UPS Basics
Uninterruptible power supplies — topologies, sizing, and run-time estimation.
Reference
Topologies
| Type | Transfer time | Output on mains | Use |
|---|---|---|---|
| Offline / standby | 4–25 ms | Filtered mains | Home PC, light loads |
| Line-interactive | 2–10 ms | AVR-regulated mains | Small business |
| Double-conversion (online) | 0 ms | Synthesized from inverter always | Data center, sensitive gear |
Sizing
- VA vs W
- VA = V · I (apparent); W = real. Size UPS so both ratings handle the load
- Power factor
- Old PSU PF ≈ 0.6; modern PFC ≈ 0.95. Match UPS rating accordingly
- Run-time
- Battery Wh / load W. Derate ~15% for inverter efficiency
- Headroom
- Size UPS 1.2–1.5× expected peak for surge
Battery
- Sealed lead-acid (SLA/VRLA): cheap, heavy, ~3–5 year life. Most consumer UPSes.
- LiFePO₄: 10+ year life, lighter, higher cost. Common in newer UPSes.
- Replace battery when runtime drops below ~80% of original.
- UPS batteries degrade faster in hot environments — rule: 10 °C rise halves life.
Rules of thumb
- PC + monitor
- 200–400 W typical
- Server
- 250–800 W per 1U
- Home network gear
- 30–80 W
- Home NAS (4-bay)
- 40–80 W
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