Thanks for quick reply!
So you're saying I might as well go for the extension lead with 3 sockets (or this one from APC: https://www.amazon.fr/APC-Multiprise-C1 ... ms?ie=UTF8)
Yes, it's only got one battery, but most of the time my load is only 50W (1 Mac mini + 24" screen), sometimes 100W (2 Mac minis + 2 screens) and very occasionally a bit more.
Main reasons for choosing this model were: good price, warranty extension possible to 5 yrs (75€, for UPS and battery), silent (no fan), sine wave, (hopefully) better management of & feedback on battery than other (cheap) UPSs.