Choosing the correct number of batteries for an Online UPS is essential to ensure that the backup time and power requirements are met without fail. Undersizing can lead to unexpected shutdowns, while oversizing slightly helps meet regulatory thresholds and provides added assurance.

In this guide, we explain how to determine battery requirements for an Online UPS system using the VAH-based method—especially relevant for government or institutional purchases on GeM (Government e-Marketplace), where sizing standards are well-defined.

The VAH Formula (As per GeM Standards)

Battery sizing for Online UPS systems on GeM is based on the following formula:

VAH = 1600 × UPS Rating in kVA × Required Backup Time (in hours)

This gives the minimum total battery capacity needed to provide the desired backup.

To ensure compliance and performance, it's always advisable to round the VAH upwards and select a battery configuration that meets or exceeds this figure.

Step-by-Step Example: 3 kVA UPS, 1 Hour Backup

Let’s say you’re installing a 3 kVA Online UPS and require 1 hour of backup time.

Using the formula:

1600 × 3 × 1 = 4800 VAH

Now assume you're using 12V, 26AH batteries, a common standard for commercial setups.

Each battery provides:

12V × 26AH = 312 VAH

To meet or slightly exceed 4800 VAH:

4800 ÷ 312 = 15.38
Round up → Use 16 batteries

This not only fulfills the VAH requirement but also aligns well with a 192V DC bus system (16 batteries × 12V = 192V), which is common in many Online UPS units.

Example: 2 kVA UPS, 1 Hour Backup

Apply the formula:

1600 × 2 × 1 = 3200 VAH

Using 12V, 26AH batteries (312 VAH each):

3200 ÷ 312 = 10.25
Round up → Use 12 batteries

This configuration meets the requirement and matches UPS systems with a 144V DC bus (12 × 12V = 144V), ensuring proper operation.

Example: 5 kVA UPS, 1 Hour Backup Using 42AH Batteries

Now consider a 5 kVA UPS with 1 hour of required backup. Let’s assume you’re using 12V, 42AH batteries, often preferred for longer runtime or space-constrained installations.

First calculate the VAH:

1600 × 5 × 1 = 8000 VAH

Each 42AH battery provides:

12V × 42AH = 504 VAH
8000 ÷ 504 = 15.87
Round up → Use 16 batteries

This configuration yields 16 × 504 = 8064 VAH—comfortably above the required minimum, and also suits a 192V DC UPS setup.

Important Notes

• Always round up the battery count to exceed the VAH minimum.

• Ensure the total voltage of the battery bank matches the DC input voltage of the UPS (commonly 96V, 120V, 144V, 180V, or 192V).

• SMF-VRLA batteries are typically preferred for their maintenance-free operation.

• Proper cabling and rack mounting are essential for system safety and battery longevity.

• If higher backup time is needed, increase the battery AH rating, or add parallel battery banks while keeping voltage constant.

Summary Guidelines

• Use the formula 1600 × kVA × hours to calculate required VAH.

• Divide the VAH by the VAH per battery (Voltage × AH) to get the number of batteries.

• Always round the result upwards.

• Match the number of batteries to the UPS’s DC voltage in multiples of 12V (e.g., 12, 15, 16).

• Ensure you slightly oversize to account for battery aging and tolerance.

By following these steps, you ensure that your Online UPS system delivers the runtime you expect, stays compliant with procurement standards, and offers peace of mind during critical outages.