I thought this may be of interest to anyone desirous of a silent UPS.
I've posted a few years ago how I cobbled together a 12v DC mini UPS set-up which has now been in continuous use powering my modem/router(s) and RPi. It has, and still, performed flawlessly, and tho it only has a 7Ah battery it has saved the day on many occasions - over 12 hours in one instance. It is truly silent. When a powercut occurs, I can use an old laptop to access the RPi which runs DSLstats, or to access the modem/router.
For quite a while I've hankered after something similar to protect my silent ITX-based-desktop and monitor (together they use around 40W); I had once used a Belkin 325W UPS but its humming (not on battery) disturbed my office quiet, and when it developed other issues it was sent to recycling. I considered quite a number of low-cost UPS units, but reviewers frequently grumbled about 'humming' mains-related, so the idea faded.
Just recently I came across PowerWalker UPS as an alternative to the main players, and when I found a basic 360W one online at a bargain price from The UPS Centre I decided to give it a go....PowerWalker Support assured me it was silent. Anyway it arrived today, and sure enough it is silent when running on mains, and very quiet when on battery. I've tested its powercut operation - completely fine. And I will get round to using its USB connector for monitoring/shutdown in due course. Though it only has a 5Ah battery, that will be easily enough for my purposes.
The model I picked is the PowerWalker VI 650 SB UK from
www.upscentre.co.uk and was just over £40 in total, including next-day delivery....its also available elsewhere.
Sorry; I forgot to add a link to its datasheet.....
https://powerwalker.com/?page=product&item=10121096