Bitmain specs say 1186 +/-7% fluctuation at 12v DC. Simply do that and add your psu's efficiency. 7% for titanium, 9% for platinum, 11% for gold and so on.

So the max it should draw is 1386w if you have a platinum psu. Psus also operate slightly more efficiently at 240v than 120v by 2%ish.

If the operating temperature is really hot I could imagine it going up to 1500w. 1350w is a more realistic day to day scenario.