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 / UPS Run-time Calculator

What this calculator does

The Battery Backup Calculator determines exactly how long your UPS, home inverter, or off-grid battery bank will power a specific electrical load. It factors in inverter efficiency and safe depth of discharge (DoD).

Inputs explained

  • Battery capacity: The rating of a single battery in Ampere-hours (Ah).
  • Battery voltage: The nominal DC voltage of your battery bank (12V, 24V, 48V...).
  • Number of batteries: Total batteries in the bank.
  • Configuration: How they are wired (Series increases voltage, Parallel increases Ah).
  • Load: The total continuous power draw of your connected devices in Watts (W).
  • Inverter efficiency: The percentage of DC power successfully converted to AC (default 90%).
  • Depth of discharge (DoD): Maximum percentage you will drain the battery (50% lead-acid, 80% Li-ion).

How it works / Method

The calculator first finds the total usable Watt-hours (Wh) in your battery bank by multiplying total Ah by voltage, depth of discharge, and inverter efficiency. It then divides this usable energy by your continuous load (W) to output the expected run-time in hours and minutes.

Formulas used

  • Total Bank Capacity (Ah): Adds Ah in parallel, keeps same in series.
  • Usable Energy (Wh) = Bank Ah × Bank V × (DoD/100) × (Efficiency/100)
  • Run time (hours) = Usable Wh ÷ Load W
  • Run time (minutes) = Hours × 60

Units: Capacity in Ah, Voltage in V, Power in W, Time in Hours:Minutes.

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Calculator Tool

Enter values to compute

Ah
W
%
%
Results
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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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Step-by-step example

Scenario: You have a single 12V 150Ah lead-acid battery. You want to run a 300W load. Your inverter is 90% efficient, and you set DoD to 50%.

Formula: Run Time = (Ah × V × DoD × Eff) / Load

  1. Total Wh = 150 Ah × 12 V = 1800 Wh total energy.
  2. Usable Wh = 1800 × 0.50 (DoD) × 0.90 (Efficiency) = 810 Wh.
  3. Run time = 810 Wh / 300 W = 2.7 hours.
  4. 0.7 hours × 60 = 42 minutes.

Result: 2 hours and 42 minutes.

Use cases

  • Sizing a home inverter system for power outages.
  • Determining UPS run time for server racks and critical IT infrastructure.
  • Planning battery banks for RVs, camper vans, and boats.
  • Off-grid cabin power planning.
  • Calculating telecom tower emergency backup duration.

Assumptions & limitations

  • This calculation is linear; it does not factor in Peukert's Law, which means very high discharge rates will reduce the effective capacity of lead-acid batteries.
  • Assumes batteries are perfectly healthy and fully charged to 100% at the start.
  • Inverter efficiency is assumed constant, though it drops significantly at very low or very high loads.
  • Power factor for inductive loads is estimated; actual VA draw might be higher.
  • Consult IEEE guidelines for stationary battery applications in enterprise settings.

Sources & references

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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.