Quick estimate Battery capacity calculator The formula Tips Next logical reads FAQ
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.
The formula's most common real-world answer: at 12.8V nominal, 100Ah is 1.28 kWh (per manufacturer spec) — the unit that makes bank-sizing multiplication concrete. Not for: the whole-house example above — a 5,556Wh bank at 48V needs a 48V configuration, and a single 12V unit caps at 1.28kWh usable. The honest tradeoff: 1.28kWh per unit means big banks stack many batteries and add wiring.
Check price on Amazon (opens in a new tab) Price & availability shown on Amazon.com — we may earn a commission.The full chain: Ah → Wh → usable Wh → runtime (worked three ways)
The calculator above compresses four steps into one click; here is the chain it is running, so you can catch your own mistakes:
Wh = Ah × V. Usable Wh = Wh × usable-fraction. Runtime = Usable Wh ÷ load watts.
- A 100Ah battery at 12V is 1,200Wh (100 × 12). At the nominal 12.8V of LiFePO4 it’s 1,280Wh — the voltage you multiply by matters, and manufacturers quote the flattering one.
- Usable fraction is a chemistry rule, not a preference: lead-acid gives ~50%, LiFePO4 gives 80–100%. So that 100Ah lead-acid battery stores 1,200Wh but delivers ~600Wh; the 100Ah lithium delivers ~1,020–1,280Wh. Two batteries with the same Ah sticker are not the same battery — this is the entire argument of the li-ion vs lead-acid comparison.
- Runtime divides, it doesn’t subtract: 1,020 usable Wh ÷ a 60W CPAP ≈ 17 hours; the same battery behind an 85%-efficient inverter feeding a 100W AC load delivers 1,020 × 0.85 ÷ 100 ≈ 8.7 hours — the inverter’s cut is real and people forget it.
Self-test (answers in parentheses): (a) 200Ah at 12V lead-acid, 50% usable, running a 400W load? (200 × 12 × 0.5 = 1,200Wh ÷ 400 = 3 hours.) (b) 100Ah LiFePO4 at 12.8V, 80% DOD, through a 90%-efficient inverter running 150W? (1,280 × 0.8 × 0.9 ÷ 150 ≈ 6.1 hours.) (c) How many 100Ah LiFePO4 batteries for 2 days of autonomy at 1.5kWh/day, 90% usable? (3,000 ÷ (1,280 × 0.9) ≈ 2.6 → 3 batteries.) If you got all three, the sizing pages on this site will read like arithmetic, which is all they are.
Tips
- Li-ion/LiFePO4 often supports 80–90% DoD with 4,000–6,000 cycles. Lead-acid is usually limited to 50% DoD for longevity — the li-ion vs lead-acid comparison works through that usable-capacity math.
- 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 Flywheel storage physics Supercapacitor banks as buffers Battery management systems explained