Debunking Power Supply Mythsby Christoph Katzer on September 22, 2008 3:00 AM EST
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All power supplies have a specific efficiency curve, which we show in our reviews. As an example, we will use the Cooler Master UCP 900W power supply. Here is the efficiency curve we measured during testing:
Efficiency is the output power divided by the input power, as there is a certain amount of power lost during the AC to DC conversion. The x-axis shows the power supply load in Watts and the y-axis shows efficiency. Let's include our three sample systems in the chart see what sort of efficiency they would get with this power supply.
The first system causes this high-performance power supply to only run at 73% to 81% efficiency, depending on input voltage. Obviously, there's absolutely no need for a 900W power supply if you're running this type of computer.
The midrange system looks quite a bit better, allowing the PSU to run at 80% to 88% efficiency, although the latter only occurs at maximum load. Considering the vast majority of systems rarely run at 100% load most of the time, real-world efficiency will average closer to 82%. Office work and Internet surfing in particular will be at that level.
For the third system, a 900W power supply actually might start to make sense. It's still more than you need, but having a bit of extra room to grow isn't a bad idea. This system idles at over 300W, so it achieves a minimum 86% efficiency with 120VAC. When running a game or other demanding task, the PSU is finally able to reach its potential and provide 89% efficiency with 230VAC (or 87.5% with 120VAC).
The quick summary then is that if you don't have a system that uses 350W of power when idle, it's probably not worthwhile to purchase this type of power supply. Our high-end sample system more or less meets this qualification, and if you were to take such a system and overclock it, these high-end power supplies are actually required. The 8800 Ultra is one of the most demanding graphics cards currently available; however, the GTX 280 appears to require even more power, making that another candidate for this sort of PSU. (Unfortunately, our power supply testing labs didn't have the latest GPUs available for testing.)