UPS Battery Sizing Guide for Industrial Loads
UPS runtime is not a rating on the box; it is the battery you hang on the DC bus. Size it from the real watt load, the backup minutes you need, and the bank voltage.
Step 1: Watts, Not VA
Convert the UPS VA rating by its power factor (0.7-0.9) to get real watts. A 3000VA UPS at 0.9 pf delivers ~2700W.
Step 2: Amp-Hours From Backup Time
Ah = (Watts x backup_minutes) / (Bank_voltage x efficiency x DoD)
Example: 2700W for 10 minutes on a 48V bank, 0.9 efficiency, 80% DoD:
Ah = (2700 x 10) / (48 x 0.9 x 0.8) = 27000 / 34.56 ≈ 781 Wh / 48V ≈ 16.3Ah.
Step 3: Bank Voltage
Small UPS use 12V or 24V; larger 48V or 96V. Match the UPS DC bus; lithium and VRLA share the same nominal.
Why does my UPS show 10 minutes but last 3?
The rating assumes a linear load at full pf. Real loads with motors or poor pf draw more, cutting runtime.
Does lithium change the math?
No - same watt-hours. Lithium just holds voltage flatter, so usable runtime is often a bit longer than VRLA at the same Ah.
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