Yes, that would be your return. I can't imagine anywhere with that much free electricity though. A school, maybe? A giant warehouse going unused?
Keep in mind 3 6990's will eat up around 1350w (if my sources are correct). That means you'll need a separate circuit for each rig, unless you're running on 220v. In which case, I'm jealous.
Actually each 6990 under load draws 300W, the 450W statistic on the AMD's website is total system load (including processor etc), not for each card. For a 3 * 6990 rig you would need a 1200W continuous output PSU.