UPS Battery Backup Calculator

Why Calculate UPS Backup Time?

An Uninterruptible Power Supply (UPS) protects equipment from power outages, brownouts, and surges. Calculating backup time tells you how long your equipment will run during a power cut — essential for servers, medical equipment, and critical workstations.

The Formula

Backup Time (hours) = (Battery Capacity Ah × Battery Voltage × Number of Batteries × 0.8 × Efficiency) / Load (W)

The 0.8 factor represents 80% Depth of Discharge (DoD) — discharging deeper shortens battery life significantly. UPS inverter efficiency is typically 85–95%.

How to Use This Calculator

Enter the battery voltage, capacity (Ah), number of batteries, UPS inverter efficiency, and the total connected load in watts. The calculator outputs backup time in minutes and hours.

Practical Examples

Example 1: 12V / 100Ah battery, 90% efficiency, 500W load. Usable Wh = 12 × 100 × 0.8 = 960 Wh. Backup = 960 × 0.9 / 500 = 1.73 hours.

Example 2: Server room with 3 × 12V/200Ah batteries, 85% efficiency, 2,000W load. Usable = 12 × 200 × 3 × 0.8 × 0.85 = 4,896 Wh. Backup = 4,896 / 2,000 = 2.45 hours.

Tips for Longer Backup Time

• Keep batteries at 20–25°C — heat dramatically reduces capacity.
• Test batteries quarterly; replace when capacity drops below 80%.
• Consider parallel battery strings for larger capacity requirements.