Most power supply units are designed to cover a wide range of applications from the 100-120 volt standard in North America and Japan to the 220-240 volt standard in Europe, Asia, Africa, and Australia. A PSU’s wattage rating is the most common specification cited by manufacturers and is based on the total maximum power of which a PSU is capable.
Typically, a desktop PSU will range from under 400 watts at the low end to over 1000 watts at the high end. Some PSUs are capable of supplying power in the 2000 watt range but these units are very expensive and used only for servers, workstations, and very extreme home computers with multiple processors and video cards.
The problem with power ratings is that there is no standard industry practice for reporting a PSUs wattage. Whereas one manufacturer may report a wattage of 800, another may report a wattage of 750 for the same unit.
Certainly, when competing on wattage output as a measure of power supply quality, manufacturers have an interest in reporting the highest wattage possible. Sometimes they get a little, shall we say, creative.
PSUs are usually made up of multiple rails or pathways through which power flows at different voltages the computer components need. This means that the total wattage delivered by a PSU can not be delivered on just one rail but must be split across several. This is an important consideration when, for example, powering multiple video cards, which require a lot of power specifically from the 12 volt rails. Luckily, most standard computers will not run into these sorts of problems.