UPS & Battery Sizing4 min read
UPS runtime and battery sizing
Understand why real watt load, VA rating, inverter efficiency, and battery age govern actual backup duration.
1. Watts vs. VA ratings
A 3 kVA online UPS with a 0.8 or 0.9 power factor supports 2,400 W to 2,700 W of active load—not 3,000 W. Exceeding the watt limit triggers overload alarms or forces static bypass transfer even when the VA percentage appears below 100%.
2. Why batteries lose runtime early
Lead-acid batteries lose usable amp-hour capacity rapidly under high discharge rates (Peukert effect) and elevated ambient temperatures. One weak battery in a series string pulls down the entire bank voltage under load, causing early low-battery shutdown.
- Measure actual connected load in watts before calculating battery Ah
- Account for UPS conversion efficiency (typically 85%–92%)
- Check battery bank date and equalize or replace matched strings together