The Powerwalker sure has lots of specs, but it's pretty difficult for me to judge whether it's better or worse than others, because the others don't give that level of spec.
Did you already read the cyberpower cp900epfclcd datasheet here?
https://www.cyberpower.com/eu/en/Fil...DS-18080023-01
By the way, I amazon-ed a little bit to search an USA cp900epfclcd's "brother" and found the cpXXXpfclcd "family". As you can see the "e" is missed and I guess it stays for european (?). If yes, in the sense if there're no other important differences, consider this
https://www.amazon.com/CyberPower-CP...dp/B00429N18S/
You can find infos he
https://www.cyberpowersystems.com/pr...e/cp850pfclcd/
and its datasheet here
https://dl4jz3rbrsfum.cloudfront.net...P850PFCLCD.pdf
The first difference I read is about the on-battery wave output: just "sine wave" and not "pure sine wave" as the EU family (cpXXXEpfclcd) BUT if we look at amazon buyers' comments, we can get these graphs:
-CP1500PFCLCD
https://www.amazon.com/gp/customer-r...SIN=B00429N19W
https://www.amazon.com/gp/customer-r...SIN=B00429N19W
https://www.amazon.com/gp/customer-r...SIN=B00429N19W
https://www.amazon.com/gp/customer-r...SIN=B00429N19W
-CP1350PFCLCD
https://www.amazon.com/gp/customer-r...SIN=B00429N19M
We'd trust as usual. Unfortunately there's nothing similar for the 850VA model within comments. My only concern is I guess there could be a certain possibility cyberpower built the "upper class" models (eg. 1500VA) with a better electronics than the "lower class" ones as the 850VA. I say that because AFAIK certain brands do this job: different models quality within the same models "family" (eg. a laptop series); what do you think about this cyberpower case?
I share your same doubts about the powerwalkers because for now it's hard to find many feedbacks to "study". Just this 800VA model
https://www.amazon.it/BlueWalker-Pow.../dp/B00K791C1U
has 101 ratings and they're "quite good". My "favorite" comment is
https://www.amazon.it/gp/customer-re...SIN=B00K791C1U
and tells the main problem is the transformer is not cool down enough by the heatsinks so the UPS won't last so much.
By the way, there's also the PSW powerwalker series
https://powerwalker.com/?page=select&cat=VI_PSW&lang=en
and it's more exspensive than the SW series I wrote above. Honestly I don't understand the differences, can you help me? Let's consider the 750VA model
https://powerwalker.com/?page=produc...120081&lang=en
with its datasheet
https://powerwalker.com/datasheets/P...SW%20Promo.pdf
I don't think the difference SW / PSW is just "sine wave" / "pure sine wave" because the SW generates pure sine waves too, so what's different to the SW series brothers?