Power rating

Computer power supplies are rated based on their maximum output power. Typical power ranges are from 500 W to lower than 300 W for small form factor systems intended as ordinary home computers, the use of which is limited to Internet-surfing and burning and playing DVDs. Power supplies used by gamers and enthusiasts mostly range from 450 W to 1400 W. Typical gaming PCs feature power supplies in the range of 500-800 W, with higher-end PCs demanding 800-1400 W supplies. The highest-end units are up to 2 kW strong and are intended mainly for servers and, to a lesser degree, extreme performance computers with multiple processors, several hard disks and multiple graphics cards. The power rating of a PC power supply is not officially certified and is self-claimed by each manufacturer. A common way to reach the power figure for PC PSUs is by adding the power available on each rail, which will not give a true power figure. Therefore it is possible to overload a PSU on one rail without having to use the maximum rated power.
Sometimes manufacturers inflate their power ratings, in order to gain an advantage in the market. This can be done due to a lack of clear standards regarding power supply labeling and testing.
Advertising the peak power, rather than the continuous power;
Determining the continuous output power capability at unrealistically low temperatures (at room temperature as opposed to 40°C, a more likely temperature inside a PC case);
Advertising total power as a measure of capacity, when modern systems are almost totally reliant on the current available from the 12 volt line(s).
This may mean that if:
PSU A has a peak rating of 550 watts at 25°C, with 25 amps (300 W) on the 12 volt line, and
PSU B has a continuous rating of 450 watts at 40°C, with 33 amps (400 W) on the 12 volt line,
and those ratings are accurate, then PSU B would have to be considered a vastly superior unit, despite its lower overall power rating. PSU A may only be capable of delivering a fraction of its rated power under real world conditions.
This tendency has led in turn to greatly overspecified power supply recommendations, and a shortage of high-quality power supplies with reasonable capacities. Very few computers require more than 300–350 watts maximum.[1] Higher end computers such as servers and gaming machines with multiple high power GPUs are among the few exceptions. Although, in recent years the power demand of "video cards" in the ability to watch high definition (HD) media has led to even the average ATX computer to consume between 400 and 500 watts. It may be expected to increase again as the media transitions to 3D.