Well it really depends on what you wanna do with it, are you saying that you want something that's going to give you 10-15 minutes of power so you can safely save work and log off or are you trying to find something that's going to keep you gaming until the power comes back on?

In either case I'm pretty sure that you're going to need a UPS that at least matches the power consumption of your rig (rig draws 1000W, UPS should output 1000W). But just because you have a 1kW PSU doesn't mean it's drawing 1kW either, so there's that. You can probably figure out what your peak power draw is and buy a UPS based on that, however if I were to be looking for a UPS I would find one that's output matches the PSU to be safe. As for peripherals like your monitors, depending on what you've got I'm assuming they don't pull a ton of power so you can probably just plug them in to your 1kW UPS with the 1kW PSU and you'll be fine considering that you're PSU isn't drawing the full 1kW (or at least shouldn't be...it goes back to what your peak power actually is).

So, you pretty much need to know how much your whole rig is drawing, and then buy a UPS that puts out at least that much...and probably more so you have a buffer (same as you would when you're buying a PSU). Look into buying a Kill-a-Watt and plug it into the socket between a power strip with everything in it and the wall so you can see what the whole rig is drawing.

Looking at something like this, puts you at 6 minutes of runtime if you're pulling the full 1000W...which is enough time to save and exit pretty much. That's already a nice chunk of change for 6 minutes at full capacity, let alone something that'll run you till the power comes back on.