GeM UPS Battery Size Calculator

Calculate the exact battery configuration your UPS needs to meet a required backup time. Enter your UPS rating, the backup duration you need, and your DC bus voltage.

The tool returns the recommended battery bank size, the number of 12V batteries in series and parallel, and the total VAH delivered. When more than one DC voltage is valid for your rating, it compares them side by side and flags the most economical option.

Calculate the exact battery configuration your UPS needs to meet a required backup time. Enter your UPS rating, the backup duration you need, and your DC bus voltage

The tool returns the recommended battery bank size, the number of 12V batteries in series and parallel, and the total VAH delivered. When more than one DC voltage is valid for your rating, it compares them side by side and flags the most economical option.

Showing all mapped voltages for 5 kVA: 144, 180, 192 V
5 kVA•60 min backup•8,000 VAH required
DC Bus
144 V
Recommended
12 × 12V 65Ah
Calculated Ah55.6 Ah
Series count12 × 12V
VAH offered9,360
DC Bus
180 V
Recommended
15 × 12V 65Ah
Calculated Ah44.4 Ah
Series count15 × 12V
VAH offered11,700
DC Bus
192 V
★ Best Value
Recommended
16 × 12V 42Ah
Calculated Ah41.7 Ah
Series count16 × 12V
VAH offered8,064
★ Best Value = most economical battery configuration

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How it works

The calculator sizes the battery bank from three inputs:

1. UPS rating (kVA) — Determines the total power the inverter must support. The VAH (volt-ampere-hour) demand on the battery bank scales directly with this rating.

2. Backup time (minutes) — The runtime you need during a mains failure. Longer backup means proportionally higher VAH, and therefore larger or additional batteries.

3. DC bus voltage — The nominal voltage of the battery string feeding the inverter. Each battery is 12V, so the string voltage divided by 12 gives the number of batteries in series. Choose Standard to see every DC voltage mapped to your UPS rating, or Custom to enter a specific bus voltage.

From these, the tool computes the required VAH, divides by the DC voltage to get the amp-hour (Ah) demand, and selects the nearest standard battery size that meets it. If a single series string can't deliver the required Ah, it adds parallel packs and reports the full count.

Reading the results

Each result card shows one DC bus voltage and its recommended bank:

  • Recommended configuration — The battery bank expressed as quantity × 12V × Ah (e.g. 16 × 12V 65Ah). Where parallel strings are used, it lists the number of packs and the batteries per pack.


  • Calculated Ah — The raw amp-hour requirement before rounding to a standard battery size.


  • Series count — Number of 12V batteries wired in series to form the DC bus.


  • VAH offered — The actual capacity the recommended bank delivers, which meets or exceeds the requirement.


  • ★ Best Value — When multiple voltages are shown, this tags the configuration with the lowest overall battery cost.

Sizing formula

The bank is sized on a nominal basis:

Required VAH = 1600 × kVA × (Backup minutes ÷ 60)

Required Ah = Required VAH ÷ DC bus voltage

The result is rounded up to the nearest standard battery rating (7, 18, 26, 42, 65, 100, 120, or 130 Ah). Parallel packs are added when the per-string Ah exceeds the largest standard battery.

Note: These figures are nominal estimates for planning and tender preparation. Real-world runtime depends on battery age, ambient temperature, depth of discharge, inverter efficiency, and actual load. Always validate the final design against the battery manufacturer's discharge curves before procurement.