As someone mentioned earlier, the purpose of a UPS is to give you enough time to save your work and shutdown the PC properly. Also, it will protect the PC from low/high spikes and brownouts. Normally, you don't use a computer for an extended period of time when running on a UPS.
For a Desktop, a ~ 650VA would be enough and it will not empty your purse! APC is quite good for home use.
If you need to work continuously for hours during a power outage, then another solution is needed.
Of course, if money is not an issue, sky is the limit!