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)
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)
it says 1275 dc + 7% at wall based on a plat psu. that 1200 watt can do 1320 watts at the wall as I think it is a gold not a plat.
a 1275 dc x 1.1 = 1402 at the wall.
so you need a better psu then those 1200 watt hp dps 1200 and remember the spec up are for the batch 2 and batch 3 s-9's
the s-9 batch 1 was specced to run at freq 650 and use 1390 watts dc +7% if a plat psu.
there is no way the hp dps1200 should be used on an s-9 unless you really clock low like 450 to 500 or so.
Usually, good quality PSU given to 1200W means it is output power. Ok for the +7%, you have right but good PSU have safety margin, it will not shut down at Full power +1%. That can work.
I have already use a HP1200W to power Garden miner who uses 1200W on 12V, ~1400W at the wall whiwout any problem.
But using a PSU @100% of it's power specification is not very good, because will be more sensitive to voltage drop or local temperature, reduce lifetime and efficiency drop.
OK got in that was weird