The power supply supplies the electrical power for a computer. It supplies power to the motherboard, drives, and certain expansion cards. It normally has at least one fan that helps cool the power supply and will assist in the task of cooling the computer. Some power supplies have an additional outlet on the back that can be used to provide power to the monitor. Power supplies come in a variety of wattages. They range anywhere from around 160 watts to about 700 watts. 350 to 400 watt power supplies are probably the most common. Some video cards need quite a bit of power in order to operate effectively. Motherboards that support nVidia's SLI, (or Scalable Link Interface), have the ability of connecting two video cards together. This adds an even greater demand on the power supply. For reasons like this, many people prefer power supplies that have at least 450 to 500 watts. A higher wattage power supply doesn't hurt anything, but a lower wattage power supply can cause problems for people with lots of devices connected to their computer. It is important to make sure that a power supply will connect properly to your motherboard before you purchase it. There are several different types of motherboard power connectors including the older AT type, the ATX 20-pin, the ATX 24-pin, and the EPS 24-pin. Another thing to consider when purchasing a power supply is its PFC, (or Power Factor Correction). PFC protects against things like voltage fluctuations and electrical irregularities that can affect the efficiency of your power supply. There are three types of PFC: active, passive, and non-PFC. Active PFC provides the best protection, while non-PFC provides the worst. A good power supply will cost between $50 and $100.