Powering Your PC: Truths and Misconceptions
A Guide To Power Supplies

Power Supply Efficiency:

A power supply's efficiency rating is determined by the ratio of AC power going in to the DC power going out. Generally this comparison is done using Watts as a common unit of measurement. Anything less than 100% efficiency is dissipated wastefully as heat. In reality no power supply will be 100% efficient, that's just a fact of life, however you can (and should) avoid power supplies with very low efficiency ratings.

If a power supply is consuming 450 Watts from the AC side, but on the DC side the output is only 400 Watts, then the math tells us that the PSU is ~89% efficient in its power conversion and the other 11% (50W) is lost as heat. That 50W of heat is no different than if you were running a 50W light bulb, you pay for that power usage on your electric bill. Not to mention the heat must be cooled, so there is an additional cooling cost that some people don't think about. So in the short-term a cheaper PSU may fit your budget better, but in the long term you will end up paying for it several times over. It is definitely worth comparing power supply efficiency ratings and choosing the most efficient power supply that your budget allows for.

Below is a table from the ATX 12V v2.01 specifications⁵ stating the required & recommended minimum efficiency levels for PC power supplies (to be compliant with the spec).

Most modern quality power supplies seem to be floating around ~75% efficiency under a typical load, with some reaching as high as ~85% for enhanced silent operation. The greater the efficiency, the less heat generated, the less cooling required, and thus the capability for a quieter PSU.

Operating Conditions:

Power supplies are meant to operate under a variety of environmental conditions that aren't always the most optimum. Most PC enthusiasts tend to go overboard in the chassis cooling department, however some OEM designs, typical servers, and even extreme environments can restrict a PC to operate in the upper environmental range of its allowable specifications.

Heat is a power supply's worst enemy, every component in a power supply is designed to work most optimum at a specific temperature, but not necessarily a realistic operating temperature (especially under heavy loading conditions). Capacity diminishes as temperature increases, and eventually the part will fail if it exceeds design parameters. Many people do not think about this, depending on the average ambient temperature inside their PC chassis, the PSU may or may not be able to operate at its full capacity (depending on how well the PSU was designed).

According to the ATX 12V v2.01 specifications, a power supply should be able to operate and maintain running at full load in 50° C (122° F) ambient conditions. There are many power supplies that often overlook this specification, and will overheat and shutdown under a full load condition before reaching anywhere close to a 50° C ambient temp. I have seen one power supply that rated its specifications under 25° C (77° F) ambient conditions, which to say the least did not hold up well when the temperature increased. The only solution would be to reduce the ambient temperature, reduce the load, or get a better power supply.