How do you pick the correct PSU wattage?

[ElectricSquid]

One thing I have noticed is that most PSU Wattage Calculators tell me some weird and low number when the card it self on the Manufacturer website tells me I need at least a higher number than what it gives me. Take this Build for Example: http://pcpartpicker.com/p/CYVtK8 It tells me 463W but I went ahead and chose an 850W. I would like some tips for picking the Wattage I need rather than guessing or trusting weird Wattage Calculators

 

[toyftw]

One thing I have noticed is that most PSU Wattage Calculators tell me some weird and low number when the card it self on the Manufacturer website tells me I need at least a higher number than what it gives me. Take this Build for Example: http://pcpartpicker.com/p/CYVtK8 It tells me 463W but I went ahead and chose an 850W. I would like some tips for picking the Wattage I need rather than guessing or trusting weird Wattage Calculators

Hi - GPU mfg's always estimate high to allow for cheap/outdated PSU's & PSU designs that don't deliver adv specs
or don't have enough +12v rail power. The CPU & GPU draw their current from the +12v rail.

Online PSU calcs tend to est on the high side as well. Prob because doing the reverse would put them out
of business.

A system with a gtx 780 needs 42 +12v amps to run properly. Most good quality modern design 650w units exceed that. Some 550w units will also have the necessary +12v amps.

A couple of good reliable sources:
http://www.realhardtechx.com/index_archivos/Page362.htm

http://forum-en.msi.com/faq/article/power-requirements-for-graphics-cards

Neither are perfect, but both are better than the others.