Hi Antje,
Please take in to account that with noAuthNoPriv the access may not protected. Depending on your UPS model anyone can power off the UPS and all connected devices. Not sure if you want that.
In SNMPv3 you have several options. In case you use authPriv you need to provide a password for authentication AND a password for privacy, which is encryption.
I would recommend using option authNoPriv which needs a password only.
We generate random complex passwords per site and I dont see any limitations with the passwords in SNMPv3.
I hope that helps
regards
Michael