I've been fiddling with simple brown-out protection and orderly power-fail shut-down (with inadvertent hard drive protection) on desktop systems for the last little while.
The intention is to keep it as dumb as possible - though the forced shutdown and saving of current work does call for dedicated SW and I'm still not even sure that windows will allow it under all circumstances. Brown-out protection and battery management is a bit easier, though it's easier still if the OS doesn't have to know about it, and an operator is available to do the dirty at the ringing of Pavlov's buzzer .......... which is a curiously prevalent situation, given the fact that computers are supposed to be machines that can perform tasks automatically.
The same sort of thing can already be done (theoretically) with an AC UPS, but I'm a little unimpressed with some of the rubbish being marketed out there. Why people continue to believe that 'everything runs off 115Volts', in spite of their massive compilation of wall-warts is beyond me.
The physical profile of this kind of hardware is largely dependent on a drive bay, or PCI slot height, given the mechanical constraints of consumer hardware, but the basic function is not dissimilar to a laptop power system.
If you're interested, I've got some free time starting in late September that could result in lower profile iterations. Obviously, run-time on any battery is system-dependent. If desktops really consumed 250W+, most consumer battery packs would be overwhelmed in the original application.
RL