UPS Sizing, Batteries and Shutdown: Getting It Right
A UPS is the component most likely to fail silently and most likely to be discovered failed during the event it exists for. This covers sizing one and keeping it working.
What a UPS is actually for
Two jobs, and confusing them leads to the wrong purchase.
Riding through short interruptions — the flickers and brief cuts that would otherwise reboot everything. This needs only seconds of runtime and is what most UPS purchases actually achieve.
Providing time for a controlled shutdown during a longer outage. This needs enough runtime for systems to shut down cleanly, plus margin.
What a UPS is not for, unless deliberately sized and budgeted: running through extended outages. That is a generator question, and a UPS bridges to it rather than replacing it.
Sizing: two numbers, not one
Load capacity — how much the UPS can supply at once, quoted in VA and watts. Watts is the number that matters; VA is larger and quoted more prominently.
Runtime at your load — how long it lasts. This is the number people get wrong, because runtime falls sharply as load rises. A UPS quoting long runtime at quarter load may give you minutes at three-quarter load.
So size by: total actual watts of everything connected, then check the runtime chart at that load, then add headroom for what you will add later. Our UPS sizing guide covers the arithmetic.
One trap: do not connect laser printers or similar to a UPS. Their startup draw can exceed the entire capacity and trip it.
Batteries are the failure point
Everything else in a UPS lasts. The batteries do not.
Batteries degrade continuously from installation, faster in heat. A UPS three or four years into its battery life may hold a fraction of its original runtime while reporting itself healthy, because a self-test draws far less than a real outage does.
Three consequences.
Replace batteries on schedule, not on failure. Our battery replacement guide covers timing and matching.
Test under real load periodically, not just by self-test.
Keep the room cool. Heat is what kills UPS batteries, and a UPS in a warm cupboard ages far faster than its rating assumes. Our equipment room guide covers this.
Shutdown integration: the part people skip
A UPS with no shutdown integration gives you runtime and nothing else. When the batteries run out, everything loses power exactly as if there were no UPS — just later.
What is needed: the UPS connected to the systems by network or USB, software on those systems configured to act on a low-battery signal, and the shutdown sequence actually tested.
Two things to get right. Shut down in the right order — virtual machines before hosts, hosts before storage. And start the shutdown with time to spare, not when the batteries are nearly flat, because shutdowns take longer than people estimate.
Where the UPS sits in the rack
Three practical points.
At the bottom. A UPS is the heaviest thing in the rack and a top-heavy rack can tip when a server is slid out. Our rack weight guide covers floor loading too.
With airflow. UPS units generate heat, and batteries in a hot pocket at the bottom of a sealed cabinet age quickly.
Feeding PDUs, not devices directly. The UPS feeds PDUs; devices connect to PDUs. And for redundant supplies, ideally two feeds from separate PDUs. Our PDU guide covers arrangement.
Buying used: what to check
UPS units themselves age well; batteries do not.
Assume the batteries need replacing and budget for it. A used UPS with old batteries is a UPS with no runtime.
Check the battery part number is still available for that model, because a UPS whose batteries cannot be sourced has a fixed remaining life.
Check the management card is present if you need network shutdown integration, since it is a separate part and frequently removed.
Our availability guide covers why checking part availability before buying matters on any older platform.
Frequently asked questions
How do I size a UPS?
By total actual watts of everything connected, then check the runtime chart at that load, then add headroom. Runtime falls sharply as load rises, so a figure quoted at quarter load tells you little about your situation.
Why does my UPS give less runtime than it used to?
Battery degradation, which is continuous from installation and faster in heat. A UPS several years into its battery life may hold a fraction of its original runtime while reporting healthy, because a self-test draws far less than a real outage.
Do I need shutdown software with a UPS?
Without it, the UPS gives you runtime and nothing else — when the batteries run out, everything loses power as if there were no UPS. Connect it to the systems, configure the low-battery action, and test the sequence.
Can I plug a laser printer into a UPS?
No. The startup draw can exceed the entire UPS capacity and trip it, taking down everything else connected.
Where should the UPS go in the rack?
At the bottom. It is the heaviest item and a top-heavy rack can tip when a server is slid out. Give it airflow, since batteries in a hot pocket at the base of a sealed cabinet age quickly.
Is a used UPS worth buying?
The unit ages well; the batteries do not. Assume batteries need replacing and budget for it, check the battery part number is still available for that model, and check the management card is present if you need network shutdown.
Tell us the total load and how long you need to shut down cleanly, and we will size the UPS against runtime at your actual load rather than the headline figure.
