Virtually all consumer UPS devices pass through the AC mains, and only provide inverter power from the batteries upon power failure. The switchover is relatively fast (milliseconds), so your device that is running on the UPS typically won't see the power interruption.
To get a continuous power UPS, you have to spend big bucks (tens of thousands typically), and they normally have much larger batteries (which are 24 or 48 volt, not 12 volt). Consumer grade UPS devices are not designed or built to run continuously. If you run them at about 25% of rated load, you can probably run them longer (using a large external battery), but they won't survive extended continuous use at or near rated output.
You can buy a consumer online UPS for a few hundred dollars. APC's "SmartUPS" line is an online UPS, for instance, which is what I use. I've never investigated the ratings to see if they're rated for 24x7x365 use and at what load, but I've used the same one at home since probably 1999 and aside from needing to replace the battery every 3-4 years it has never failed.
If you want something that never goes down, even for battery replacement, then yeah you need to go beyond consumer products and look at something like APC's Symmetra, but such a need for online battery swap is way beyond a typical consumer requirement. And of course your bigger expense will be the diesel generator that backs it up since you wouldn't typically plan to run on batteries for longer than it takes to start up the generator.
Edited by slice1900, 14 April 2014 - 09:07 PM.