To choose a UPS, add up the watts the connected equipment actually draws, pick a unit whose watt rating (not VA) covers that with 20 to 30 percent to spare, and then choose the runtime you need: a few minutes to ride out flickers and shut down cleanly, or longer to keep the network up through an outage. For office desks and network closets a line-interactive UPS with a pure sine wave output is the usual choice; online double-conversion units are for servers and sensitive equipment.
Decide what the UPS is for
A UPS does three jobs, and which one you care about changes what you buy. First, it bridges short interruptions and flickers so equipment does not reboot. Second, it gives time to shut down cleanly during a longer outage. Third, it conditions the incoming power, smoothing sags and surges. Most desks need the first two. A network closet with the router, switches and Wi-Fi controller needs enough runtime that staff on notebooks keep working through a short outage. Servers and storage need clean power and an automatic shutdown signal.
Watts versus VA: size by watts
UPS models are sold by VA (volt-amperes), but the number that limits what you can plug in is the watt rating, which is usually lower. Ignore the VA figure for sizing. Add up the real power draw of the equipment (a plug-in power meter is more accurate than the number on the label, which is a maximum) and choose a UPS whose watt rating exceeds that total by 20 to 30 percent.
Typical desk loads are modest: a notebook, dock and two monitors draw far less than a desktop workstation. A small network closet with a router, switches and modem is usually light too, unless the switches supply a lot of PoE power, which counts against the UPS.
Runtime: how long do you need?
Runtime depends on the load: the same UPS lasts far longer at 20 percent load than at 80 percent. Manufacturers publish runtime charts by load, and those charts are the numbers to use. For a desk, five to ten minutes at your load is enough to save work and shut down. For a network closet, decide how long you want phones and Wi-Fi to survive; thirty minutes to an hour is a common target, which usually means a larger unit or an external battery pack. Every UPS should be paired with software or a network card that shuts servers and desktops down cleanly before the battery is empty.
Line-interactive or online
Standby (offline) UPS units are the least expensive and switch to battery only when power fails; they suit basic desks. Line-interactive units add automatic voltage regulation, which corrects sags and surges without draining the battery, and are the standard choice for offices and network closets. Online double-conversion units rebuild the power continuously so the equipment never sees the utility supply at all; they cost more, generate heat and fan noise, and are meant for servers, medical or lab gear and areas with unreliable power. Look for a pure sine wave output on any UPS that will run a desktop or server: many modern power supplies misbehave on the stepped output of entry-level units.
| Type | How it works | Suits |
|---|---|---|
| Standby | Passes utility power, switches to battery on failure | Basic desks, point-of-sale terminals |
| Line-interactive | Adds voltage regulation; battery for outages | Office desks, network closets, small servers |
| Online double-conversion | Continuously converts AC to DC to AC | Servers, sensitive equipment, poor utility power |
Outlets, form factor and management
Count the outlets you need and check how many are battery-backed versus surge-only; the surge-only outlets are for printers and lamps that should not drain the battery. In Canada, desk and closet units use standard NEMA 5-15 plugs and outlets on 120 V; larger units may need a NEMA 5-20 or 30 A circuit, so confirm the receptacle you have before buying. Tower units sit under a desk; rack-mount units (1U or 2U) go in a network cabinet alongside switches. A network management card lets you monitor the UPS remotely and trigger shutdowns across several machines. Batteries are consumables that last roughly three to five years; choose a model with user-replaceable batteries and note the replacement cost.
Do not plug laser printers into the battery outlets; their heaters draw surges that can trip a small UPS.
Quick checklist
- Measure or add up the real watt draw; buy a UPS with 20 to 30 percent watt headroom.
- Use the manufacturer's runtime chart at your load; add an external battery pack for long runtimes.
- Line-interactive with pure sine wave for most desks and closets; online for servers.
- Enough battery-backed outlets, correct plug type for your circuit.
- Tower or rack-mount to suit the location; management card for closets.
- User-replaceable batteries; plan a replacement every three to five years.
See the range of uninterruptible power supplies, and if you are protecting many desks or several sites, ask NetraHub for a volume quote.
Common questions
What size UPS do I need for a home or small office?
Add up the watts of what you will plug into the battery outlets, then choose a UPS whose watt rating is at least 20 to 30 percent higher. A notebook, dock and two monitors are a light load; a desktop workstation is heavier. Then check the runtime chart at that load. Do not size by VA alone, because the watt rating is what limits the load.
Why is the watt rating lower than the VA rating?
VA is apparent power and watts is real power; the ratio between them is the power factor of the UPS output. Most UPS units are rated with a power factor between about 0.6 and 1.0, so a 1500 VA unit might support anywhere from 900 W to 1500 W. Always check the watt figure, since it is the one your equipment will hit first.
Do I need a pure sine wave UPS?
For desktops, servers, network storage and anything with an active power-factor-corrected supply, yes. Those supplies can shut down or buzz on the simulated (stepped) sine wave that entry-level UPS units produce on battery. For a router, switch and modem, a simulated sine wave is usually fine. If in doubt, pure sine wave is the safe choice.
How long will a UPS keep my router and switch running?
It depends on the unit's battery capacity and the load. Network gear draws little power, so a mid-sized line-interactive UPS often runs a router, modem and small switch for well over half an hour, and adding an external battery pack extends that to hours. Check the manufacturer's runtime chart at your measured load rather than relying on the headline runtime.
How often do UPS batteries need replacing?
Sealed lead-acid batteries typically last three to five years, less in a warm closet. Most units run a self-test and warn when the battery is failing, and many let you replace it without shutting down. Some newer models use lithium batteries with longer life at a higher price. Put battery replacement on the same schedule as other office maintenance so it is not forgotten.