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.

Three Seasonic ATX Power Supplies Reviewed - Cases and Cooling 61

According to the ATX12V Power Supply Design Guide, power supply efficiency should be a minimum of 70% at full load, 60% under a typical load, and 50% while under a light load.  The input voltage was set to 115 VAC.  I measured the AC power input with a WattsUp? Pro watt meter and calculated the combined DC power output by summing the products of all the DC outputs (volts x amps).

Three Seasonic ATX Power Supplies Reviewed - Cases and Cooling 62

As you can see, all three Seasonic PSUs exceeded the recommended 60%~70% range for a ‘typical’ load (237.3 watts).  Due to Seasonic’s efficient circuit designs, these power supplies averaged about 5% better efficiencies than other power supplies in their class.  Better efficiency translates to lower operating costs.  Spending a little more money up front to purchase an efficient power supply may very well pay for itself over the lifetime of the PC.

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.


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