A UPS for network equipment keeps the devices that carry your internet and phones running through short power outages and long enough to shut down cleanly in long ones. Put the carrier's equipment, your firewall, your core and PoE switches and any backup internet router on it. Size it by adding up the real watts those devices draw, keep the load well under the unit's rating, and check the manufacturer's runtime chart for the minutes you need. Then replace the batteries on a schedule, because a UPS with a dead battery is just an expensive power strip.
Many businesses pay for a second internet circuit and then plug both routers into the same wall outlet. When the building loses power, the backup line may still be up on the street, and nothing in the office can reach it. A battery backup unit in the network closet closes that gap.
What a UPS does, and what it does not
An uninterruptible power supply sits between the wall and your equipment. When mains power drops, sags or spikes, it switches to its battery, so the devices behind it never notice. It rides through the flickers and short outages that otherwise reboot your firewall and drop every call, buys time in a long outage to keep working or shut down cleanly, and filters the surges and brownouts that shorten the life of network gear.
It is not a generator. A typical small UPS is sized in minutes, not hours. If the office must run through a long outage, that is a generator question, and the UPS still bridges the seconds before a generator starts and takes the load.
What to put on battery
Start with the path a packet takes from your desk to the street, and protect every powered box along it:
- The carrier's equipment at your demarc: the ONT, network interface device, modem or carrier router. If this loses power, nothing behind it matters. Our guide to the demarc and inside wiring explains what usually sits there.
- Your firewall or router.
- The core switch and any PoE switches. Desk phones, Wi-Fi access points and cameras powered over Ethernet all go dark when their switch does, so one UPS on the switch protects all of them.
- The backup internet device, whether a second circuit's equipment or a cellular router. See do you need backup internet for the options.
- Anything that must stay up for your business to work, such as an on-site phone system, a small server or a network storage box.
Leave off laser printers, space heaters and anything with a heating element. They draw large surges and will drain or overload the unit. Many units have some outlets with surge protection only; use those for the monitor and the label printer.
The carrier side of the outage
A power outage in your building does not always mean the carrier's network is down. Fiber networks often keep running when one building goes dark, but the equipment at your end still needs local power. Coax networks rely on powered equipment in the neighborhood, usually with its own backup, which may or may not last through a long, widespread outage; that varies by provider and area. Ask your carrier whether its on-site equipment includes any battery of its own and how long it is designed to last, rather than assuming.
Sizing: watts, VA and headroom
UPS units are rated in two numbers: volt-amperes (VA) and watts. The watt rating is the one to size against, because it is the real power the unit can deliver, and it is always equal to or lower than the VA figure. A unit labelled 1500 VA might be rated for somewhere around 900 to 1,000 watts, depending on the model.
To size one:
- List every device that will plug into it.
- Find each one's power draw from its label or datasheet. Datasheet maximums overstate normal use, which gives you a safety margin. If you want the real figure, a plug-in power meter or the UPS's own load display will tell you.
- For PoE switches, count the switch's own draw plus the power it sends to phones, access points and cameras. The switch datasheet lists both its idle draw and its maximum with a full PoE load. We explain PoE budgets in managed vs unmanaged switches.
- Add headroom. Common practice is to keep the load comfortably below the watt rating, leaving room for growth and for devices that draw more at startup. Running near 100% shortens runtime sharply and gives you nowhere to go when you add an access point.
Runtime: read the chart, not the box
Runtime depends on the load. The same unit that lasts a long time with a light load can last only a few minutes at full load, and the relationship is not a straight line: halving the load usually more than doubles the runtime. Every manufacturer publishes a runtime chart or calculator for each model. Look up your total watts on it and read off the minutes.
Then decide how many minutes you actually need:
- Ride-through only: enough to cover flickers and short outages, plus time for a clean shutdown. A few minutes may do.
- Keep phones and internet up during a typical local outage: longer. Ask your facilities team or landlord how long outages usually last at your building.
- Bridge to a generator: enough to cover the generator's start and transfer time, with margin in case it does not start first time.
If the chart shows less runtime than you need, the options are a larger unit, an extended battery pack where the model supports one, or reducing the load on battery.
