I’m using an Eaton 9PX1000iRT2U (that’s the 230V model, the 120V I think is 9PX1000RT2U without the i).
I figured the printer probably isn’t too sensitive to minor fluctuations (will have a switchmode PSU after all) but a dual-converstion UPS was better than a line-interactive - considering the cost of the printer even the more expensive UPS seems a pretty trivial cost.
For just my F3, it should last more than 3 hours during a print - the printer uses quite a bit of power at the beginning while it’s heating, mixing, and whatever else, but settles down to very low consumption for the remainder of the print.
If you need longer, can stack up to 4 additional battery packs onto it.
I posted some graphs of power and estimated runtime here: The printer (form3) not continue printing after power outage - #3 by DKirch