineering disclaimer
PV/BESS planning estimate only. Do not use UPS runtime or UPS battery calculators for this intent. Confirm chemistry limits, temperature, and OEM usable energy curves before procurement.
Bank size vs autonomy (same load)
At your current DoD / η / daily load—how bank kWh maps to days (planning screen only).
| Bank kWh | Autonomy days |
|---|
People also ask
- How do I calculate solar battery bank size? Daily kWh × days ÷ (DoD × η).
- What is a good size battery bank for solar? Policy-driven (often 1–3 days of critical load)—not a single retail kWh.
- How many panels to charge the bank? Use panel sizing + MPPT after bank kWh.
- Same as UPS runtime? No—different domain; do not merge.
Planning guidance
Solve required bank kWh from policy days, or reverse for autonomy days from a known bank. Keep PV storage separate from UPS minute-runtime tools.
- Load first: Build daily kWh in off-grid load.
- Bank kWh: Use target days here, then read the method in how to size a solar battery bank.
- Facility BESS: Peak-shave / PCS architecture → industrial BESS kWh sizing.
- Do not: Link this intent into UPS runtime or UPS battery tools; do not chase lithium shopping SERPs.
- Hub: Solar calculator.
Last updated: 2026-08-07. Planning estimate only—OEM usable energy, temperature, and C-rate limits govern procurement.
Typical scenarios
- 2-day commercial hybrid: Cover cloudy stretches without full diesel runtime.
- 3-day remote hut: Conservative DoD and winter PSH together.
- Li vs lead: Higher usable DoD shrinks required bank kWh for the same days.
- Telecom / critical comms: Longer autonomy buffers (often 4–8 days) with conservative DoD.
Recommended autonomy days by application
Autonomy days are a policy input, not a physics constant. Start here, then solve required bank kWh with the calculator above.
| Application | Typical autonomy | Why |
|---|---|---|
| Residential / commercial hybrid (grid primary) | 1–2 days | Grid is primary; battery covers outages / peak shave |
| Off-grid cabin | 3–5 days | Weather variability; no grid fallback |
| Telecom / critical comms | 4–8 days | High reliability; extreme weather events |
| RV / marine | 2–3 days | Limited charging windows; load shedding common |
DoD impact on usable bank capacity
usable_kWh = bank_kWh × DoD (then apply η in the full formula). Higher usable DoD shrinks the nameplate bank needed for the same autonomy days.
| Battery type | Planning DoD | Usable multiplier |
|---|---|---|
| Flooded lead-acid | ~50% | 0.50 |
| AGM / Gel | ~50–60% | 0.55 typical |
| Lithium LiFePO4 | ~80–90% | 0.85 typical |
Example: 10 kWh nameplate → Li @ 80% DoD = 8 kWh usable before η; lead-acid @ 50% = 5 kWh. Method detail: how to size a solar battery bank.
Temperature derating and chemistry (short)
- Lead-acid: capacity falls in cold (planning: ~80% at 0 °C, ~60% near −20 °C vs 25 °C baseline—OEM curves override).
- LiFePO4: often milder capacity loss, but charging may need heaters below freezing—follow OEM limits.
- Formula screen: actual_capacity ≈ rated_capacity × temp_factor; fold conservatism into DoD or a lower η rather than inventing precision.
| Dimension | Lead-acid | LiFePO4 |
|---|---|---|
| Planning DoD | ~50% | ~80–90% |
| Cycle life @ planning DoD | Lower (hundreds–~1k) | Higher (thousands) |
| Autonomy buffer | Often +15–25% | Often +10% |
Worked examples
- Off-grid cabin: 3 kWh/day, 10 kWh LiFePO4 @ 80% DoD, η 0.90 → usable 7.2 kWh → 2.4 days. For a 3-day target: required bank ≈ 3 × 3 ÷ (0.80 × 0.90) ≈ 12.5 kWh.
- Telecom site: 1.5 kWh/day, 20 kWh AGM @ 50% DoD, η 0.90 → usable 9 kWh → 6.0 days.
- RV: 2 kWh/day, 5 kWh Li @ 80% DoD, η 0.90 → usable 3.6 kWh → 1.8 days (use Target 2–3 days preset to back-calculate a larger bank).
- 10 kWh bank vs house load: usable ≈ 10 × 0.80 × 0.90 = 7.2 kWh. At 3 kWh/day ≈ 2.4 days; at 10 kWh/day ≈ 0.72 days—enter your daily kWh above.
