Testing – Cont’d

Testing — Efficiency


The efficiency of a power supply is defined by the power output divided by the power input and is usually expressed as a percentage.  If a PSU were a 100% efficient (which none are) 400 watts of AC power going in would result in 400 watts of DC power coming out.  In the real world there are always inefficiencies and power is lost in the form of heat during the conversion process.


Seasonic S12-600 watt Power Supply - Cases and Cooling 29


According to the ATX12V Power Supply Design Guide, the minimum measured efficiency is required to be 70% at full and typical loads and 60% while under a light load.  The input voltage was set to 115 VAC.  I measured the AC power input with the Seasonic PowerAngel watt meter and calculated the combined DC power output by summing the products of all the DC outputs (volts x amps) for three different DC loads.  In addition to using my own home made load tester (262.6 W), I also used the load tester kindly provided by Seasonic.


Seasonic S12-600 watt Power Supply - Cases and Cooling 30


As you can see, the Seasonic S12-600 easily exceeded the recommended 60%~70% range for all three loads tested.  Due to Seasonic’s efficient circuit designs, their power supplies typically produce higher efficiencies than other power supplies in the same class.  Better efficiency translates to lower operating costs. 


As mentioned in the previous section, having active PFC onboard does not ‘technically’ make a power supply more efficient because efficiency is based on true power — watts, not apparent power — VA.  In fact, active PFC can have a slight negative affect on overall PSU efficiency due to the extra circuitry.  However, having a power supply with active PFC will reduce the computer’s overall current draw and is more environmentally friendly to the AC distribution system.


I suspect this increased efficiency of the S12-600 is due more to the special Seasonic circuit design and has little to do with active PFC. 


Seasonic S12-600 watt Power Supply - Cases and Cooling 31





The 80 Plus Computer Power Supply Program



There is a growing awareness among users, PC manufacturers and electric utilities regarding the money and natural resources that could be saved by adopting higher efficiency power supplies.  One group that is spearheading this new movement is Ecos Consulting.  You can learn more about their efforts to promote power supplies with better than 80% efficiency by visiting the 80 Plus Program website.


Spending a little more money up front to purchase a high efficiency power supply may very well pay for itself over the lifetime of the PC… 🙂

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