Backup power (UPS) for the office, server and security systems — how to choose

Most offices buy a UPS the same way: someone remembers that the power once went out, picks up "something with enough volts" and puts it under the desk. A year later the unit either beeps for no reason, lasts three minutes, or fails to help in a real outage. The problem is rarely the device itself — more often it lies in the choice of type, the power calculation and the lack of maintenance. Here is what is worth knowing before you buy.

Why power problems cost more than they seem

When we talk about power problems we usually mean a complete outage, but the mains has several ways of harming equipment:

• Complete outage. A computer or server shuts down abruptly. The file system and databases can be left in an unfinished state.
• Short voltage sags. Sometimes the lights merely flicker, yet some devices restart. They do not all do it at the same moment, and the system is left in an unclear state.
• Spikes and surges. Switching on heavy loads nearby, a fault on the grid or a thunderstorm can briefly deliver a higher voltage than power supplies are rated for. The result is often a damaged power supply with no visible cause.
• Noise and unstable mains. A quieter but constant form of the problem — unstable voltage that wears out power supplies and leads to unexplained restarts.

The cost is not only in the damaged equipment. After a restart, routers and switches take several minutes to boot, during which nobody can work. A video recorder may miss footage exactly when it is needed most. That is why power protection is part of the reliability of the whole system, not an accessory to it.

UPS types — how they work and when each fits

There are three main topologies. The difference lies in when the battery comes into play and how the input voltage is treated.

Offline (standby). While the mains is normal, the load receives mains voltage directly and the battery only charges. When power fails, the unit switches to an inverter, which takes a few milliseconds. It is the simplest and cheapest option, suitable for a home computer.

Line-interactive. It adds an automatic voltage regulator (AVR) that raises or lowers the input voltage within certain limits without using the battery. That way minor deviations do not drain the battery. For most offices and video surveillance systems this is the sensible middle ground.

Online double-conversion. The input voltage is continuously converted to DC and back to AC. The load is always supplied by the inverter, so there is no switchover during an outage and the output is isolated from mains problems. The price is higher, the unit is noisier, gives off more heat and is less efficient. It suits servers, sensitive equipment and places with a very poor supply.

Another important detail is the shape of the output voltage. Some cheap models produce an approximated (stepped) sine wave when running on battery. Many modern power supplies with active power factor correction tolerate it poorly — they may shut down or hum. For a server and newer computers, look for a model with a pure sine wave on battery as well.

VA, watts and how to calculate the power you need

The box of a UPS almost always carries two numbers — one in VA and one in watts. VA (volt-amperes) is the apparent power, the product of voltage and current. The watt is the real, active power that a load actually uses. The relationship depends on the power factor: watts = VA × power factor. That is why a 1000 VA unit cannot power a 1000 W load — its real capacity is lower and is stated in the technical data.

The calculation is simple:

1. Add up the wattage of all the devices you will connect. The figures are on the label or in the documentation; the label shows the maximum, while real consumption is usually lower.
2. With a PoE switch, also count the power it supplies to cameras and access points — it is drawn from the same supply.
3. Add headroom, usually 20 to 30 percent, for future additions and switch-on peaks. A UPS that constantly runs at its limit wears out faster.
4. Compare the result with the wattage of the model you are considering, not with the VA.

An illustrative example with hypothetical numbers: a 40 W recorder, a 120 W PoE switch under real load and a 15 W router add up to about 175 W. With 30 percent headroom that is about 230 W, so a sensible choice is a unit with at least 300 W of real power. Check the figures for your own equipment before buying.

Runtime and battery ageing

The time a UPS can sustain the load depends on the energy in the battery and on the load itself. The relationship is not linear — at half load, runtime is usually more than twice as long. So a "minutes" figure only means something if you know at what load it was measured. Before choosing a model, define the goal:

• Bridging short interruptions. A few minutes are enough — most disturbances last seconds.
• Controlled shutdown. The server or recorder needs time to close its files safely. For most computers that is a few minutes; for systems with databases, more.

If you need hours of operation, a small UPS is not the answer. In that case, use models with external battery modules or a combination with a generator, where the UPS covers the time until the generator starts.