Required bank kWh → Ah @ 12 / 24 / 48 V
Ah ≈ (kWh × 1000) ÷ V. Example screens (re-run with your required bank from the calculator):
| Bank kWh | Ah @ 12 V | Ah @ 24 V | Ah @ 48 V |
|---|---|---|---|
| 10 | 833 | 417 | 208 |
| 20 | 1,667 | 833 | 417 |
| 27.8 | 2,317 | 1,158 | 579 |
| 40 | 3,333 | 1,667 | 833 |
Formula (quick reference)
Days = (Bank kWh × DoD × η) ÷ Daily kWh
Required bank kWh = Daily kWh × Target days ÷ (DoD × η)
Method walkthrough: how to size a solar battery bank (PV autonomy — not UPS runtime).
Formula and sources
Planning model only. Chemistry, temperature, and C-rate limits from OEM datasheets override spreadsheet DoD defaults.
- Battery University — lead-acid charging / DoD context
- NREL PVWatts — array production context after bank energy is set
- NREL System Advisor Model (SAM) — off-grid / storage simulation reference
Frequently Asked Questions
Is this a battery bank calculator / battery bank size calculator?
Yes for solar and off-grid bank kWh + autonomy days (UnboundSolar/SolarMathLab-style sizing). It is not a UPS “how many 12 V blocks” layout tool—use the UPS battery bank calculator for series/parallel strings.
How do I convert bank kWh to amp-hours?
Ah ≈ (kWh × 1000) ÷ V. Example: 27.8 kWh at 48 V ≈ 579 Ah (before DoD already applied in bank kWh). Use the Ah to kWh converter for bidirectional unit math.
How do I calculate solar battery bank size?
Required bank kWh ≈ (Daily kWh × Autonomy days) ÷ (DoD × η). Example: 10 kWh/day × 2 ÷ (0.80 × 0.90) ≈ 27.8 kWh. Enter target days above to back-calculate, or enter a known bank to read days.
How do I calculate solar battery autonomy days?
Days = (bank kWh × DoD × η) ÷ daily kWh. Example: 40 × 0.80 × 0.90 ÷ 10 ≈ 2.88 days.
What is a good size battery bank for solar?
There is no universal retail answer. Size from critical daily kWh and outage policy (hybrid often 1–2 days; off-grid cabins 3–5; telecom longer). Then apply DoD and efficiency—see the application table above.
How long will a 10 kWh solar battery last?
At DoD 0.80 and η 0.90, usable ≈ 7.2 kWh. Autonomy = 7.2 ÷ daily_kWh. At 3 kWh/day ≈ 2.4 days; at 10 kWh/day ≈ 0.72 days. Enter your daily load in the calculator for an exact screen.
How many solar panels do I need to charge my battery bank?
This tool sizes storage energy, not array watts. After bank kWh is set, use the solar panel sizing calculator and MPPT sizing with peak sun hours.
What DoD should I assume for lead-acid vs lithium?
Flooded lead-acid planning often uses ~50% usable DoD; AGM/gel ~50–60%; many LiFePO4 systems allow ~80–90%. Confirm OEM usable energy and warranty constraints—do not assume 100% DoD.
How is this different from UPS runtime?
UPS tools estimate minutes of backup for IT/facility UPS systems. This tool sizes multi-day PV storage bank energy—keep the intents separate.
Is this for lithium solar battery shopping?
No. It sizes bank kWh and autonomy days—not product pages for lithium packs or portable power stations. For industrial BESS architecture see the industrial BESS kWh sizing guide.
What is the next step?
Confirm bank kWh here, walk through how to size a solar battery bank, then size MPPT/array for recharge.
What is the solar battery calculation formula?
Bank kWh ≈ (daily kWh × autonomy days) ÷ (DoD × η). Example: (10 × 2) ÷ (0.80 × 0.90) ≈ 27.8 kWh. Convert to Ah with Ah ≈ (kWh × 1000) ÷ V. Method write-up: how to size a solar battery bank. Recharge time uses a different formula on the solar panel charge time calculator.
How it works
Usable energy is nameplate bank kWh reduced by depth-of-discharge and round-trip efficiency. Autonomy days are usable energy divided by daily load. Required bank kWh reverses the same equation from a target day count. This is not a UPS minute-runtime model.