TryBuildCalcHow Many Batteries Do I Need? (Battery Bank Sizing for a Target Runtime)
Find how many batteries you need.
🕒 Last updated: August 28, 2026
Inputs
Equipment to Power
Pick a preset for a typical starting point, or choose Custom and enter your equipment's actual nameplate watts.
Equipment 1
Equipment 2
Equipment 3
Equipment 4
Battery Configuration
ℹ️Sets the default depth-of-discharge limit — override it below if your manufacturer specifies something different.
ℹ️Leave blank to use the selected battery type's recommended default.
ℹ️The voltage of ONE battery unit — commonly 6V, 12V, or 2V cells.
ℹ️Your inverter/UPS's DC bus voltage — must be an exact multiple of the single battery voltage above (e.g. 12V batteries build a 12/24/48V bank).
ℹ️Typically 80-90% — check your inverter/UPS's datasheet.
ℹ️A design safety margin so the system still performs adequately as the battery ages — not a claim about a battery's current condition.
Cost
Recommended Battery Bank: 3 × 100Ah
Delivers 4.52 hours (target was 4.0 hours) at 325 W load
Load Breakdown
| Equipment | Watts | Qty |
|---|---|---|
| Refrigerator | 150 W | × 1 |
| LED Bulb | 10 W | × 6 |
| LED TV | 100 W | × 1 |
| Router | 15 W | × 1 |
| Total Load | 325 W |
Battery Bank
Battery type: Sealed Lead-Acid / AGM / Gel (VRLA) (depth of discharge: 60%)
System voltage: 12V — 1 × 12V in series
Required capacity: ~266 Ah — rounded up to 3 × 100Ah = 300 Ah total
Total batteries needed: 3 (1 series × 3 parallel)
Efficiency: 85%, aging/end-of-life margin: 80%
Informational Inverter/UPS Sizing
Suggested inverter/UPS rating: 600 VA (assumes 0.8 power factor, 25% headroom)
Assumptions Used
Assumes a sustained/continuous load — motor-driven appliances (like a refrigerator's compressor) cycle on and off, so actual runtime for such loads can vary from this estimate. This is a reference estimate — confirm with a licensed electrician for a permanent installation.
Need longer, whole-house backup instead of a battery bank? Generator Size Calculator →
Sizing your dwelling's full electrical service? Panel Size Calculator →
Looking for the verification checklist, reference tables, tips, or common mistakes?See the complete UPS / Inverter Battery Capacity Calculator.
Battery bank sizing for a target runtime
This page is pre-set to Sizing mode with a 4-hour backup target — a common planning figure for a short-to-medium outage.
Edit your equipment list, target runtime, and battery specs above to match your actual needs.
- Enter your target backup duration and the calculator works backward to the required battery Ah.
- Results round up to a whole number of batteries — the actual runtime shown may be slightly more than your target.
- Change the battery chemistry to see how much a higher-DoD battery (like lithium-ion) reduces the number of batteries needed.
UPS Battery Formula: How Is Capacity Calculated?
The core formula is the same in both directions — only which variable you're solving for changes.
Step 1 — Total Load
Total Load (W) = Sum of every equipment row's Watts × Quantity
This is the total connected load you want backed up, treated as a sustained/continuous draw.
Step 2 — Usable Energy
Usable Energy (Wh) = Battery Ah × System Voltage × Depth of Discharge × Aging Factor × Efficiency
Depth of discharge, an aging/end-of-life margin, and inverter conversion efficiency all reduce how much of the battery's rated capacity actually reaches your equipment.
Step 3a — Runtime Mode
Runtime (hours) = Usable Energy ÷ Total Load
Given a battery bank's Ah and voltage, solve directly for how long it backs up your load.
Step 3b — Sizing Mode
Required Ah = (Total Load × Target Runtime) ÷ (System Voltage × Depth of Discharge × Aging Factor × Efficiency)
Batteries Needed = Series Count (for voltage) × Parallel Count (rounded up, for capacity)
Given a target runtime, solve backward for the required battery capacity, then round up to a whole number of batteries.
Worked Example
This example walks through your current inputs above, using the same steps as the Formula section.
Input Values Used
| Input | Value | Why it is used |
|---|---|---|
| Equipment | 4 items, 325 W total | Sets total load |
| Battery | Sealed Lead-Acid / AGM / Gel (VRLA), 60% DoD | Sets usable energy per Ah |
| Mode | Sizing (given target runtime) | Sets which variable is solved for |
Steps 1-2 — Load and Usable Energy
| Calculation | Result |
|---|---|
| Total load | 325 W |
| Target runtime | 4.0 hrs |
| Required capacity | 266 Ah |
Therefore, for a 4.0-hour target at 325 W, you need 3 × 100Ah batteries (delivering 4.52 hours).
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Disclaimer: This calculator provides approximate results for planning and estimation purposes only. Actual requirements may vary based on site conditions, materials, workmanship, and local building regulations. Always consult a qualified engineer, architect, or construction professional before making final decisions.