Battery Backup Calculator estimates UPS or inverter run-time from battery Ah, system voltage, load watts, and inverter efficiency with formulas, examples, FAQs, and references.

Battery Backup Calculator - UPS Runtime and Inverter Battery Size

Agarapu Ramesh — Editor and content reviewer
Editorially reviewed Reviewed by Agarapu Ramesh, science educator (chemistry). LinkedIn
Last reviewed: May 2026 | Standard electrical engineering formulas

Battery backup time is stored energy divided by load. The rough math is simple: battery Ah x battery volts gives watt-hours, then you cut that down for depth of discharge and inverter losses. Real batteries are less polite than formulas, especially old lead-acid ones.

Formula at a glance

  • stored Wh = Ah x V
  • usable Wh = Ah x V x DoD x inverter efficiency
  • runtime hours = usable Wh / load W

Field note: If the UPS is three years old, assume the battery is weaker than the sticker says. A load test tells the truth quickly.

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Calculator Tool

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Ah
W
%
%
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Formulas

Usable WhAh × V × DoD × Eff
Run TimeUsable Wh / Load W
Req Ah(W × Hrs) / (V × DoD × Eff)

Quick Reference: Typical Loads

ApplianceWatts
LED Bulb9W
Wi-Fi Router12W
Laptop Charger65W
Ceiling Fan75W
LED TV 43"100W
Fridge150W
1.5T Split AC1500W

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How to use the Battery Backup Calculator

Use this as a runtime estimate, then discount it for battery age, discharge rate, temperature and inverter losses. Battery labels are optimistic on their best day.

Worked example

Example: a 150 Ah, 12 V battery has 1,800 Wh on paper. At 50% DoD and 90% inverter efficiency, usable energy is 810 Wh. A 300 W load runs about 2.7 hours.

Practical checks before you trust the number

  • Lead-acid usually gets 50% DoD if you want it to live.
  • Lithium can often use 80% or more, but check the BMS and manufacturer sheet.
  • Fridges, pumps and laser printers have starting surges. Runtime math does not start motors for you.

Common mistake

If the UPS is three years old, assume the battery is weaker than the sticker says. A load test tells the truth quickly.

Sources and references

Related calculators

Frequently Asked Questions

Backup time (hours) = (battery Ah × battery V × DoD) ÷ load watts. For a 12 V 100 Ah lead-acid battery at 50% DoD running a 100 W load: (100 × 12 × 0.5) ÷ 100 = 6 hours. Without DoD applied you'd see 12 hours, which is misleading because draining lead-acid that deep ruins the battery in months. Always apply realistic DoD: 50% for lead-acid, 80% for lithium.

UPS runtime (minutes) = (battery Ah × battery V × DoD × 60) ÷ load watts. Example: 7 Ah × 12 V × 0.5 × 60 ÷ 200 W = 12.6 minutes for a 200 W desktop on a small 7 Ah UPS. Multiply by inverter efficiency too — usually 0.85 for cheap UPS, 0.90 for sine-wave units. The math is simple, but the surprise comes from inrush at startup, which can drop runtime by 10–15%.

Required Ah = (load watts × backup hours) ÷ (battery V × DoD × inverter efficiency). For a 500 W load needing 8 hours at 12 V, lead-acid 50% DoD, 90% inverter: (500 × 8) ÷ (12 × 0.5 × 0.9) = 740 Ah. That's a heavy bank, so going to 24 V or 48 V cuts it down significantly. We almost always recommend 24/48 V battery banks for backups beyond 4 hours to keep cable size and copper cost reasonable.

Several reasons. The Peukert effect reduces effective capacity at higher discharge rates, so a 100 Ah battery at C/5 might only deliver 80 Ah. Inverter efficiency eats another 10–15%. Cable losses, ambient temperature, and battery age also bring the actual time down. A 3-year-old lead-acid battery may only have 70% of its rated capacity. Always test with a real load test once a year so customers don't get a shock during an actual outage.

Multiply the calculated battery energy by the inverter efficiency. Effective backup hours = (battery Ah × V × DoD × η_inverter) ÷ load watts. If η is 0.9, a 100 Ah 12 V 50% DoD bank running 200 W gives (100 × 12 × 0.5 × 0.9) ÷ 200 = 2.7 hours instead of the naive 3 hours. Cheap modified-sine inverters can drop to 75%. We always insist on pure sine wave for fridges, microwaves, and motor loads — the efficiency and load behavior are both better.

It depends on the actual load drawn through the inverter, not the inverter's rating. If you draw the full 500 W: backup = (100 × 12 × 0.5 × 0.9) ÷ 500 ≈ 1.08 hours. If you only draw 200 W: about 2.7 hours. The inverter's maximum rating just sets the ceiling. Tell the customer their actual runtime in terms of their fans and lights, not the nameplate, otherwise they'll always feel cheated when reality kicks in.

Yes, a comparison calculator can show usable Wh side by side. Lead-acid: usable Wh = nominal Ah × V × 0.5. Lithium: usable Wh = nominal Ah × V × 0.8. So a 100 Ah 12 V lead-acid gives 600 Wh usable, while the same lithium gives 960 Wh. Lithium also lasts 5 to 10 times as many cycles. The upfront cost is higher, but life-cycle cost on lithium is now usually lower for daily-cycle UPS or solar applications.