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UPS Backup Time Calculator

Calculate UPS runtime for a load from its installed battery capacity.

About UPS Backup Time Calculator

UPS runtime is strongly non-linear in load: halving the load more than doubles the runtime, because lower discharge current recovers capacity lost to the Peukert effect. This non-linearity is why a UPS rated for 10 minutes at full load often gives half an hour at a typical 40 per cent load.

Formula

UPS Backup Time

Runtime = usable Wh ÷ load watts, with usable capacity reduced at high discharge rates

usable capacity
Fraction of rated capacity available at UPS discharge rates

Worked example

With the default values below, UPS Backup Time Calculator returns Backup Runtime of 120.96. Change any input above and every figure here is recalculated in your browser.

Inputs used

Installed battery capacityminimum 0 Ah
100 Ah
Battery string voltageminimum 1 V
48 V
Loadminimum 1 W
1,500 W
Inverter efficiencyaccepts 0–100 %
90 %
Usable capacity at high discharge rateaccepts 0–100 %
70 %

Results

Backup Runtime
120.96 min
Backup Runtime
2.0160 h
Usable Energy
3,024.00 Wh
Discharge Current
34.72 A
Runtime at Half Load
278.21 min

Step by step

  1. 1Values used
  2. 2UPS Backup Time
  3. 3Backup Runtime= 120.96 min
  4. 4Backup Runtime= 2.0160 h
  5. 5Usable Energy= 3,024.00 Wh
  6. 6Discharge Current= 34.72 A
  7. 7Runtime at Half Load= 278.21 min

Assumptions and limits

  • The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax or compounding frequency into account.
  • Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is UPS Backup Time calculated?

Runtime = usable Wh ÷ load watts, with usable capacity reduced at high discharge rates UPS runtime is strongly non-linear in load: halving the load more than doubles the runtime, because lower discharge current recovers capacity lost to the Peukert effect.

Why does UPS Backup Time matter?

This non-linearity is why a UPS rated for 10 minutes at full load often gives half an hour at a typical 40 per cent load.

What values do I need to enter?

This calculator takes 5 inputs: Installed battery capacity, Battery string voltage, Load, Inverter efficiency, Usable capacity at high discharge rate. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.

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