Quick estimate
Use this calculator to find the minimum battery bank size for a given daily load. It converts required watt-hours into amp-hours at your chosen system voltage.
Battery capacity calculator
The formula
Battery Wh = (Daily Wh × Days of autonomy ÷ Inverter efficiency) ÷ Depth of discharge
Example: 2,000 Wh × 2 days ÷ 0.90 ÷ 0.80 = 5,556 Wh total battery bank.
- Usable capacity is what you can actually draw before recharging.
- Total capacity is the nameplate you must buy to keep DoD healthy.
- Higher voltage means lower current for the same power, so less voltage drop and thinner cable. See 12V vs 24V vs 48V solar and how to choose system voltage.
1.28 kWh, exactly
LiTime 12V 100Ah LiFePO4
The formula's most common real-world answer: at 12.8V nominal, 100Ah is 1.28 kWh — the unit that makes bank-sizing multiplication concrete.
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- Li-ion/LiFePO4 often supports 80–90% DoD with 4,000–6,000 cycles. Lead-acid is usually limited to 50% DoD for longevity.
- Factor inverter efficiency for AC loads. DC loads skip that loss.
- Size for critical loads first, then add discretionary loads.
- Cold temperatures reduce usable capacity and can slow charging. Size for the coldest month if off-grid year-round.
- Match your bank to inverter and cable sizing so protection stays realistic.
Next logical reads
How to size a solar system Li-ion vs lead-acid Solar system cost breakdown How to choose system voltage
FAQ
Do I size from daily or peak load?
Daily Wh is the baseline. Add surge headroom at the inverter, not the battery, unless surge is sustained.
What if I only need backup for 4 hours?
Set days of autonomy to 0.17 (4 ÷ 24). The calculator will still apply DoD and inverter efficiency.
Can I use a 12V battery for a whole house?
Usually no. Whole-home storage runs at 48V or 400V to keep current and cable sizes manageable.
Why does the calculator show more Ah at lower voltage?
For the same energy, lower voltage means higher amp-hours (Wh = V × Ah). Higher voltage gives lower Ah and usually easier wiring.