Quik quastion about PSU

I need to get this straight:

Lets say my PC uses 500 W power.

I buy 700 W PSU with 90% efficacy. As I think it should waste 10% of power, than i get 630W power and it would waste 70W as heat.
If i buy 1200w PSU with 90% efficacy, than it will waste 10% = 120w as heat and my PC will use 500W ? Or my PC will use 500W and PSU waste 50w (10% of 500w) as heat?

My main question is: PSU waste 10% of power that PC is using (in this case 500w) or it will waste 10% of all PSU capacity (in this case 1200w)?

Im planning to buy FSP Aurum Pro 1200W if its matter u to answer correctly :)

Thank U for your help
3 answers Last reply
More about quik quastion
  1. Why buy a 1200w psu? A 90% efficient psu will use 10% more power than the PC needs as you are right, it gets wasted as heat. So a PC using 500w will put 550w from the wall with a 90% efficient psu. That's no matter what wattage psu you have.
  2. So that 10% is from W that PC actually uses, NOT PCU MAX W? That what i need to know. Main reason why i will buy 1200w: Im planning to add more rams and videocards (my PC actually is server) :) And i will get that 1200w PCU very cheap, I asked because I need to know if that 10% isn't from PCU max 1200W that is 120w. If so, than i really shouldn't buy it because my bill for electricity will be huge :)
  3. P. S. can anyone say if FSP Aurum Pro 1200W is compatible with

    intel S5520SCR 905722 :

    http://ark.intel.com/products/36600/Intel-Workstation-Board-S5520SC

    This PSU has all thingies that this mobo needs? Im not so good at this, this is my first built :)
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