Computer power supply units are rated in Watts, according to their maximum possible power output. While it sounds simple, it is not. Most companies give shady or no specifications regarding if they are talking about the maximum continuous or maximum peak output, which are entirely different values. The maximum continuous power indicates the power the device can continuously output while operating at a given ambient temperature (see below), while the maximum peak output is the maximum possible power the device can output for a given period of time (usually less than half a second!) while the system is starting up. The peak power output only last for a very brief period of time and is usually much higher than the actual continuous power output, which is why cheap units are often advertised focusing on the peak output rating.
At the same time, almost no companies specify the ambient temperature in which the power supply can perform as such. You see, higher temperatures make a huge impact on the power supply’s performance. A power supply rated to output 500W at 25°C will not be able to offer more than 60-65% of that value at 50°C, while a power supply rated to output 500W at 50°C will vastly exceed its specifications at 25°C. Therefore, cheaper units have ratings which can be very inaccurate when used inside actual systems where the ambient temperature will certainly be quite higher than the average room temperature.
What’s even more disturbing is that most people have been mislead to believe that a more powerful power supply will always be better than its inferior (in terms of maximum power output) counterpart. Not only that is not true but a too powerful power supply will probably deliver inferior performance.
Here is why: All computer power supplies are designed to deliver their maximum possible performance at 40-60% load. No design can be used to deliver its maximum output for a very long period of time, plus a headroom is necessary to ensure stability against devices which require large starting currents (e.g. hard drives) and other power fluctuations. Therefore only at 40-60% load the electrical and thermal efficiency of a power supply is at its peak and that is because engineers rightfully design the retail products like that. As anybody can understand, a too powerful power supply will be delivering less than 40% of its actual capacity; or even worse, less than 20%, where the efficiency graphs literally take a dive. Choosing the power output of your power supply properly and wisely is far more vital to the overall performance of your system than most people like to think.