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General Quality Specifications
There are some power supply specifications that don't pertain specifically to its operation, but are just indicative of its overall quality. Be sure to pay attention to these, as they can give you a good indication of the power supply's general quality level.
Noise Level: Normally rated in dB (decibels). The larger this number, the more noise the power supply puts out. Early PCs had only two components that were constantly moving, and therefore generating noise: the hard disk drive motor, and the power supply fan. In contrast, today's PC are a veritable cacophony: higher-speed disk drives (multiple in many systems), removable drives, power supply fans, case cooling fans, CPU fans. As a result, users have begun to notice that their PCs are loud, and many have started to take the effort to buy or build systems that will cut down on the racket. While most power supplies of the same form factor and similar output will produce similar amounts of noise, some are better than others. In particular, look for power supplies with "low noise" or "silencer" specifications. The component of the power supply that primarily influences its noise level is, of course, the power supply fan.
MTBF / MTTF: Mean time between failure / mean time to failure (essentially, these are the same thing; not exactly, but close enough). These figures are an estimate of the number of hours that this type of power supply, statistically, will run before failing. Typical values for power supplies are 30,000 to 50,000 and up. It's important to realize that these numbers are both estimates and averages--they are not a guarantee for each power supply unit. Also, these numbers reflect average failure rates within the normal service life of the power supply--a 100,000 MTBF figure does not necessarily mean the manufacturer is saying it expects the power supply to run continuously for 11.4 years, even on average. See the discussion of MTBF in the hard disk area for details.
Warranty: The term, in months or years, during which the manufacturer will agree to repair or replace the power supply if it fails. While to some extent the warranty is a marketing tool, it is still the best indication of how good the manufacturer thinks its own products are--no company will put a three-year warranty on a product it believes will fail in 18-24 months. Obviously, look for the longest warranty period you can find, but bear in mind that not all warranties are created equal. Check the terms and conditions of the warranty, and also check out the reputation of the company, especially with regard to warranty service.