Back-UPS XS 1000 (it's the lower-end version of the RS1000) hooked to a first-gen G5 DP2.0 and 20" aluminum ACD. I can't seem to find the power measurements I took, but I seem to remember it being around 150W at idle, 250W full-bore. When new I want to say it got about 10 minutes of runtime, but my memory sucks.
The battery actually needs replacing about now; it will still run the computer, but it started failing the self-test on startup about half the time a couple months ago, meaning it's borderline. Meaning I got just a tad under 6 years out of it before needing new batteries.
I also, to note, have a 2009 mini plus a couple of external drives, an AEBS, a DSL modem, and an answering machine hooked to one of the new "Green" APC UPSes (there's a brick covering up the model number, but whatever the bigger one is--750, I think). That shut itself down smoothly after five minutes on battery with no additional software on Snow Leopard during a recent outage, and the UPS kept the internet and wireless up and running for another half hour until the power came back on.
The XS1000 provided enough power to shut down the G5, at least, so it's still doing its job.
I haven't done any careful measurements of how much power a "Green" UPS saves versus an older one or the big dogs we use at work, but based on how much heat some of the 4U rackmount RS series or TrippLite ones put out I'm guessing it's nontrivial. Depending on the particular model, how much power you use, and the efficiency/phantom load to keep the batteries floated, you could be spending a LOT on electricity with bigger hardware.
I should probably do some real measurements so I can give more concrete statistics on that, though.