TryBuildCalcUPS / Inverter Battery Capacity Calculator (Runtime & Battery Bank Sizing)
Calculate UPS battery capacity instantly.
🕒 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
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).
ℹ️How many battery strings you have (or plan to have) wired in parallel.
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
Estimated Backup Runtime: 4.55 hours
From 200 Ah (2 × 100Ah battery/ies) at 215 W load
Load Breakdown
| Equipment | Watts | Qty |
|---|---|---|
| LED Bulb | 10 W | × 5 |
| Ceiling Fan | 75 W | × 2 |
| Router / Modem | 15 W | × 1 |
| Total Load | 215 W |
Battery Bank
Battery type: Sealed Lead-Acid / AGM / Gel (VRLA) (depth of discharge: 60%)
System voltage: 12V — 1 × 12V in series
Parallel strings: 2 × 100Ah = 200 Ah total
Total batteries needed: 2 (1 series × 2 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 →
What Is a UPS / Inverter Battery Capacity Calculator?
A UPS/inverter battery capacity calculator answers two related questions: given a battery bank, how long will it back up your equipment (Runtime mode); or given a target backup duration, what battery bank do you need to buy (Sizing mode). Both use the same underlying formula, accounting for battery chemistry's recommended depth of discharge, an aging/end-of-life safety margin, and inverter conversion efficiency.
This is a different calculation from the Generator Size Calculator (which sizes a fuel-powered generator's wattage, including motor starting surge) — use this tool for battery-based backup power, where sustained runtime from stored energy is what matters, not a brief startup transient.
Why getting battery capacity right matters:
- An undersized battery bank runs out well before your outage ends, right when you need it most
- An oversized bank costs significantly more than necessary for the same backup duration
- Depth of discharge is the single most commonly overlooked factor — using a battery's full rated Ah instead of its safe usable capacity badly overstates runtime
- Battery chemistry (lead-acid vs. lithium-ion) changes both the usable capacity and the cost per usable Ah substantially
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 | 3 items, 215 W total | Sets total load |
| Battery | Sealed Lead-Acid / AGM / Gel (VRLA), 60% DoD | Sets usable energy per Ah |
| Mode | Runtime (given battery) | Sets which variable is solved for |
Steps 1-2 — Load and Usable Energy
| Calculation | Result |
|---|---|
| Total load | 215 W |
| Total battery capacity | 200 Ah |
| Usable energy | 979 Wh |
Therefore, 200 Ah at 12V backs up 215 W of load for 4.55 hours.
Essential Checklist+−
Complete these critical checks before approving the work or proceeding to the next construction stage.
✓Load Input Accuracy+-
- Actual nameplate/rated watts used for major equipment, not guessed from a generic chart
- All equipment you actually intend to keep running is included, including easy-to-forget items
✓Battery Configuration & Depth of Discharge+-
- Battery chemistry selection matches the actual battery type purchased or planned
- Planned usage confirmed not to regularly discharge the battery beyond its recommended DoD
- Series/parallel wiring configuration confirmed against the actual battery's voltage rating
- System voltage confirmed against the inverter/UPS's actual DC input requirement
✓Runtime / Sizing Calculation+-
- Correct calculation mode selected for the actual planning question being asked
✓Installation & Safety+-
- Battery placement and ventilation confirmed per the manufacturer's guidance
- DC-side wiring and cable gauge sized for the actual current draw, not just the AC-side circuit
- Overcurrent protection (fuse or breaker) confirmed on the battery bank, sized for the DC circuit
- Permanent or whole-panel backup installation reviewed by a licensed electrician
Full QC Checklist+−
Verification checklist for UPS/inverter battery capacity sizing — covering load input accuracy, battery configuration and depth of discharge, runtime/sizing calculation, and installation/safety. Use the Essential Checklist for critical checks before purchasing; expand to Full QC Checklist for complete verification.
