UPS Sizing & Runtime Calculator
Size a UPS from your load in watts with proper headroom, or estimate battery runtime from Ah and voltage. Honest derating for inverter loss and high-rate discharge. Free.
VA vs watts, and why runtime is separate
UPS units are sold by VA (apparent power) but your gear draws watts (real power). VA = watts ÷ power factor, and modern power-factor-corrected IT loads sit around 0.9–1.0. Size so your real load lands at or below ~80% of the UPS rating — running a UPS flat-out shortens both its battery life and its ability to handle inrush.
Runtime is a different question entirely. It's set by the battery, not the VA rating, which is why two UPS units of the same VA can have very different runtimes. A UPS holds your load up long enough for a graceful shutdown or a generator to start — most double-conversion and line-interactive units give only a handful of minutes at full load by design.
| Typical load | Approx. watts | Suggested UPS |
|---|---|---|
| Single desktop + monitor | 150–300 W | 650–850 VA |
| Small NAS + network gear | 100–200 W | 650–1000 VA |
| Wall-mount rack (switch, firewall, NAS) | 300–500 W | 1000–1500 VA |
| Half-rack of servers | 800–1500 W | 2200–3000 VA |
Planning UPS coverage for a server room or closet? See UPS sizing for a small-business server room for load auditing, runtime targets, and when to step up to an external battery pack.