Sometimes the specifications are at odds with vendor technical preferences. This explains the situation typical in all cheap PSUs - whose production is more about quantity than quality - that consume more energy than is really necessary, or that fail when used in certain system configurations. On the other hand, high-quality PSUs tend to function flawlessly, in our experience. This also reflects the last endurance testing (by our German staff from February 2007) in which five out of nine models rated at up to 550 W failed before the 24-hour testing period expired.
In part, some vendors tend to exaggerate PSU power outputs. This leads to official ratings that are simply unreachable in actual use. Alas, this can lead to underpowered system components, and contribute to intermittent instability problems. In addition, the components inside a PSU consume energy themselves, which can lead to greater or lesser power consumption and heat production, depending on the quality of those components. As with motherboards, defective condensers in a power supply can bring an entire system crashing down (for more details, see our article How to Fix Your Motherboard for $15).
Power Supply Rating Myths
In theory, the power that a power supply delivers can only be as great as the power the device itself consumes. In reality, this represents 100% efficiency, a performance level that power supplies can never attain. The transformation of 110 V or 220 V A/C power into various D/C voltage levels inside a PSU involves some waste, with the majority of such waste energy making itself felt in the form of heat produced inside the PC's case. This means that the rated wattage that a power supply can deliver must be strictly less than the energy it consumes before it starts the voltage transformation process.
By calculating the ratio between energy consumption and energy production, we produce a number somewhere between zero and one. Thus for example, net energy production of 450 W divided by gross consumption of 550 W at maximum load produces a value of 0.818. This number represents the efficiency of the power supply. Commonly this efficiency index is represented as a percentage value, which may be calculated by multiplying the previous ratio by 100, to produce in our example a value of 76.4%.
Vendor wattage ratings on PSUs always represent the maximum output that the device can deliver. A 350 W PSU with an efficiency rating of 70% must therefore consume a maximum of 500 W, though this occurs only when the components that the power supply drives actually consume the entire 350 watts. The real efficiency of a PSU is not a constant value either; rather, it changes with the amount of power that the device delivers at any given moment. The ATX12V Power Supply Design Guide requires that PSUs deliver minimum efficiency of 65% under light load, 72% under normal load, and 70% at peak load. There is also a recommended efficiency regimen that ups these levels to 75% for light loads, 80% for normal loads, and 77% at peak loads. Here, the term "load" must be understood as the power consumption of the system, as measured in amperes.