✓Load Input Accuracy+-
- Actual nameplate/rated watts used for major equipment, not guessed from a generic chart
- All equipment you actually intend to keep running is included, including easy-to-forget items
- Motor-driven, cycling loads (refrigerator, sump pump) flagged as producing a variable, not perfectly steady, draw
- Quantity field used correctly for repeated items, not double-counted as separate identical rows
- Non-essential loads excluded, rather than listing everything in the house "just in case"
✓Battery Configuration & Depth of Discharge+-
- Battery chemistry selection matches the actual battery type purchased or planned
- Planned usage confirmed not to regularly discharge the battery beyond its recommended DoD
- Series/parallel wiring configuration confirmed against the actual battery's voltage rating
- System voltage confirmed against the inverter/UPS's actual DC input requirement
- Aging/end-of-life margin considered appropriate for the expected battery lifespan and how it will be used
✓Runtime / Sizing Calculation+-
- Correct calculation mode selected for the actual planning question being asked
- Efficiency percentage reflects the actual inverter/UPS's rated efficiency, not left at a generic default
- In Sizing mode, the actual achieved runtime (not just the target you entered) confirmed adequate
- Understood that motor-driven/cycling loads' real-world backup time can differ from this steady-load estimate
- Runtime and Sizing mode results cross-checked against each other where practical
✓Installation & Safety+-
- Battery placement and ventilation confirmed per the manufacturer's guidance
- DC-side wiring and cable gauge sized for the actual current draw, not just the AC-side circuit
- Overcurrent protection (fuse or breaker) confirmed on the battery bank, sized for the DC circuit
- Inverter/UPS VA rating cross-checked against the actual connected load, not assumed automatically adequate
- Permanent or whole-panel backup installation reviewed by a licensed electrician
- Battery disposal/recycling plan confirmed per local regulations at end of life
Battery Chemistry Depth-of-Discharge Reference
Recommended maximum depth of discharge by battery chemistry — used as this calculator's default, overridable if your manufacturer specifies something different.
| Battery Chemistry | Recommended DoD | Notes |
|---|---|---|
| Flooded Lead-Acid | 50% | Lowest usable capacity per Ah; needs ventilation |
| Sealed Lead-Acid / AGM / Gel (VRLA) | 60% | Maintenance-free, common for UPS/inverter use |
| Lithium-Ion (LiFePO4) | 85% | Highest usable capacity per Ah; typically higher upfront cost |
When should you use this UPS battery calculator?
- Finding how long an existing or planned battery bank will back up your equipment.
- Sizing a new battery bank for a target backup duration.
- Comparing how battery chemistry (lead-acid vs. lithium-ion) changes the required battery count.
- Planning backup power for a home office, refrigerator, or critical equipment during outages.
- Estimating battery bank cost alongside the technical sizing.
Quick UPS Battery Sizing Tips
- Never assume a battery's full rated Ah is usable — always apply the correct depth of discharge for its chemistry.
- Use actual nameplate watts for major equipment instead of relying on generic presets.
- Confirm your system voltage matches your inverter/UPS's actual DC input requirement before buying batteries.
- Consider lithium-ion for a smaller, lighter battery bank at the same usable capacity, if budget allows.
- Size with a realistic aging margin if you don't plan to replace batteries proactively as they age.
Common Mistakes
- Sizing off a battery's full rated Ah instead of its safe usable capacity after depth of discharge.
- Ignoring inverter/UPS conversion efficiency, overstating actual runtime by 10-20%.
- Mixing up series (for voltage) and parallel (for capacity) wiring when planning a multi-battery bank.
- Regularly discharging a battery well beyond its recommended DoD, significantly shortening its lifespan.
- Assuming a plug-in UPS/inverter needs no transfer switch consideration when hard-wired into household circuits.
Limitations
- Treats equipment load as steady/continuous — motor-driven, cycling loads (like a refrigerator) have a variable real-world draw this calculator approximates as constant.
- Does not model battery starting/inrush current for motor-driven loads — for that, and for engine-driven generator sizing, use the Generator Size Calculator.
- Does not model Peukert's effect (capacity loss at high discharge rates) or ambient temperature derating — both real factors for detailed engineering-grade sizing.
- Practical parallel battery count is capped for a reasonable single bank — very large installations need an engineered design.
- This is a planning reference estimate, not a substitute for a licensed electrician's review, especially for a permanent installation.
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.