The quietest power supplies are usually temperature controlled, that means that the warmer they get, the faster they spin the fan. If your pc generates a lot of heat, then a temperature controlled PSU may end up just as noisy as a cheaper standard PSU, but for most computers with should result in a much quieter machine.

Don't be fooled however by the descriptions manufacturers give their products (such as super quiet or ultra silent), check out the noise level figures for the PSU. These are usually measured in decibels (db). A reading of 25db is a quiet whisper, which you can just make out from a couple of meters away. 30db you can hear from the other side of the room, and may be too loud for a computer situated in the lounge. Any manufacturer not willing to provide real figures for the powersupplies noise level, is not worth considering. Check how the noise level rises as the temperature increases. You can usually find out the temperature inside your case, either from the computer BIOS, motherboard monitoring utility, or by simply using a thermometer. This should help you gauge how noisy the psu will really be.

To keep the noise down, we only want one fan in the power supply, and the larger the better. If the fan is mounted at the back of the psu, then it can be no more than 80mm in diameter. Some powersupplies have larger 120mm fans mounted on the bottom of the power supply, which should provide a quiter airflow. There is however usually a higher cost for these quiet PSUs. Remember to get a powersupply that has an adequate power rating for your computer. A 200 watt psu will be no use if you are running the latest Pentium 4 or Athlon processor. Likewise a 400 watt PSU is not necessary for 1Ghz celeron, or low power Via C3 processor. By matching the power supply to your system, you will be running the PSU at a level it was designed to work well at, and so should be at its quietest.

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