Add up what the equipment actually draws, compare it against both figures a UPS publishes — the volt-amperes and the watts — and choose on the one you exceed first. Then read the maker’s runtime chart at your load, not at the headline figure.
Two published figures, one load calculation and one chart. In that order.
Work out the load first
Add up what each piece of equipment draws in normal operation, from the figures its maker publishes. A network enclosure, a router, a switch and a modem are a modest total; the enclosure is usually most of it, and it is highest at start-up when every drive spins at once.
Use the equipment makers’ published consumption figures rather than the rating printed on a power supply, which states what the supply can deliver rather than what the device takes.
Both figures on the UPS matter
A UPS publishes two numbers and they are not the same quantity. Volt-amperes describe apparent power — the product of voltage and current — and watts describe real power, which is what the equipment consumes. The relationship between them is the power factor, and it is why a unit’s watt figure is always the lower of the two.
Whichever of the two your load reaches first is the limit. Sizing against volt-amperes alone is the common error, because it is the larger and more flattering number and it is the one printed largest.
Then read the runtime chart
A stated runtime always assumes a load, and the maker names it. Runtime rises steeply as the load falls, which is why the headline minutes on a carton correspond to a load nobody runs.
Makers publish a chart or a calculator giving runtime against load for each model. Find your total on it. That is the honest answer to how long you have, and it is the number to plan the shutdown around.
What the runtime is actually for
Not for working through a power cut. For two things: riding through the brief interruptions that make up most outages, and giving the enclosure enough time to close its volumes and shut down cleanly when the outage is not brief.
An enclosure interrupted mid-write is the case a UPS exists to prevent, and a shutdown that completes is worth far more than extra minutes that nobody uses.
Which kind of UPS
Standby units switch to battery when the mains fails. Line-interactive units also correct voltage that is high or low without going to battery, which matters where the supply sags. Online units run everything from the inverter continuously.
Makers state which their model is, and the middle kind is the usual answer for a small network. What the equipment tolerates in the shape of the output waveform is stated by the equipment maker, and that is a line to check for a machine with an unusual power supply.
And the part that is not a comparison-page matter
A UPS and everything around it is installed, sized, connected and replaced as its own instructions say. Where those instructions refer to the building’s electrical installation, that is a matter for someone qualified to work on it, and no specification sheet substitutes for the manufacturer’s own documentation.
The manufacturer’s own documentation comes first, and this page is not a substitute for it. Every rate, capacity, class figure, endurance rating, port speed and power figure on this page is reported from the manufacturer's own documentation, with its unit written as published, and named as such — none of it is measured here. A stated capacity is reported as the maker counts it, and is never restated in another unit, because the two are not the same figure. And for anything wired to the mains: a UPS and a surge protector are installed, sized and replaced as their own instructions say, and nowhere else. Volt-amperes are not watts, a stated runtime assumes a load the maker names, and a joule rating describes what a protection device absorbs before it stops protecting.
The questions that come up before an order
Should the UPS also carry a desktop machine and a monitor?
That depends what you want from it. A UPS sized for a NAS, a router and a switch is buying an orderly shutdown and a ride through short interruptions. Adding a desktop and a screen raises the load considerably and shortens the runtime for everything on it, so it is a decision to make deliberately rather than by adding sockets.
Does the enclosure need to be told about the UPS?
For an orderly shutdown, yes. Most network enclosures can watch a UPS over its data connection and shut themselves down when the charge falls to a level you set. That link is what turns a UPS from a delay into a protection, and both makers publish which models they support.
Last reviewed 10 September 2026