Use the EVGA Power Meter or the OuterVision Power Supply Calculator which is more detailed, to see estimated system wattage and to see the recommended PSU wattage.
 
Check out some PSU reviews of the one/s you have in mind.

DarkDiverWould you opt for an RM850x in my situation or is the G3 750W good enough?

 
If I could go back I would 100% go for the RM850x. My G3 750 can't even handle a stock 3080 FE, randomly powers off, most often when switching from a menu/loading screen to the game. No error, bluescreen, or anything, just powers off (even the case light powered by molex instantly turns off).
 
EU Support don't seem to care either, 8 days without any response at all..

DarkDiver
How is that even possible... the guy i bought my 3080 from has used this GPU with a 650W PSU and he didn't encounter any problems. Is the G3 750W such a ****ty PSU that it can't live up to official Nvidia 750W recommendations, which are already conservatively chosen?

Why is the G3 750W still appearing as recommended on the Evga power calculation website even though it cant even handle the 3080 let alone the 3090?? Why not skip straight to the 850w+ models as soon as someone chooses the 3080 or 3090 on the power calculator tool?

I wish I knew. I thought the 750 G3 would be a safe bet too, since its pretty much a Superflower Leadex II, and it was 100% solid with my 1080ti, I'd have recommended it to anyone, until I slotted in my 3080.
 
Perhaps mine is just a bad example, and its faulty in some way (or was part of a bad batch or whatever) and the design itself is fundamentally sound (the reviews would imply it is). Maybe, if EU support ever get back to me, I can get it replaced and the new unit will work fine, I have no way to know right now, but I can say for certain that the current unit I have is not up to the job.

Yea, thanks for the tip, but I've tried everything. ECO mode on/off, two separate cables from the PSU to the GPU (its a 3080 FE so only takes two 6+2 pins). I've tried all 4 cables that came with the PSU (and combinations of each), tried all 4 VGA ports on the PSU, removed/unplugged everything from the PC I didn't need, disabled HT on my CPU. It does it at stock, and even while undervolted (1800mhz @ 900mv).
 
It can happen anytime, but most commonly when switching from a menu/loading screen to rendering the game.
 
Not sure if it could cause damage to the rest of the components, there is a chance of anything, but probably not, as I don't think its the PSU 'failing' to provide the required power (brownout), its just its deciding to turn itself off because it mistakenly thinks that there is a problem (OCP/OPP too sensitive/broken), so its probably unlikely to be sending a spike of voltage at any point, but I could be wrong.
 
I'd be much more comfortable if it wasn't doing it a few times a night, that's for sure.

EDIT: Someone else with a Supernova 750 G3 that posted a few days ago has the same problems I have, so its not just me.
forums.evga.com/FindPost/3191479
 

Yea, it seems with some designs there is quite a difference between what the PSU can provide, and what it is willing to instantaneously provide.
 
I can run a 200w load on the CPU (Cinebench R23) and simultaneous 370w load on the GPU (3dmark Firestrike stress test) for hours at a time, and its rock solid, but then it'll power off when exiting from a loading screen while my GPU is at 60% usage even within the first 5mins of starting a game up (or two hours later, depending on what mood its in).
 
I do appreciate the 3080 didn't exist when the G3 was launched, but it was EVGA who said it can provide 750w and who decided to sell it with a 10 year warranty, so while unfortunate for them as a company, its on them to resolve. Promising something will work for 10 years is always a gamble, but its one they choose to make.
 
Unfortunately though, their current course of action appears to be to hope I get bored and buy a good PSU from another vendor and leave them alone.