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.
 
I'm hearing that 1200W is adequate yet I regret buying the Corsair RM1200x SHIFT as the first component of my new build now that the HX1500i SHIFT has been released... Granted, it took me a year to gather everything, but the additional peace of mind of 1500W would've been appreciated now that I've settled on a 5090 too.
 
As others have said 1200w is absolutely enough.

The 5090 rarely runs at 600w, and when gaming the 9800X3D runs at under 100w, even overclocked. The only time it goes to about 120/140w is when compiling shaders.

That leaves a massive amount of overhead.

The only thing I’d add is I wouldn’t only go for the headline power of the psu. Make sure you buy a quality unit and don’t get caught up buying pretty third party cables. Good quality units come with their own braided cables.
 
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1200 is plenty, I've built 9800X3D/5090 rigs running on lower wattage supplies without issues, obviously you need to go for a high quality PSU.

I'd highly recommend power limiting the card to around 450W/undervolting anyway, there's a number of advantages to doing so. Obviously you'll use less power, but you'll also run cooler and as a result enjoy higher boost clocks, it also notably decreases the chances of the 12VHPWR connector developing issues.

I've never known a 5090 that wasn't absolutely fine at 450W, boggles the mind that they default it to 600W given the known problems their ridiculous connector has.
 
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