For the reason you mention even a quality powerbank is enough for a pi3+. Even less than 850va would be enough. The ups u posted is 850va, not watt. Generally, unless stated otherwise the power factor is 0,6 for va to watt, so 850va would be about 450w. More than enough for your needs. Whatever u buy from the ones u mention would be a good choice, but there are some complaints for the cheap Eaton. Personally, I had bad experience with this one. So, I would suggest the Powerwalker which seem to be decent in this price tag. Avoid any Energenie/Gembird.

Oh my bad you are absolutely right, i meant to say 480W. The load on the UPS will not be just a Rpi3+ but also the DS920+, 12W router/modem, 3W switch. 

I was wondering more if anyone has actually opened any of the UPS mentioned and checked the caps and other components used. The Powerwalker indeed seems the better choice and it has the extra advantage of the 2 Schuko outputs.

Επίσης σε πολύ καλή τιμή βρήκα και αυτό (και 600W αντί για 480W!):

Powerwalker Basic VI 1000 SB UPS Line-Interactive 1000VA 600W με 2 Schuko Πρίζες - Skroutz.gr

Also, i found this one too which is at 600W instead of 480W:

Powerwalker Basic VI 1000 SB UPS Line-Interactive 1000VA 600W με 2 Schuko Πρίζες - Skroutz.gr

Powerwalker UPS are decent for their money, and I never had any problems with them.

awesome guys, thank you! They just seem to cheap to be true 😁. I'll go ahead and order the 1000VA and will get back to you if any issues arise. 

@pt4r keep us updated. 850va would be enough. No need to go for 1kva. But whatever u want.

Posted by: @pt4r

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Hello everyone!

 

Do you guys have any recommendations on which UPS has some good capacitors for a normal price tag (less that 100€).

 

I think a 850W UPS would be enough for the ds920+, router/modem, 4 port switch and a rpi 3B+.

 

At what voltages ds920+, router/modem, 4 port switch and a rpi 3B+ are working. Maybe it's not best to use UPS, but Li-Ion batteries with BMS!