You also forgot to factor in 3000w worth of PSU, or something close to that if you intend on over clocking. My two S1's are pulling in over 900w combined.
All of our ants @ 393mhz get 200GH/s, and pull 410W at the wall with an 80 Plus Gold 750W Rosewill PSU, which is right at 90% efficient at. That means our Ants are pulling 370W from the PSU, and this includes removing the blue/white wires from the fans so they're running at 100%.
If your 2 ants are pulling 900W, you need a better (more efficient) PSU.
I have a 1000 watt seasonic plat 2 s-1s clocked to 400 mhz giving me 406 gh.
I use 805 watts and I have no issues with this psu. it is the best for 2 s-1s overclocked.