When selecting a PSU, it is important to consider the wattage, efficiency ratings, form factor, and price. The wattage determines the power the PSU can provide to the computer. Efficiency ratings, such as 80 Plus, measure the efficiency of a PSU and can affect the amount of electricity it consumes and the amount of heat it generates. The form factor determines the size and shape of the PSU and whether it will fit in the computer case. Price is also an important factor to consider, as higher-quality PSUs tend to be more expensive and more reliable and have longer lifespans.