Choosing an appropriate power supply for a desktop requires estimating how much power the computer will use; experts typically recommend choosing a model from a brand with a reputation for reliability. Purchasing a power supply with extra capacity makes upgrading to more power-hungry components easier.
Computer components vary by how much power they draw, so it's important to choose a power supply with enough capacity to power all components. Power supplies are typically rated for their peak power performance, although they generally operate at peak level for only short periods of time, so it's important to use a power supply rated above the system's typical consumption.
Power supplies are a common point of failure on computers, and experts recommend spending a bit extra for a reliable brand. In general, power supplies are closed systems that are not designed to be repaired. As a result, any one of the many electrical components failing can lead to an expensive replacement. Reliable brands use more durable components, which can reduce the likelihood of a power supply failure.
Newegg and other companies provide calculators to help users choose the right power supplies for their desktops. Gaming-caliber graphics cards often force users to purchase more capable power supplies, especially top-of-the-line models and builds that use multiple graphics cards.