TryBuildCalcInverter Battery Backup Time Calculator (Runtime From a Given Battery)
Find your inverter battery's backup time.
🕒 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).
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: 1.84 hours
From 150 Ah (1 × 150Ah battery/ies) at 400 W load
Load Breakdown
| Equipment | Watts | Qty |
|---|---|---|
| LED Bulb | 10 W | × 6 |
| Ceiling Fan | 75 W | × 3 |
| LED TV | 100 W | × 1 |
| Router | 15 W | × 1 |
| Total Load | 400 W |
Battery Bank
Battery type: Sealed Lead-Acid / AGM / Gel (VRLA) (depth of discharge: 60%)
System voltage: 12V — 1 × 12V in series
Parallel strings: 1 × 150Ah = 150 Ah total
Total batteries needed: 1 (1 series × 1 parallel)
Efficiency: 85%, aging/end-of-life margin: 80%
Informational Inverter/UPS Sizing
Suggested inverter/UPS rating: 850 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.
Inverter battery backup time
"How long will my inverter battery last?" comes down to your battery's usable energy divided by your connected load — this page is pre-set to a single 150Ah battery running common household loads.
Edit your equipment list and battery specs above to match your actual setup.
- Usable energy accounts for depth of discharge, not just the battery's full rated Ah.
- Inverter efficiency (typically 80-90%) further reduces usable energy from the DC to AC side.
- Add more batteries in parallel to see how backup time scales with battery bank size.
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, 400 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 | 400 W |
| Total battery capacity | 150 Ah |
| Usable energy | 734 Wh |
Therefore, 150 Ah at 12V backs up 400 W of load for 1.84 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.