Short for power supply and sometimes abbreviated as PSU, which is short for Power Supply Unit. A power supply is an internal hardware component that supplies components in a computer with power. The power supply converts a 110-115 or 220-230 volt alternating current (AC) into a steady low-voltage direct current (DC) usable by the computer and rated by the number of watts it generates. For example, the image to the right, is of an Antec True 330, a 330 Watt power supply and an example of a computer power supply.

Computer power supply
Caution: Do not open the power supply, it contains capacitors that can hold electricity even if the computer is off and unplugged for a week.
Tip: You can protect your power supply and your computer from a surge and voltage drops by investing in a UPS. If you cannot afford a UPS, you should at the very least have the computer plugged into a surge protector.
Parts found on the back of a power supply

Below is a list of parts you may find on the back of the power supply.
A connection for the power cord to the computer.
A fan opening to draw air out of the power supply.
A red switch to change the power supply voltage.
A rocker switch to turn the power supply on and off.
On the front-end, which is not visible unless the computer is opened are several cables that connect the power supply to each of the devices and the computer motherboard. A power supply connects to the motherboard using an ATX style connector, other connectors include an auxiliary connector, Berg connector, Molex connector, and P4 connector.
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How big of a computer power supply should I get?

There are several factors to consider when determining how big a power supply you should get for your computer. These factors include:
1 Computer Case Size
2 Style of Power Supply
3 Wattage Requirements
4 Computer Case Size

Computer Case Size

There are a number of different sizes available for computer cases. Some computer cases can accommodate a full size power supply while other cases can only accommodate a specific size of power supply. Still other cases only work with a proprietary style of power supply, one that is designed to fit that case only (a specific brand of case or a specific computer series).
Power supplies
It is important to determine this first, before looking at any other factor. If you are building your own computer, most standard computer cases can accommodate a standard sized power supply. If you are replacing a power supply in an existing computer, one that is OEM, it may be best to check with the manufacturer to determine which power supply is best for your computer. At the very least, open the case and examine the type of power supply yours uses first.
Style of Power Supply

The style of the power supply is somewhat based on the size and style of the computer case. Many power supplies are the standard shape, more square in shape. However, there are other types of power supplies, including some that are more rectangular in shape, elongated to fit a smaller case size.
Again, if the computer is of a specific brand (Hewlett Packard, Compaq, Dell, Gateway, etc.), it may be best to check with the manufacturer to determine which power supply is best for your computer.
Another aspect of the style of power supply is based on the application, or use, of the power supply. Some power supplies are designed for basic use, not strenuous performance needs and therefore are not equipped with higher speed fans. For more strenuous performance needs (e.g. gaming, graphic design, and video editing), some power supplies will come equipped with high speed fans and better cooling capabilities. They also produce higher wattage for video cards and other computer hardware that have high power demands.
Wattage Requirements

Computer power supplyDifferent computer hardware requires different power supply wattage to operate correctly and efficiently. Often most users who are upgrading to a new video card will want a new power supply since the original power supply will lack the required wattage. Since basic video cards typically don’t require very high wattage to operate. A 300W power supply can often times be adequate for a basic video and computer used for email, internet surfing, and word processing.
As users start getting into video gaming, video editing and graphic design, video card requirements start to increase. Typically the more powerful and advanced a video card is, the more wattage it requires to run. These video cards may also require a direct power connection to the power supply, instead of relying on the computer slot it sits in to supply power to it. Computers with higher end video cards often require a minimum of at least a 500W or higher power supply. For computers with two or three video cards in them, it may be necessary to have a 1000W or even 1200W power supply.