is it possible to power an s5 with a 550W gold psu? Seasonic G-550 80+ Gold Modular
Only if underclocked. Using HP server supplies my S5's running stock speed pull 605-610 watts from the wall as reported by the UPS's on some of them.
One NEVER ever should continuously run any PSU at or close to it's max output. It simply will not last or be stable. For any constant load eg miners, general rule of thumb is to load a PSU - and the line feeding it - to 80% or less of the maximum rating. So for an s5 I'd go with at least a 750w supply to keep everything happy.
Agreed 80% is optimum depending on efficiency rating on the PSU. 85% is doable too but overloading a PSU in any case is not good and really just gambling with equipment lifetime.