How to Choose and Size a UPS
Most undersized UPS units were sized by adding up the numbers printed on the back of each device. Those are maximum ratings, not what the equipment draws, so the result is usually far too large — and occasionally, when someone applies a discount by instinct, far too small. Measuring takes ten minutes and removes the guesswork entirely.
The sizing method in four steps
- Measure the actual load in watts, rather than summing nameplate ratings.
- Add headroom — 25% is a reasonable working figure for growth and battery ageing.
- Choose runtime based on what has to happen during an outage.
- Check the watt rating, not the VA rating, against that total.
Measure, do not add up nameplates
A server's power supply might be rated 750W. Under normal load it may draw a fraction of that. Sizing on the label produces a UPS several times larger than needed, which costs money up front and, less obviously, runs inefficiently at very low load.
- Use a plug-in power meter or a metered PDU Measure what each device actually draws at the wall during normal operation. This is the number to size against.
- Measure at a busy time, not overnight Servers draw more under load. Take the reading when the systems are doing real work.
- Account for startup surge Devices with motors or spinning disks draw more at power-on than in steady state. This matters when everything restarts together after an outage.
- Add the measurements to get total load in watts Keep it in watts throughout. Converting to VA introduces exactly the confusion you are trying to avoid.
- If you cannot measure, use nameplate with judgement Summing nameplates and taking a portion of it is a rough guide only. Where the load matters, measure — a metered PDU pays for itself the first time it stops you buying the wrong UPS.
It covers three things at once: batteries losing capacity as they age, equipment added over the UPS's life, and the surge when everything restarts together after an outage. A UPS loaded to 75% also delivers meaningfully longer runtime than one at 100%, because runtime scales non-linearly with load.
Choosing runtime
Runtime is a function of load, so it is quoted as a curve rather than a single figure. Decide what you want to happen during an outage, then read the curve at your load.
| Goal | Runtime needed |
|---|---|
| Ride through brief interruptions | A few minutes. Covers the large majority of real outages, which are seconds long. |
| Graceful automatic shutdown | Enough for your slowest system to close cleanly, plus margin. Time an actual shutdown rather than estimating. |
| Bridge to a generator | Enough to cover the changeover, with margin for a failed first start attempt. |
| Keep working through an outage | Extended battery packs, and a serious cost. Usually the wrong goal unless the work genuinely cannot pause. |
| Network gear only, keep connectivity up | Small load, so long runtimes are achievable cheaply. Often worth protecting separately from servers. |
Graceful shutdown takes as long as your slowest system needs, and virtual hosts with several guests can take considerably longer than people assume. Run a real shutdown with a stopwatch, then add margin. Sizing runtime against a guess is how a UPS runs out of battery halfway through the shutdown it was bought to enable.
What else to specify
- Network management card. Lets the UPS be monitored, alert on battery faults, and trigger shutdowns independently of any host. On unattended equipment this is close to essential — it is also how you find out a battery has failed before you need it.
- Outlet groups. Better units allow outlets to be switched independently, so non-critical loads can be dropped to extend runtime for what matters.
- Extended battery pack capability, if longer runtime may be wanted later.
- Rack or tower, and the rack's weight rating. UPS batteries are heavy; mount low.
- Input circuit. Larger units often need a dedicated higher-voltage circuit with a specific plug. Confirm with an electrician before ordering, because this can be the long pole.
- Maintenance bypass, for anything where the load cannot be dropped to service the UPS.
- Replaceable battery cartridges, and whether they are hot-swappable. Batteries are consumables — see battery testing and replacement.
What to protect, and what not to
| Equipment | Guidance |
|---|---|
| Servers, storage, virtual hosts | Battery protection with automatic shutdown. The core case. |
| Network switches and firewalls | Protect these too. A running server with no network is not much use, and they draw little. |
| PoE switches | Remember the PoE budget is part of the load — see planning a network deployment. |
| Point of sale terminals | Worth protecting — an outage mid-transaction is a real problem. |
| Laser printers | Surge-only outlet, never battery. The fuser surge can trip an overload. |
| Space heaters, kettles, vacuums | Never. This sounds obvious and it happens constantly in offices. |
Six months later, nobody remembers which outlets are battery-backed and which are surge-only, and somebody plugs a printer or a heater into the wrong one. A label at install time prevents an overload that takes down the equipment the UPS was bought to protect.
A shortlist that avoids the usual regrets
- Measure the real load in watts at a busy time.
- Add around 25% headroom.
- Time a real shutdown and set runtime from that.
- Pick the topology from your power quality and equipment sensitivity — see topologies and VA vs watts.
- Check the watt rating, not the VA rating.
- Include a management card for anything unattended.
- Confirm the input circuit and plug with an electrician before ordering.
Official manufacturer resources
Runtime curves, watt ratings, and input requirements are model-specific. Confirm against the datasheet for the exact part number before ordering.
Send us your measured load
Total watts, what has to stay up, and how long you need. We will size it properly, flag whether the input circuit needs electrical work, and tell you if a smaller unit would do the job.
Still stuck? Talk to someone who works on this hardware daily.
Tell us the model, what you have already tried, and what you are seeing. We will tell you whether it is a setting, a consumable, or a part — and we will say so if you do not need to buy anything.
