I am a bit confused over the terminology and efficiency rating of a power supply. For instance, under 100% load my 1200W 80+ Gold power supply is listed to operate close to 88% efficiency. What does the 88% efficiency rating apply to: the power consumption measured at the wall, or the power consumption measured at the interface points to the computer?

I initially assumed it was the latter but I put together a system that measures a 1040W power draw at the wall (with a Kill-A-Watt meter). At this point, the system starts to become unstable and eventually shuts down. I suspect the need for more power since 88% of 1200W is 1056W -- conspicuously close to my measurement.

Edit: I am using a Corsair AX1200 80+ Gold power supply.


The efficiency is a measure of the conversion between the mains to output.
The higher the efficiency the less power loss as wasted heat.

Your system may be becoming unstable because you are probably drawing too much power on one of the power supply rails.

You haven't said exactly which power supply you have but as an example

My Silverstone strider ST1500 is rated at 1500W. However look at these numbers:

Power Capacity  1.5 kW
Output Current  +3.3V - 40 A / 0.8 A 
+5V - 40 A / 0.5 A 
+12V1 - 25 A / 0.1 A 
+12V2 - 25 A / 0.1 A 
+12V3 - 25 A / 0.1 A 
+12V4 - 25 A / 0.1 A 
+12V5 - 25 A / 0.1 A 
+12V6 - 25 A / 0.1 A 
+12V7 - 25 A / 0.1 A 
+12V8 - 25 A / 0.1 A 
+5VSB - 6 A / 0 A 
-12V - 0.5 A / 0 A

If I try to draw more power than 40A on the 5V rail or more than 25A on a 12V rail I should expect problems. I think this is what you're seeing.

If you happen to have spare power rails that you're not using then you may want to add another cable say onto the graphics card.

  • Thank you for your insight! I have two graphics cards connected to the same peripheral cable via a molex->PCIe. I had a tiny suspicion thought this might be the problem and will have to check the amperage rating of each rail now. – Steve Guidi Sep 26 '11 at 23:44
  • Most likely that's your issue. – Matt H Sep 26 '11 at 23:48
  • I've determined that there is only one 100.4A rail in my Corsair power supply, so I don't think it should matter how I've connected the graphics cards. I suspect that I may indeed be hitting a power draw limitation -- is the amount I've measured at the wall the ouptut of the supply? – Steve Guidi Sep 27 '11 at 21:09
  • Steve, Unlikely that it's power draw. I suspect the problem is your power supply is either running out of spec or a motherboard problem. If you remove one of your graphics cards does it run stably? Also check the BIOS settings and make sure your system is not shutting down due to temp setting in the bios. – Matt H Sep 29 '11 at 0:50
  • 1
    hmm, lights should not flicker at all. They should be on their own circuits. – Matt H Nov 7 '11 at 0:05

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