Actually a lot of equipment is more efficient given a higher input voltage, so if you ran your UPS off 230 instead of 115 it would probably use less Watts in standby/charging mode.
There are three types of UPS:
1- the lowest tech type is the battery backup which stays off line (on charge) unless the AC power drops below a certain set voltage - can take a few millisecs to switch over,
2- line interactive, the midtech type - this one can adjust the voltage within a narrow range w/o relying on the battery by switching between windings on the AC transformer - switches to battery when AC drops below that range - shorter switching time - seems to be the preferred choice of PC users in the US,
3- online or true UPS - this one runs the load full time off the battery - AC is just used to keep the batteries charged - this one is for areas where the power is really messy as the load is totally isolated from the mess - significantly more expensive than the other two - zero switching time.
If your PC's PSU has autoranging AC sensing, you can more safely use the cheap battery backup type as your PSU can compensate itself - some down to 100, some down to 90VAC. Of course your monitor may not be able to do the same, so you'll need the better, line-interactive UPS, unit to take care of that. Active PFC also helps to get the most uptime out of a UPS battery - usually comes along with the autoranging AC sensing in PC PSUs like my Zippy HP2-6460P.
.bh.