When a power supply is marked as 750W, does this mean that it draws 750W from the wall at 100% or that it delivers 750W at 100% to the computer, which would in this case draw like 850W from the wall because of its 80-90% efficiency?
A PSU that runs at its peak generates more heat. Heat is the no.1 degrader of components, esp caps. So running it for extended periods at peak can shorten its lifespan.
Always allow for some headroom, to be able run a 750w system at full load on a 750w PSU would be incredibly efficient, which systems are not, and PSUs are not Also hardware degrades over time, so after not too long, the PSU wouldn't be able to provide that full 750w a 900w+ PSU would have plenty of headroom to duck and dive so to speak Plus if you wanted to add another HDD or upgrade something, you'd be ok within a margin The PSU will only pull from the wall what it needs for the system, so don't worry that a 900w PSU will bankrupt you vs a 750w one
Than I guess I should buy a 1250W instead of 1000W for a 4930k and 2x 780ti on water. Sucks because it's more noisy.
I would myself not wory about using a powersuply at full load as most are tested to be able to suply full load at the listed efficiency. I if its a decent make you should be fine