What Is a UPS?

What a UPS Does Exactly

A UPS sits between the wall outlet and the equipment. As long as there is power, it passes it along and at the same time charges the battery inside it. The moment the power drops, it switches to the battery within milliseconds — fast enough that the router and the switch don't notice at all, and don't reboot.

This matters more than it seems. Even a one-second interruption, when the lights just flicker, is enough to shut down a router. After the power returns, the router takes a minute or two to boot up and reconnect to the internet, and the phones need more time to register again. During that time, every call in progress is cut off and the office is unreachable. A UPS prevents all of this.

It also has a second, quiet job: protection. Power surges, brief voltage drops and "noise" on the power line wear out power supplies and shorten the life of equipment. A UPS filters out most of it. In an old building or in an area with frequent outages, the difference is noticeable.

Three types of UPS

Not every UPS is built the same way. The difference is in how it behaves when the power is on:

For most office communications cabinets, Line-interactive is the right balance between price and protection. An Online UPS is more expensive and noisier (it has a fan that runs all the time), but it delivers the cleanest power.

How long it lasts, and how to calculate it

Every UPS is labeled with two numbers: VA (volt-amperes) and watts. They tell you how much equipment you can connect to it at once — not how long it will last. Backup time depends on the ratio between the battery size and what is connected.

Example: the cabinet of a mid-sized office, with a router, a fiber converter and a PoE switch powering 15 phones, might draw 100–150 watts. A small cabinet UPS will keep it running for roughly twenty minutes to an hour. Every additional device you connect shortens that time. A desktop computer with a monitor can draw more by itself than the entire cabinet.

The practical rule: connect to the UPS only what must keep working — the router, fiber converter, switch, and maybe a gateway for analog lines. No laser printers (they draw a great deal of power at startup) and no air conditioner. Want a longer runtime? Some models support additional external batteries.

UPS and PoE: why the phones keep working

This is where the big advantage of PoE shows. An IP phone powered by a PoE switch isn't plugged into a wall outlet at all. If the switch is connected to a UPS, all the phones in the office are backed up through it — with no small UPS at every desk.

The UPS sits at the bottom of the cabinet, because it is the heaviest item. Above it is a power strip fed from it, and the switch, router and fiber converter are plugged into that. This way the whole chain — from the internet to the phone on the desk — keeps working when the power goes out.

It is important to remember the ends of the chain. If the building has another switch on a different floor, or a fiber converter in the building's communications room, they need backup too — otherwise the UPS in your cabinet is protecting a network with no way out.

And what happens in a long outage?

Here the difference between an on-premises PBX and a cloud PBX is especially large. A physical PBX in the office shuts down when the UPS runs out, and all the lines go with it. A cloud PBX sits in a data center with its own power backup, and is not affected by the power outage at your site at all.

Even after the battery in the office has run out, calls keep reaching the PBX. You can set it up in advance so that when the phones in the office are unavailable, calls are sent to a mobile phone, to another branch or to a voicemail box. The UPS gives you the first few minutes; the cloud PBX takes care of the rest.

In institutions that have a generator — a yeshiva with a dormitory, a hospital, a factory — the UPS acts as a bridge: it keeps the network up for the minute or two until the generator starts and stabilizes the voltage. Without it, every outage would restart the router even with a generator.

Maintenance: the battery doesn't last forever

• Batteries wear out. In most small UPS units the battery lasts about 3–5 years, depending on heat and the number of outages. A UPS that beeps for no apparent reason sometimes signals that it's time to replace it.
• Heat shortens its life. A closed, unventilated cabinet, or one next to a heater or in the sun, wears out the battery faster.
• Test it from time to time. Unplug the UPS from the wall for a minute and check that everything keeps working. It's better to discover a dead battery in a test than in a real outage.
• Don't connect more things "just for a moment." A splitter with a kettle on a communications cabinet UPS is a story that ends badly.

A worked example: choosing a UPS for a small cabinet

Let's take the communications cabinet of an office with eight phones and choose a UPS for it from start to finish. The first step is to list what is connected and what each item draws. You can find the numbers on the supplier's label or in the spec sheet; when they aren't available, use the estimates in the table:

Step two: output power. The UPS should comfortably deliver more than 85 watts, and almost every model does, since even the small ones deliver a few hundred watts. Note that the big number on the box is VA. The actual watts are usually about 60 percent of that in the basic models. 650 VA is roughly 360–390 watts.

Step three, which is what really decides it: time. The battery capacity is listed in the specs as voltage and amp-hours. A typical battery in a small model is 12 volts and 7 amp-hours, which is about 84 watt-hours on paper. In practice you get roughly half to two thirds of that, because of conversion losses and shutdown before the battery is fully drained. 84 times 0.6 is about 50 watt-hours, divided by 85 watts: about 35 minutes. If you want an hour, choose a model with two batteries or with an option for an external battery, not a model with a higher VA rating. VA determines how much equipment it can power, not for how long. Manufacturers publish a runtime-versus-load chart for each model, and it is worth checking it before you buy.

And step four: what else will be connected a year from now. A cabinet that starts with 8 phones grows to 16, and cameras show up without notice. Leave a margin of about 30 percent in the load.

What to connect, what not to, and which outlet to use

Most UPS units have two types of outlets, and they are not always clearly marked: battery + surge protection outlets, which keep working during an outage, and surge protection only outlets, which just filter voltage spikes and shut off along with the power. Equipment that must keep running goes into the battery outlets. Equipment you only want to protect goes into the protection-only outlets.

A few small rules that prevent problems: keep the UPS on the floor or at the bottom of the cabinet, because it is the heaviest item and the battery likes a cool spot; never plug a UPS into another UPS, or into a splitter with a switch; plug it into a properly grounded wall outlet, not a cheap power strip; and stick a note on the cabinet showing what is plugged into which outlet, so that on the day of an outage nobody unplugs the switch to charge a phone.

Battery care: what shortens its life and what extends it

The battery in most small UPS units is a sealed lead-acid battery, a close relative of a car battery. It has three enemies: heat, deep discharge, and neglect.

• After a long outage, give it time to charge. A full charge takes 8–12 hours. A second outage on the same day may catch the battery half-empty, and that is not a fault in the device.
• The beeps mean something. A short beep every few seconds: running on battery. A continuous beep: the battery is about to run out. A beep every few minutes while power is on, or a steady orange light: the battery has reached the end of its life. The exact meaning is in your model's manual, but the pattern is similar on all of them.
• After a long outage, let it charge. A full charge takes 8–12 hours. A second outage on the same day will find the battery half empty, and that is not a fault in the device.
• After a long outage, give it time to recharge. A full charge takes 8–12 hours. A second outage on the same day will find the battery half empty, and that is not a fault in the device.
• The beeps tell you something. A short beep every few seconds means it is running on battery. A continuous beep means the battery is about to run out. A beep every few minutes while there is power, or a steady orange light, means the battery has reached the end of its life. The exact meaning is in the manual for your model, but the pattern is similar across all of them.
• Replacement. On most models you only replace the battery, which takes a few minutes and sometimes doesn't require turning the unit off. Take the old battery to a collection point, since it contains lead and acid.

And a small detail many people don't know: a new UPS taken out of its box after months in storage arrives with its battery half empty. Plug it in for a full day before relying on it.