For as long as the maker’s runtime chart says at your particular load, which is usually a very different figure from the one on the carton. Runtime falls steeply as load rises, and the headline minutes assume a load almost nobody runs.
There is no single answer, and the reason is worth understanding rather than working around.
Runtime is a function, not a number
A UPS holds a fixed amount of energy. How long that lasts depends entirely on how quickly it is being drawn, and the relationship is not a straight line: a battery supplying a heavy load yields less total energy than the same battery supplying a light one.
So makers publish a chart — runtime against load — rather than a single figure. Any number quoted on packaging corresponds to one point on that chart, and it is normally a point far to the light end.
How to get your own figure
Add up the published consumption of everything you intend to plug in, in watts. Find that total on the maker’s chart or calculator for the model you are considering. Read the minutes off.
Then subtract some, because the battery will not be new for long and because a load rarely stays at its quiet figure. An enclosure spinning up several drives at once is drawing far more than the same enclosure idling.
What you should be buying the minutes for
Most power interruptions are short, and a UPS covers them completely; the equipment never notices and neither does anybody else.
For the ones that are not short, the useful outcome is an orderly shutdown. The enclosure closes its volumes, finishes what it was writing and powers down cleanly, and comes back up without a check when the supply returns.
That is why the data connection between the UPS and the enclosure matters more than the last few minutes of runtime. Both makers publish which units they support, and the setting that says at what point the shutdown begins is on the enclosure.
What shortens it, sometimes surprisingly
Adding a monitor or a desktop machine to the same unit, which can multiply the load.
A battery several years old. Capacity falls with time and with each discharge. The maker publishes a self-test and a replacement interval, and a unit whose battery has never been thought about is a unit whose runtime is unknown.
Temperature. Batteries are sensitive to the conditions they sit in, and the maker states the range the unit is specified for.
What extends it
Removing things from it. A UPS carrying only a router, a modem, a switch and the enclosure lasts much longer than one carrying a workstation as well, and the argument for keeping the connection alive is a different argument from the one for keeping a desktop on.
Buying a unit with more headroom, which puts your load further down the steep part of the chart.
Nothing else. In particular, no setting on the UPS creates energy that is not in the battery.
And the boundary
How the unit is installed and sited, what its own instructions permit to be connected to it, and when its battery is replaced are all matters for the manufacturer’s documentation. Anything that touches the building’s electrical installation is work for somebody qualified to do it.
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
Why does the runtime fall so much faster than the load rises?
Because a battery delivering a larger current yields less total energy than the same battery delivering a small one. That is a property of the chemistry rather than of the unit, and it is why makers publish a chart across a range of loads instead of one figure with a division to do.
Does the runtime get shorter as the unit ages?
Yes, and that is expected. Battery capacity falls over time and with each discharge, so a unit several years old will not hold what it held when new. Makers publish a self-test and a replacement interval for their own batteries, and following theirs is the whole of the maintenance.
Last reviewed 10 September 2026