That depends on how much power your PC uses.
If it's a totally insane rig that actually needs all 1000W that the PSU can supply, you'll need to look at a minimum of a 1500VA UPS.
I'd advise looking at something like PC Parts Picker to get an estimate of the system's maximum power usage, taking into account possible future upgrades such as adding another GPU, then looking at the power ratings of UPSes to see what meets the need.
Stephen