The math I did was a little different, piecing together measurements from what I've seen online (or speculations, in the case of the 3950x), as I am trying to estimate the power load at max when overclocking on water.
The 2080 ti when it's flashed and overclocked can pull near-400W, the 3950x could possibly pull as much as 250W when overclocked on full load, the RAM I put at 3W per 8G so 24W total, this x570 mobo has killer vrms so I put it at like 30W (according to the Buildzoid video you could go even higher than this?), custom loop I've seen anything from 20W to 50W so I erred on the high side, PCIe 4.0 M.2 NVMe SSD I put at 10W, fans I put at 1.8W per fan and just rounded up to say 10W.
Adding it all up that's 400 + 250 + 24 + 30 + 50 + 10 + 10 = 774W
And if I want that to be at the 50% efficiency point doesn't that put me near-1500W?
I know this is much much much higher than the usual recs but I want to know where my logic is off.
In the past every time I've taken recs it ends up being underpowered because people assume I am not multitasking as much as I say I am (I game on one monitor and usually have lots of things going on on the other screen, ranging from resource-hungry code on multiple IDEs for all my projects, to video editing, to virtual machines, to Photoshop, to Chrome with lots of tabs to streaming etc).
I want to make sure that even when my computer is cranking away at a high load, there will be more than enough power to run it all efficiently.
How do I better calculate all of this?
The wattage numbers you're adding up for CPU and GPU are peak loads under stress test conditions and as said by many above, i say it again: just because those watts were achieved under lab conditions doesn't mean that you'll also get same power consumption out of them.
I'll run the numbers again:
GPU is 260W, with OC let's put it 360W (adding another 50W to my initial estimation)
CPU is 105W, with OC let's put it 205W (adding another 50W to my initial estimation)
Open loop is 50W
Rest of the system i estimated at 150W but since you dig out exact numbers, it's 74W
Making the total for OCd PC: 689W
Here, as i said above, 850W unit is more than enough, leaving 161W as spare headroom. Even if i were to take your peak wattage number (774W), for 850W PSU, free headroom would be 76W. Also, peak load doesn't continue for minutes or hours. Peak load is only milliseconds in duration which good quality PSU can handle just fine.
Do note that with CPU, it's the luck of silicone lottery. If you get lucky, you'll get CPU which you can OC to e.g 5.2 Ghz. But if you don't get lucky, max OC where your CPU would be stable may be only 4.8 Ghz. Lower OC level doesn't consume as much power as higher OC level. So, there's that.
If you don't believe our expert estimations then you can use online power calculators as well,
link:
https://outervision.com/power-supply-calculator
Also, you got the PSU's efficiency wrong. When i said that:"PSU's are most efficient when load on them is between 50% - 80% of their max rated wattage", what it means is that for 850W PSU, it's the most efficient when the load on it is between 425W and 680W. If the load on PSU would be outside of those numbers, PSU's efficiency, at most, would drop 3%.
PSU's efficiency works like so:
850W 80+ Bronze PSU - 20% load (170W) with efficiency of 82% will deliver 170W to components while drawing 200W from the wall. 30W would be wasted as heat.
850W 80+ Gold PSU - 20% load (170W) with efficiency of 87% will deliver 170W to components while drawing 192W from the wall. 22W would be wasted as heat.
850W 80+ Titanium PSU - 20% load (170W) with efficiency of 92% will deliver 170W to components while drawing 183W from the wall. 13W would be wasted as heat.
850W 80+ Bronze PSU - 50% load (425W) with efficiency of 85% will deliver 425W to components while drawing 488W from the wall. 63W would be wasted as heat.
850W 80+ Gold PSU - 50% load (425W) with efficiency of 90% will deliver 425W to components while drawing 467W from the wall. 42W would be wasted as heat.
850W 80+ Titanium PSU - 50% load (425W) with efficiency of 94% will deliver 425W to components while drawing 450W from the wall. 25W would be wasted as heat.