Articles :: Power Supplies 101: A comprehensive guide ::

Jon Gerow · 06-26-2006 · Category: Guides

Power Supply Efficiency

The efficiency of a power supply is determined by taking the DC output of a power supply and dividing it by the AC input. If a power supply draws less AC to produce more DC, then that power supply is more efficient then one that draws more AC to produce the same amount of DC.

(DC Output) / (AC Input) = Efficiency

So if we have a power supply that was currently putting out 250W, and it's pulling 300W from the wall, then we can figure out it's efficiency by dividing 250 by 300, like so:

250 / 300 = 83%

The higher this percentage of efficiency is, the less power a power supply is going to use to run your computer. Typically, the efficiency of a quality power supply is between 75 and 85%.

Another advantage to better efficiency, outside of a lower electric bill, is less heat. The 15 to 20% of power that's unaccounted for is dissipated as heat. So a more efficient power supply should run cooler, require less cooling and run altogether quieter.

Something to remember about efficiency is that efficiency is not a straight line. It's not even a slope, really. It's more like a bell curve, with a high point somewhere in the middle and a low point at the beginning and an even lower point at the very end.

Let's say we have a 500W power supply with a minimum efficiency of 70%. If we were to graph the efficiency, we would likely see the 70% at 500W, 75% at 100W and below and, who knows, perhaps 78% at 250 to 350W.

This is the efficiency "bell curve" from an Ultra X2 550W power supply.

What's good to know about this is that your computer isn't likely going to pull either the minimum or maximum wattage of a particular PSU. (unless you have a PSU that's considerably under powered for your system) It's most likely that your PC is only going to be typically using about 50% to 70% of the wattage that your power supply is capable of producing, at the most.

This brings us back to labels and how different companies do things differently.

Some power supplies might say "70% minimum efficiency at maximum load." Another might say, "maximum 80% efficiency." Which power supply is more efficient? Would you believe they're both the same? Two power supplies, from the same factory, labeled differently by two different marketing groups. So do make sure that you notice if the power supply's label is telling you the minimum or maximum efficiency of the power supply and don't assume that 70% simply means 70% efficient and that 80% means 80% efficient. Personally, I wish all power supplies would actually rate for maximum efficiency. I know it might seem misleading because it's only a number representative of the best capability of that power supply, but you know that bell curve I told you about? You're bound to be on that curve and not somewhere on either end of it.


  1. Introduction
  2. The PC power supply:
  3. The PC power supply label:
  4. Defining the connectors of an ATX power supply
  5. ATX power supplies DO NOT turn on at the flip of a switch
  6. Testing your power supply's voltage: Software vs. Multimeter
  7. Power Supply Efficiency
  8. The Derating Curve
  9. Power Factor Correction
  10. The resistance of modular connectors, adapters and splitters
  11. Conclusion

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