Originally Posted by Recursion
I have just bought a 6950 but cannot unlock the shader. me too, I have a xfire 2x 6950 and my first is unlock to more shader. my psu is a 500 watt 80+ gold and my calculation is like this:
2x 200 watt xfire 2x gpu 6950
1x 150 watt cpu amd x2 phenom be 55
1x 50 watt fans and disks
600 x 0,90 efficiency = 540 watt maximum
that's a bit more but only about 10%. When you google for xfire 6950 you can see often 400 watt at full load. Hence a 500 watt is plenty let alone a 750 watt.
Your 500 watt 80+ gold PSU is rated at 500 watts total OUTPUT. So, according to your math, you could be drawing 600 watts out of the 500 watt power supply. That's 100 watts, or 25% over the rated power output. The 90% efficiency rating just means that to deliver 500W output, it's going to draw 500/0.90 = 555.55 watts from the wall = 500W DC output + 55.55 watts of wasted heat.
PSU are ALWAYS rated by power output, not power input.