What happens if my load exceeds the UPS watt rating?

The UPS will refuse the load or shut down on battery. Move to a higher-wattage unit or shed load.

How much UPS runtime do I actually need?

Enough to ride through the majority of outages that last under a few minutes, plus time for a clean shutdown — for a home server, 10–15 minutes with automatic shutdown (apcupsd/NUT) beats an hour of blind runtime.

Should I get a UPS or a power station?

A UPS switches in milliseconds and can signal your server to shut down; a power station holds 5–20× the energy. Serious setups use both: UPS for the cutover, power station for the long haul.

WattNeeded / UPS runtime calculator

How long will your UPS actually last?

The runtime printed on the box assumes a brand-new battery at half load. This calculator uses your real devices, inverter losses, and battery age — and tells you what you'd need for the runtime you actually want.

1 · What's plugged into it?

	Device	Qty

Add your own

2 · Your UPS

Size Battery age

Battery Wh values are typical for consumer line-interactive UPSes (12 V × 7–9 Ah blocks). If you know your exact battery, use Custom: Wh = volts × amp-hours × number of blocks.

Your runtime

0 minutes

Total load0 W

Load vs UPS max output0%

Usable battery energy0 Wh

Want minutes instead?

You'd need 0 Wh of battery —

[Affiliate recommendations slot — UPS models matching the required Wh/W, populated once affiliate programs are joined.]

How this calculator works (the honest math)

runtime = battery Wh × 0.85 inverter efficiency × age factor ÷ load watts × 60. Real runtimes at low loads are slightly worse than this (fixed electronics overhead); at very high loads slightly worse again (battery Peukert effect). Treat the answer as an honest central estimate, not a guarantee.

The VA rating on a UPS is not watts and says almost nothing about runtime — it's the electrical ceiling of the inverter. Runtime comes from the battery's watt-hours, which manufacturers rarely print on the front. Two 1500 VA units can differ 2× in battery size.

Battery ageing is the number everyone ignores: sealed lead-acid UPS batteries lose roughly a fifth of their capacity every two years even when treated well. If your UPS is four years old, size your expectations (or your replacement battery order) accordingly.

My load exceeds the UPS watt rating — what happens?: The UPS will refuse the load or shut down on battery. The calculator warns you; move to a higher-wattage unit or shed load.
How much runtime do I actually need?: Enough to ride through the 95% of outages that last under a few minutes, plus time for a clean shutdown of anything with state — for a home server, 10–15 minutes with automatic shutdown (apcupsd/NUT) beats an hour of blind runtime.
UPS or power station?: A UPS switches in milliseconds and talks to your server for auto-shutdown; a power station holds 5–20× the energy. Serious setups use both: UPS for the cutover and shutdown signal, power station for the long haul. Our home backup calculator sizes the second half.