THG tested 13 power supplies with rated outputs from 400 to 520 Watts in order to determine the top performer. However, a study of the model identification stickers immediately revealed limitations. The combined output on the +3.3, +5 and +12-Volt lines is, in most cases, 20 Watts below overall output. This output gap is reserved for the standby line and the -5 and -12-Volt lines no longer needed by most systems. On the other hand, the power supplies' plus-Volt lines are significant for powering the components of a PC. There were clear differences among the test candidates in this area. The Zalman ZM400A-APF, for instance, is designed for a combined maximum load of 380 Watts for the +3.3-, +5- and +12-Volt lines, while the Chieftec HPC-420-302 DF delivers 400 Watts. This is why, when buying a new power supply, it's very important to familiarize yourself with the +-Volt lines that supply the processor, cards, hard drive and other drives.

Combined Output

Combined output measures the total power produced by combining the +3.3- and +5-Volt lines and the +3.3-, +5- and +12-Volt lines. In this arena, several power supplies have severe limitations. One of these is the Seasonic SS-400FS; only 180 Watts are available for both the +3.3- and +5-Volt lines. Since graphics cards and drivers, in particular, use these two lines, it means that the output is quickly exhausted. In the 450-Watt-and-above class, a 200-Watt restriction is very low, such as with the Enermax and Noisemagic power supplies. Also, the combined load of the three plus-Volt lines for the Enermax EG465AX-VE(W) FMAX is rated at a mere 432 Watts. That means that output is a far cry from that suggested by the product name.

Even more problematic is the fact that the amperage ratings of the individual lines are likewise limited to the highest values. There is a difference of up to ten amps among the separate power supplies on the +3.3- and +5-Volt lines.

The Topower device, for example, limits the +3.3-Volt line to 26 Amperes. That is a low value for this class, which is normally available with reliable 400-Watt power supplies. On the Herolchi HEC-450LR-AT, the +12-Volt line is limited to just 17 Amps. Values above 30 Amps are the rule in the 450-Watt-and-above class.

A plus point is when manufacturers such as Zalman provide in-depth details on the power supply's identification sticker. The devices from Fortron Source and Seasonic, conversely, only list the combined output of the +3.3- and +5-Volt lines.