The 5090 might most of the time draw like 350W but like many top-end cards (also from AMD) power draw can spike really high and can reach double that even for very short moments. So you need a beefy power supply regardless. For a 5090 in combination with a top-end 16 core CPU I wouldn’t recommend anything under 1200W (so you still have some wiggle room. Power supplies are also at most efficient when they’re not at ~95-99% capacity but at ~80%).