Got an APC C5-650 UPS from my dad when visiting last week, he'd rescued it from a skip so it was free. Battery looks in good condition so no idea why it was thrown away. Will test it out when I'm in long enough to keep an eye on it.
Works a treat and so NAS box, ESXi server and switch have been moved onto it. Should also put one of the monitors on it otherwise I have to open the laptop if the power does ever go and I want to log in and shut things down cleanly.
No idea how long it would last but it certainly lasted >30s whilst I was moving some cables around.
Also meant I could plug in an inline power meter to give me an idea[1] of the draw of the home office.
170W for two 23" monitors, work laptop, NAS box (nigh on idle), ESXi box (nigh on idle), speakers, HP OfficeJet printer, UPS keeping itself topped up, MSI min-laptop keeping itself topped up.
1. I am aware of their limitations due to power factor magic.