What a battery backup actually protects
Picture a retail shop that loses power for two seconds during a thunderstorm. The lights flicker, the register reboots, and the owner assumes that is the only interruption. What actually happened is that the cable modem lost power, the router lost power, and the credit card terminal lost its network connection mid-transaction. The sale did not go through. The customer's card was not charged, but the terminal shows an error and no one is sure whether to retry or whether that will double-charge. The customer leaves. The register was on a battery backup, but nothing else was.
Most people think a UPS is for keeping the computer running long enough to save work and shut down. That is one use, but in a small business the more common problem is that a momentary power loss takes down the network, the phones, and anything that depends on internet access. A modern point of sale system does not process cards locally. It sends the transaction over the network to a payment processor. If the network is down, the sale stops. The register can stay on, but it cannot complete a transaction. The same is true for a VoIP phone system. If the modem and router lose power, the phones stop working even if the phone devices themselves are still powered.
What to put on a UPS
The modem and router are the first priority. These are the devices that provide network access to everything else. A typical cable modem draws 10 to 20 watts. A small business router draws 10 to 30 watts. A managed switch adds another 20 to 50 watts depending on the number of ports and whether it is providing power over Ethernet to other devices. Add those up and you have the baseline load.
If you have a VoIP phone system, the phone server or base station should be on the UPS as well. A cloud-hosted phone system only needs the modem and router to stay up, but an on-premises system needs its own hardware powered. Individual desk phones can usually be left off the UPS unless they are critical, but the main system should stay up.
Credit card terminals that connect over Ethernet or Wi-Fi depend on the network, so keeping the modem and router up keeps the terminals working. Terminals that connect over a phone line need the modem to stay up if you are using VoIP. Old analog phone lines do not need a UPS because the phone company powers the line, but almost no one has those anymore.
How to size a UPS
Add up the wattage of everything you want to keep running. Most devices have a label that lists watts or amps. If it lists amps, multiply by 120 to get watts. If the label only lists volts and amps, multiply those together. A modem, router, and switch together typically draw 50 to 100 watts total. A UPS rated for 350 to 600 volt-amperes will handle that with room to spare.
Runtime depends on the battery capacity and the load. A 600 VA UPS with a 50-watt load will run for about an hour. With a 100-watt load, it will run for 30 minutes. You do not need hours of runtime for network equipment. You need enough time for a brief outage to pass or for you to realize the power is out and shut things down properly. Fifteen to thirty minutes is usually enough. If you need longer, you need a generator, not a bigger UPS.
The mistake most people make is buying a UPS, plugging it in, and forgetting about it. The batteries inside a UPS last about three years. After that, they start to lose capacity. A UPS that used to provide thirty minutes of runtime might only provide five minutes, or it might fail immediately when the power goes out. The UPS does not warn you that the batteries are weak until you actually lose power and discover it does not work. Some models have a self-test feature or a battery indicator light, but most people do not check those.
Where this fits
When I set up a network, the UPS goes in at the same time as the modem and router. I size it for the actual load, label what is plugged into it, and set a reminder to check the battery every two years. If you are running your own equipment and have not thought about this, check what is currently on a UPS and what is not. If your payment processing or phones go down during a power blip, you now know why.
A UPS is not complicated, but it only works if it is protecting the right things and if the batteries are still good. Most of the time, neither of those is true until someone fixes it on purpose.