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

Output Power Ratings:

A PC power supply has a maximum power rating which they are all classified by, usually expressed in Watts. In the first example picture below, the power supply is rated for a maximum total power of 470 Watts. However, crunching some numbers shows that the sum of the rated power for each rail does not equal 470W, in fact the total would be in the neighborhood of about 706W! But wait there's more, if you notice that the maximum wattage for the 3.3V & 5V combined is 280W, but adding up those two rails gives a combined 312W. Nothing adds up! Confused yet?

The reason the numbers do not add up is because of the whole power triangle between the three primary voltages (as illustrated by the graph in the previous section), the load demands on each rail have a direct effect on each others maximum output. As a PC performs different tasks, it will consume different amounts of power on different rails, everything is in a constant state of flux. However, on average the 12V rail is the most important for a modern PC. So always pay attention to the 12V rating, regardless of what the overall PSU rating is (Some PSUs have an oversized 5V rail, which pumps up the total power rating, but does the end user little good in a modern PC).

Unfortunately there is no clear standardized system of labeling or terminology when talking about power output, consumers are at the mercy of what the manufacturer may or may not provide. For instance, from the numbers below in the first picture, we do not know if any of those figures are continuous output or peak. For that matter we do not know what one manufacturer's definitions of "maximum" or "peak" are compared to another manufacturer. Maximum may mean peak for one manufacturer, while another may consider maximum to mean continuous, it is all one big gray area (which can lead to some shady practices by manufacturers).

If there are no labels stating maximum, peak, or continuous, then to be safe you should assume that the individual current ratings (expressed in Amps) are peak, and the combine (or total power) ratings (expressed in Watts) are continuous ratings. When wanting to determine continuous power, but only have peak (or unknown) numbers listed, a good guestimate is to take 90% of the number given.

The picture above is an example of the information provided on a typical power supply, minimal but still enough to make informed decisions. The picture below is an example of a power supply providing a little more information that may or may not be useful. The power supply pictured above is meant to conform to ATX 12V v2.0 specs, while the power supply below only conforms to v1.3 specs. Because of the different loading requirements between the two specs, there are different levels of emphasis placed on each voltage rail. Even though the PSU below has a higher overall power rating, the difference in power is mostly on the 5V rail (which may or may not be what you are looking for depending on the age of your PC).

The numbers can be further scrutinized when considering the operating environment a PSU must deal with. Heat will reduce the total capable power output, so you may or may not be getting the full rated capacity out of your power supply. This is what separates the quality power supplies from the cheap ones. The topic on operating conditions will be discussed in a couple pages, so I shall say no more at the moment.