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UPS Battery Size Calculator

Size UPS batteries for a required runtime at a given load.

About UPS Battery Size Calculator

UPS batteries discharge at very high C-rates for short runtimes, which reduces effective capacity. Sizing must also allow for capacity loss as the batteries age toward replacement. A UPS that just meets its runtime when new will fail to do so within two years, which is why end-of-life derating is standard practice.

Formula

UPS Battery Size

Ah = load × minutes ÷ (60 × efficiency × voltage) ÷ (1 − end-of-life derate)

end-of-life derate
Margin so the UPS still meets runtime as batteries age

Worked example

With the default values below, UPS Battery Size Calculator returns Battery Capacity Required of 14.47. Change any input above and every figure here is recalculated in your browser.

Inputs used

Loadminimum 1 W
2,000 W
Required runtimeminimum 1 min
15 min
Battery string voltageminimum 1 V
48 V
Inverter efficiencyaccepts 0–100 %
90 %
End-of-life capacity deratingaccepts 0–100 %
20 %

Results

Battery Capacity Required
14.47 Ah
Energy Required
555.56 Wh
Discharge Current
46.30 A
Discharge C-Rate
3.2000 C
Batteries at 12 V and 100 Ah
4

Step by step

  1. 1Values used
  2. 2UPS Battery Size
  3. 3Battery Capacity Required= 14.47 Ah
  4. 4Energy Required= 555.56 Wh
  5. 5Discharge Current= 46.30 A
  6. 6Discharge C-Rate= 3.2000 C
  7. 7Batteries at 12 V and 100 Ah= 4

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 Battery Size calculated?

Ah = load × minutes ÷ (60 × efficiency × voltage) ÷ (1 − end-of-life derate) UPS batteries discharge at very high C-rates for short runtimes, which reduces effective capacity. Sizing must also allow for capacity loss as the batteries age toward replacement.

Why does UPS Battery Size matter?

A UPS that just meets its runtime when new will fail to do so within two years, which is why end-of-life derating is standard practice.

What values do I need to enter?

This calculator takes 5 inputs: Load, Required runtime, Battery string voltage, Inverter efficiency, End-of-life capacity derating. 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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