UPS Battery Runtime

The question every UPS purchase eventually comes down to: if the power drops right now, how long do I actually have before this thing dies? Enter the battery capacity, voltage and your connected load, and this tells you — because "big enough" isn't a spec, and finding out your runtime was three minutes short during an actual outage is not how you want to learn that.

The maths behind the number

Runtime comes from the battery's energy capacity in watt-hours: Wh = Voltage × Amp-Hours × Efficiency. A 12V 7Ah battery holds about 84Wh. Feed that a 50W load at 95% UPS efficiency and you get roughly 1 hour 35 minutes — but that load figure needs to include everything actually plugged in: the server, the switch it talks to, any attached storage, not just the one box you were thinking about when you bought the UPS.

Which type you actually need

• Standby (offline) — cheapest, switches to battery on failure with a 5–25ms transfer time. Fine for a desktop, overkill concern for anything you actually depend on.
• Line-interactive — regulates voltage without switching to battery for minor fluctuations, 2–8ms transfer time. The sensible default for servers and network gear, especially anywhere the mains supply isn't rock solid.
• Online double-conversion — always running on battery, zero transfer time. Costs more, runs hotter, but it's what you want for anything genuinely critical — medical, telecoms, storage arrays that can't tolerate even a millisecond blip.

Sizing rules that actually get followed (and ones that don't)

Size for 70–80% of rated VA, not 100% — running a UPS flat out shortens battery life and generates more heat than the spec sheet's happy path accounts for. Make sure runtime covers however long a graceful shutdown actually takes (usually 10–15 minutes for a server, longer if it's got a lot to flush to disk). And the one that gets skipped most often: batteries degrade — replace every 3–5 years, and actually test the UPS on a schedule rather than assuming it'll perform to spec the one time you need it, because a battery that's quietly lost 40% of its capacity looks completely fine right up until the power actually goes out.