Psu for build
hows does one determine the psu you need to power your build?
There are psu calculators out there. Unfortunately most of us can't accurately determine such factors as cpu usage, capacitor aging and other inputs to the models.
I find it simpler to use a rule of thumb.
The psu requirements are mostly determined by the graphics card/s.
Assuming a normal complement of hard drives, fans, dvd' etc,
Here is what EVGA recommends for their graphics cards:
GTX550ti needs 400w with 24a on the 12v rails plus one 6-pin PCI-E power lead.
GTX560 needs 450w with 24a on the 12v rails plus two 6-pin PCI-E power leads.
GTX560Ti needs 500w with 30a on the 12v rails plus two 6-pin PCI-E power leads.
GTX570 needs 550w with 38a on the 12v rails plus two 6-pin PCI-E power leads.
GTX580 needs 600w with 42a on the 12v rails plus one 6-pin and one 8-pin PCI-E power lead.
GTX590 needs 700w with 50a on the 12v rails plus two 8-pin PCI-E power leads or 4 6-pin power leads.
It is not wrong to overprovision your psu .
But... do not try to economize on a cheap psu.
Get a quality unit, and you will get what you pay for.
My short list of quality psu's would include Seasonic, XFX, PC P&C, Corsair, and Antec.
If you want to objectively, accurately and scientifically determine what PSU power is required for your Vid card and PC in both watts and 12v rail amps., the forum Utility link below will show you how easy it is to calculate this information and objectively determine which PSUs are quality built, reliable PSUs that can meet your needs. Be advised that the available 12v rail amps. is just as important as the total PSU wattage. You need both to be correct.
Put your specs in the calculator and then select a quality brand! http://extreme.outervision.com/psucalculatorlite.jsp
What Rolli said.....don't forget fans, connected USB (KB, Mouse, phone, etc) ... figure 15% for capacitor aging on a tier 1 or 2 PSU.
Given current pricing there's no reason to get anything below 650 watts as, for a quality unit, there's no real cost savings.
750 watter is good for choice for twin 6950's / 560's
850 watter is good for twin OC'd 6970's / 560's / 570's
100 watts needed for twin 580's