Batteries are consumables. Most office UPS units contain sealed lead-acid batteries, which typically last several years, and less at higher temperatures. Heat is the main enemy. Over time capacity drops, and under heavy load the voltage collapses and the unit shuts down sooner than you expect. Signs it is time for replacement: sharply shortened runtime, a self-test failure, a constant alarm, a swollen or unusually warm case. Replace with batteries of the same specification, and hand the old ones in for recycling rather than throwing them out with household waste. Write down the installation date.

What to connect to a UPS and what not to

One limitation is often overlooked: a UPS only keeps your own site powered. If your internet provider has equipment in a street cabinet that also loses power, the connection will drop regardless of your backup. Where remote access is critical, consider a second connection, such as a mobile one.

Safe shutdown and monitoring

A UPS that simply holds for a set time and then cuts out abruptly only postpones the problem. The real value comes when devices know that the power is gone and shut down properly before the battery runs out.

For this, the UPS is connected to the computer or server by a USB or serial cable, or, on larger models, through a network card. Dedicated software monitors the status and starts a normal shutdown at a set battery level. There is software from the manufacturers as well as free open-source solutions. With several devices, a network card is convenient: one UPS tells all the machines that they must shut down.

Notification is also useful — an email or message when the UPS switches to battery. It is not unusual for a UPS to sit with a worn-out battery unnoticed because it lives in a cabinet. Set the shutdown threshold with a margin, and test the chain with a real shutdown, not only on paper.

Placement, ventilation, testing and maintenance

A UPS gives off heat and is sensitive to it. Place it somewhere with good airflow, away from radiators, direct sunlight and tight enclosed niches. In a comms cabinet, keep the ventilation openings clear. Avoid dusty and damp places.

Connect equipment directly to the UPS outlets. Do not plug one UPS into another, and do not connect it through a power strip or extension lead. Use a grounded outlet. A UPS is not a substitute for lightning protection — with outdoor cameras and long cable runs, separate protection is appropriate.

Maintenance is short but regular:

• Run a self-test at a set interval — many models have an automatic function.
• At least once a year, check the runtime under the real load and compare it with the previous result.
• Keep a note: installation date, battery replacements, test results.

The most common mistakes and a checklist

UPS problems usually come from a few recurring mistakes:

• Choosing by price or by VA alone, without calculating in watts.
• Connecting a laser printer or a heater to the same UPS.
• Never testing and never replacing the batteries until a failure happens.
• Protecting only the recorder while the switch and router are left without backup.
• No connection to the server, so the shutdown is still abrupt.

Before buying, go through these questions:

1. Which devices are critical and what is their total power in watts?
2. What type of UPS suits the equipment and the quality of the mains?
3. How much time is needed — to bridge an interruption or to shut down safely?
4. Is there 20–30% headroom, and a pure sine wave if needed?
5. How will the status be monitored and how will devices be shut down?
6. Where will it sit — ventilation, temperature, access for maintenance?
7. Who will test and replace the batteries, and when?

If you are not sure how to do the calculation for a specific site, the Select IT team can inspect the installation and help with the selection.

Frequently asked questions

What is the difference between a UPS and a surge protector?

A surge protector only limits voltage spikes and stores no energy. A UPS has a battery that keeps power flowing when the mains fails, and most often includes some form of surge protection too. A surge protector does not help during an outage.

Can I connect a printer to a UPS?

Small inkjet models are usually acceptable, but laser printers have a high peak draw while warming up and are better left off the UPS. A printer is not a critical device during an outage anyway.

How often should the batteries be replaced?

There is no single date. Lead-acid batteries usually last several years, but heat and frequent discharges shorten that. It is more reliable to go by the result of a runtime test than by the calendar.

Is one UPS enough for the whole office?

It depends on the power and on what is critical. Often it is better to have one unit for the comms cabinet and recorder and separate ones for groups of workstations. One large UPS is a single point of failure, while several small ones are more flexible but need more maintenance.

Will a UPS keep my internet working during an outage?

Only if the provider's equipment along the route also has power. A UPS protects your part of the chain. If the provider's street cabinet loses power, the connection will drop no matter how large your backup is.