Hello,

Technically, if the S9 really use 1275W At the wall, it's use 1275 x 0.93 = 1185W on 12V. So a good quality 1200W PSU is enough but be the smallest unit you can use.

But usually, best efficiency is obtain at the half of power. At full power, you usually loose between 1 and 2% efficiency.

So a gold PSU used at 50% is very near a platinum PSU used at 100% (and will probably be more reliable)