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The short answer
Add up the wattage of what you will connect, then buy a UPS rated comfortably above it, because runtime falls sharply as load rises. If you have a modern power supply with active power factor correction, buy a pure sine wave model, since a stepped approximation can cause those supplies to shut down.
An uninterruptible power supply is a battery that sits between the mains and your equipment. Its job is not to keep you working through a long outage but to ride out brief interruptions and give you a few minutes to save and shut down properly. It also smooths out brownouts, which are more common than full cuts and harder on equipment.
Sizing it
List what you will connect
A desktop computer, a monitor, a router and a network switch is a typical set. Printers and laser devices do not belong on a UPS.
Add up the wattage
A modest desktop plus monitor is often 150 to 250W under normal use. A gaming machine can be 400W or more.
Buy a UPS rated well above that
Running a UPS near its limit gives very short runtime. At half load, runtime is typically several times longer.
Check the runtime chart
Manufacturers publish minutes at given loads, which is far more useful than the headline VA figure.
Decide how long you need
Five minutes is enough to save and shut down. Half an hour of continued working needs a much larger and more expensive unit.
VA and watts are not the same. A unit sold as 1500VA might deliver 900W. The watt figure is the one to compare against your equipment, and the VA number is the one printed largest.
Sine wave matters
Cheaper units produce a stepped approximation of a sine wave when running on battery. Most equipment tolerates it; computer power supplies with active power factor correction, which is most modern ones, frequently do not. The symptom is the computer shutting down or the UPS reporting an overload the moment it switches to battery, which defeats the entire purpose.
If you are protecting a desktop computer built in the last decade, buy a pure sine wave model. For a router and a network switch alone, a stepped unit is fine.
Units worth considering
CyberPower CP1500PFCLCD pure sine wave UPS battery backup
The default recommendation for protecting a desktop. Pure sine wave output avoids the compatibility problem that makes cheaper units shut a computer down at the moment it is supposed to keep it alive. The display showing load and estimated runtime is genuinely useful for sizing.
APC BE600M1 UPS battery backup and surge protector
Keeping a router and a mesh node powered through brief cuts is an underrated use, because a five-second interruption otherwise takes the whole house offline for several minutes while everything reboots. A small unit does that for very little money.
CyberPower CP1500AVRLCD3 intelligent LCD UPS
Voltage regulation is the feature that earns its keep day to day, correcting sags and surges without switching to battery. That extends battery life and protects equipment from the slow damage that undervoltage causes.
APC BX1500M UPS battery backup and surge protector
A practical mid-range unit for a desk with a computer, a monitor and networking equipment. Check the output type against your power supply before choosing, since stepped output units are the ones that cause compatibility surprises.
CyberPower ST425 standby UPS battery backup
Worth considering for a network cabinet, a small NAS or anything that simply must not lose power abruptly. Runtime is short and adequate for riding out the brief interruptions that make up most power events.
What we liked
- Prevents data loss and abrupt shutdowns
- Voltage regulation protects against brownouts
- Keeps the internet alive through brief cuts
- Includes surge protection
What we didn't
- VA and watt figures are easy to confuse
- Stepped sine wave units can be incompatible with modern PCs
- Batteries need replacing every few years
- Runtime falls sharply near the rated load
What not to plug in
- Laser printers, whose fuser draws a very large current spike that will overload most units.
- Heaters and anything with a heating element.
- Another UPS or a surge strip, which causes unpredictable behaviour.
- Anything you do not need during an outage, since every connected device shortens the runtime for the ones that matter.
Most units have some outlets with battery backup and others with surge protection only. Put the printer and the desk lamp on the surge-only side.
Living with one
Install the monitoring software
It tells the computer to shut down cleanly before the battery runs out, which is the whole point of the purchase.
Connect the data cable
USB from the UPS to the computer. Without it, the software cannot trigger a shutdown.
Test it deliberately
Pull the mains plug and see what happens, once, when you are ready rather than during a real outage.
Note the battery replacement interval
Typically three to five years. A UPS with a dead battery is a surge strip that beeps.
Keep it ventilated
Batteries age faster when warm, and a UPS in a sealed cupboard will not last as long.
Respond to the self-test warning
Units test themselves periodically and beep when the battery fails the test. That beep is the one to act on.
What size UPS do I need?
Add up the wattage of what you will connect and buy a unit rated comfortably above it. Check the manufacturer’s runtime chart at your load, since runtime falls sharply near the rated maximum.
What is the difference between VA and watts?
VA is apparent power and watts is real power. A 1500VA unit might deliver 900W. Compare your equipment against the watt figure, not the VA one.
Do I need pure sine wave?
For a modern desktop computer, yes. Power supplies with active power factor correction often refuse to run on a stepped approximation, which causes a shutdown at the worst moment.
How long do UPS batteries last?
Three to five years typically, less in a warm environment. Most units test themselves and warn when the battery fails, and replacement batteries are available for most models.
Can I plug a laser printer into a UPS?
No. The fuser draws a large current spike that overloads most units. Put it on a surge-protected outlet without battery backup, which many UPS units provide.
How to choose
Add up your wattage, buy well above it, and read the runtime chart rather than the VA figure. Choose pure sine wave for a desktop computer and a basic unit for networking equipment. Install the shutdown software and connect the data cable, keep the printer off it, and test it once deliberately so you know what happens before it matters.
