9800X3D + 5090

You'd be completely fine running an 800W PSU with the above, just make sure it is from a good brand.
I bet the PC would not even top 750W peak power, assuming no crazy setup with 17 hard drives!
 
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For a 5090, nvidia are suggesting a minimum of a 1000 watt. Realistically you could probably go as low as 850 if the PSU is of high quality and is gold+. At 1200 watt you’ll be fine.
 
1200W would be more than enough I would imagine.

The 9800x3D doesn't consume that much power after all unless if you REALLY try to overclock it. With almost 600W for the 5090, assuming you could +10% power, that's up to 660W, but lets say that's 700W for now for easier math. That coupled with the 120W of the 9800x3D and you're looking at 820W odd, throw in some extra % for the other parts of your rig, and you're looking at around 900W for constant CPU and GPU use at full tilt. Add a little more for any spikes and to cover the age of any older PSU not being as efficient, 1000W that NVidia are recommending, wouldn't be too far off here either.

As above though, if you've got some serious PSU build you can probably go for less and not be a problem, although age will eventually factor in as time goes on. For example, my TX700 Fanless Seasonic might be 700W officially, but it can throw out up to 950W odd safely before feeling it's in trouble. It's also only 4 odd years old out of 12 years warranty, so most likely will be fine for a bit longer. Nearer the end of those 12 years, I certainly wouldn't attempt to use this PSU to drive such a rig that's at full tilt all day long. But right now? Sure, I'll feel safe with it attempting to do so.
 
I've been debating the same recently on a new build and trying to decide between 1000W and 1200W PSU. Not sure if the headroom on a 1000W PSU will be large enough at peak load.

It might be more sensible to go for 1200W, which from my research should be more than ample.
 
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