The three types of UPS
| Type | How it works | Good fit | Trade-off |
|---|---|---|---|
| Standby (offline) | Passes wall power through and switches to battery when it fails | A single modem and router at a small office | Least protection against sags and surges; often a stepped output waveform |
| Line-interactive | Adds voltage regulation that corrects sags and surges without using the battery | Most small business network closets | Still switches to battery briefly when power fails |
| Online (double conversion) | Runs the equipment from the inverter all the time, so there is no switchover | Server rooms, sites with poor power, sites with a generator | Costs more, runs warmer, uses more electricity |
Look at the output waveform too. Units described as pure sine wave produce power much like the grid. Cheaper units produce a stepped approximation, which most simple network gear tolerates, but some power supplies, particularly those with active power factor correction, can misbehave on it. For a closet of switches and firewalls, a line-interactive unit with sine wave output is a sensible default.
Shutdown, alerts and load shedding
A UPS can do more than keep the lights on. Many models offer:
- A USB or network connection that tells a server or storage box to shut down cleanly when the battery runs low.
- A network management card that sends alerts by email or through your monitoring system when the UPS goes on battery, fails a self-test or needs a new battery. If you already watch your network, add the UPS to the list; see network monitoring for small business.
- Outlet groups on some rack-mount models, which switch off less important equipment after a set time on battery, so the remaining charge goes to the internet and phones. Some PoE switches can do the same for chosen ports, such as storeroom cameras.
Batteries wear out
The sealed lead-acid batteries in most small and mid-sized UPS units commonly last around three to five years, and heat shortens that. A UPS in a hot closet with no ventilation will need batteries sooner. Lithium-ion models last longer and weigh less but cost more upfront.
Put the replacement date on a sticker on the unit and in your calendar, let the UPS run its self-tests, and replace the battery as soon as it warns you or fails a test, not after the next outage. Keep the closet ventilated.
A worked example
Hypothetical: the office, devices and power figures are invented to show the method. Use your own datasheets.
A 20-person office has a single network closet with:
| Device | Power used for sizing |
|---|---|
| Carrier ONT and router | 25 W |
| Firewall | 30 W |
| 24-port PoE switch, its own draw | 40 W |
| PoE load: 16 desk phones at 5 W and 4 access points at 15 W | 140 W |
| Cellular backup router | 10 W |
| Total | 245 W |
The owner wants phones and internet to stay up for at least 30 minutes, since most outages at the building are short, and the office has no generator. A line-interactive unit with a watt rating around four times the load keeps it at roughly a quarter of capacity, leaving plenty of headroom. The owner checks the manufacturer's runtime chart at 245 W. If it shows less than 30 minutes, the choice is an extended battery pack or a schedule that switches off the four ceiling cameras' PoE ports after five minutes on battery. The unit's network card is set to send an alert when it switches to battery, so someone knows to start the outage plan.
Then comes the test nobody likes: on a quiet evening, unplug the UPS from the wall and confirm that the internet, a desk phone and the Wi-Fi all stay up, and that the alert arrives.
Where the UPS fits in the wider plan
Battery backup is one line in a business continuity plan for connectivity, alongside the backup circuit and call forwarding. VoIP desk phones go dark without power, so decide whether the switch that feeds them is on battery, and whether staff can call 911 from a mobile if not. Our guide to E911 rules for business phones covers the location rules for multi-line systems.
Questions to ask your provider or installer
- Does your on-site equipment include its own battery backup, and how long is it designed to last?
- What does each device in the closet draw, and what is the total with a full PoE load?
- How many minutes of runtime will this UPS give at our load, from the manufacturer's chart?
- When are the batteries due for replacement, and who tracks that?
Getting the closet and the circuits right together
Our managed network service covers the switches and Wi-Fi in the closet and monitors them, and we price the internet circuit and a backup line across several Tier 1 carriers. The carrier pays FiberX, so nothing is marked up, and if what you have already wins we will tell you. The quote is free with no obligation, you usually hear back the same day, and Phil Morales is your one point of contact. Send us your address, call 478-758-8091 or text (347) 870-0965.
