Isn't it an external, independent, device? Are there any models that require that the OS does something in order to work?

http://apcupsd.sourceforge.net/ supports some UPSes (not only USB), they provide power use information and notify the system when external power fails (useful e.g. to safely shutdown the system). This isn't required, although could be useful for someone choosing an UPS.

Hello ! Some UPSes are "seen" by the system, some do not, so sharing information about them would be useful...