Logic Behind Power Supply Recommendation

[astromechanic]

So I have the following system:
3770 i7 Processor 3.4GHz
12GB Generic DDR3
EVGA GTX 670 FTW 2GB
256 Crucial SSD
3TB WD HDD
6 Case Fans
(I don't remember the exact motherboard, but it's not high end)
While I know it's probably overkill I chose a 850W XFX Power Supply so I have plenty of room to grow if I want to in this computer case since I like it a lot.
Anyways, I plugged my computer into a Kill-A-Watt P3 unit which is suppose to measure power usage within 0.02%. I started up 3DMark and ran the Fire Strike benchmark which maxes out my CPU and GPU according to OpenHardwareMonitor. The Kill-A-Watt only showed 251 Watts Max usage with around 2 AMPS. So while my power supply is over kill, I imagine 500-600W PSU would be recommended with a good amount of amps on the 12v rail for my video card. So what's with actual usage not coming anywhere close to power supply recommendations. I know you want some headroom on top, but 250W+ of headroom. Do PSUs like create more power or something and from the PSU on it's actually using more power, that doesn't make sense to me, but my knowledge of the actual workings of a power supply are limited. I know how to mount it and find the right plugs for components but that's about it. Sorry if this is in the wrong forum, couldn't find power supplies on the drop down menu when posting.

 

[jimthenagual]

i7-3770 TDP is 77W
670 FTW Tdp is 170W
Rest of system 100W
Total max draw: 347W. That's if you can get all components to draw 100% power which is not realistic. As you said, you could only get it to draw much lower than that. Still, in calculations, it is typically the TDP that is taken into account when calculating power supply needs. As you are seeing, Computers can run on MUCH less than what is typically recommended.

Still, I usually take that number above and add 150W for overhead to keep the PSU running at low loads, so I personally would be looking to a 500W psu to run your system.