Power Rating
As all of the rails come from one transformer and primary-side switching components, there is an overall maximum power limit. Power requirements for a modern desktop personal computer may range from 300 watts to more than 1000 watts for a high-performance computer with multiple discrete graphics cards. The power rating of a PC power is rated by the manufacturer. Simple, general purpose computers rarely require more than 300–500 watts maximum.
It is possible to overload one voltage from a power supply well below the total rating of the power supply. For example, most PSUs create their 3.3 V output by regulating down their 5 V rail. As such, 3.3 V and 5 V typically have a combined limit as well. A 3.3 V rail may have a 10 A rating by itself (33 W), and the 5 V rail may have a 20 A rating (100 W) by itself, but the two together may only be able to output 110 W. In this case, loading the 3.3 V rail to maximum (33 W), would leave the 5 V rail only be able to output 77 W.
Since supplies are self-certified, a manufacturer's claimed output may be double or more what is actually provided. Although a too-large power supply will have an extra margin of safety against overloading, a larger unit is often less efficient at lower loads and therefore wastes more electricity than a more appropriately sized unit. For instance, an 80 PLUS, 520 watt supply could still be 70% or less efficient at 60 watts (a typical idle power for a desktop computer). Some power supplies have no-overload protection.
The most important factor for suitability for certain graphics cards is the PSUs total 12V output. If the total 12V output stated on the PSU is higher than the suggested minimum of the card, then that PSU can fully supply the card. However a system will have other loads on the 12 volt supply.
Power supplies are usually sized so that the typical calculated system consumption is about 60% of the rated capacity, and the maximum system demand does not exceed the rated capacity of the supply. The power supply ratings often given by the manufacturer of single component, typically graphics cards, should be treated with skepticism. These manufacturers want to minimize support issues due to under rating of the power supply specifications and advise customers to use a more powerful power supply to avoid these issues.
Read more about this topic: Modular Power Supply Unit
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