PSU will normally hold over minor spikes - anything big enough to cause it to restart would normally be noticeable as lights dim or flicker and/or other electronic devices exhibit problems. I'm not sure a UPS will solve your issue.

If the PC is suddenly turning off completely in use then it is more likely the PSU is failing, faulty RAM or a bit less likely faulty CPU or GPU.

 

You'd need a UPS that is rated for the kind of actual power draw the devices are consuming - it won't matter if the UPS can only manage a few minutes of power on time. That unit looks like it is rated for 480 watt so it would probably be a bit hit and miss with a reasonable spec gaming PC if you were in the middle of a demanding game, etc.

Most UPS are generally pretty quiet - many of the BackUPS series are sealed units with no fan and aside from a little bit of faint hum/whistling when charging pretty much silent aside from the odd click when doing a self test and/or swapping over power. Some can have a noisier fan though often that is only used when the device is charging and/or temperatures rise to 40+C which they'd only hit on a warmer day.

 

Sound level depends on the UPS. With my current UPS you cannot hear it while main power is on but the noise level goes up when there is a power cut and its working. That is acceptable to me.

The other UPS I have and no longer use is so loud its around 44db even when mains is on.

 

If your PC was drawing under 480 watt than yes the PC shouldn't see any interruption of the supply and continue as normal - "clean" power is a bit of a complex one as many cheaper UPS actually don't do much proper filtering as such other than providing continuous power with many of them putting out a somewhat dirty modified square wave rather than a nicely cleaned up sine wave.

EDIT: As an aside - I'm not sure how true it is but apparently a lot of PC monitors do NOT like the output from an average UPS even with the additional conversion and prolonged use driven from the UPS output (when it is running from battery) can destroy them.

 

You'd need a pretty serious brownout to knock out a PC. The PSU is probably rated to work down to 100V.

 

Alarm for blackout/battery operation is no doubt louder than that in most UPSes.
But for normal mains operation off-line/line-interactive UPSes don't have reasons to keep noise.
Though some server/workstation grade line-interactive UPSes have always on fan.

I guess you would like standard UK power socket to avoid needing different cables?

APC Back-UPS has that closed plastic box insulating heat inside to cook crap capacitors of control electronics.
So wouldn't recommend those.

Cyberpower's such UPSes have some ventilation gaps:
https://www.cyberpower.com/uk/en/product/series/Backup UPS Systems/BRICs LCD


And just forget that VA rating used for marketing numbers.
In AC world VA and W don't mean same thing, even if being equally big units.
With power factor correction in PSUs, PC's don't have any real reactive power component, needing only watts.
So of course marketing has again started hiding actual watt ratings deeper into specs and are pushing for VAs. (especially APC)

I guess you're going to be keeping relatively high end gaming PC?
(even if not updating parts constantly)
In that case you'll want watt rating of UPS to be around 500W, so that it can handle transfer to battery during high loads.

 

Qualify 'extremely expensive'.

 

I've got a number of the tower style Back-UPS units in use and so far they've lasted fine though they do get warm especially if in an enclosed space during this warmer weather and that is all the time not just charging (technically they are always charging when not running off battery) or on battery.

We've been through a few of the ones that are like an oversized power extension strip and they do not last very well - I'm guessing the path the heat takes through the chassis is a bit different between the units.

 

That one is tower model with slots for some airflow.
But it has IEC sockets, so your normal power cords don't fit.
Though you really need to get IEC power cord only for PC and monitor.

 

Those flat boxes are designed to fail with total lack of any airflow and garbage capacitors.

 

Not connecting monitor to UPS and inability to close programs and Windows in controlled way eats half the purpose of UPS.

Actually checking from Geizhals.eu site Cyberpower's BR1000ELCD-UK (with UK power sockets) should be available for nearly same price level as that BX950UI.

 

An APC Back-UPS kept my PS4 Pro and 4k TV going strong during about 600 power cuts (no exaggeration) while living in India for 6 months this year, would have brought it back but it weighs a ton..
Also, it used to click a few times a minute or so in advance of a power cut when there was no outward sign of one, giving a handy warning!

A few times there were almighty sand storms blowing outside in truly apocalyptic scenes, with power out for miles, while I'm on the couch in my boxers playing Dark Souls 3, thinking, if I'm going to die this isn't such a bad way...

 

For those countries with hash weathers, a good idea is to have a beefy ups that is generator friendly.. Then run near enough everything through the ups in the house, or the important stuff. Then use a generator to power the ups during long powercuts. You then have constant safe power 24/7

You just need to work out a easy way to to connect the ups to and from mains/generator power.