How big of a computer power supply should I get?
Several factors should be considered when determining how big a power supply you need for your computer. These factors include computer case size, the style of power supply, and your machine's wattage requirements. We recommend you read both sections of this page before selecting a new or replacement power supply for purchase.
Computer case size and shape of the power supply
Computer cases come in many different sizes, as do power supplies. Some computer cases can accommodate a full-size power supply while others require a smaller, more specific size such as those sometimes found in brand-name computers. Proprietary style power supplies, such as those found in computers manufactured by HP (Hewlett-Packard), Compaq, Dell, Gateway, etc., may not be compatible with other systems. As you can see in the image below, power supplies not only differ in size, but in shape as well. The power supply on the right is has a rectangular, elongated shape to fit in a more slender or smaller case.
If you are building a custom computer, most standard computer cases can accommodate a standard sized power supply. If you are replacing the original, OEM (original equipment manufacturer) power supply in an existing computer, it's best to check with the manufacturer for proper sizing. At the very least, open the case and examine the power supply to get a rough idea of which one to purchase.
Wattage requirements
Different computer hardware combinations have varying operating wattage requirements. For instance, a user who's going from an onboard graphics chip to a dedicated GPU (graphics processing unit) or a larger video card may need a new power supply. Systems with a powerful video card can use significantly more electricity those with a mid-range or no card.
A 300 watt power supply is usually adequate in a computer used for e-mail, Internet surfing, and word processing. However, systems with high-end or multiple video cards generally require a minimum of 500 watt power supply. Wattage requirements for each GPU is found with an online search.
What should I do if my power supply is not powerful enough?
If your computer requires more power, more wattage than what the power supply can provide, you must replace the power supply with a bigger, more powerful one. It is not possible to upgrade an existing power supply to output more power. You can only replace a power supply in a computer.
For example, a computer has a 300 watt power supply and the hardware installed in the computer requires 350 watt of power to run correctly. To fix the problem, the 300 watt power supply would need to be replaced with a new power supply that provides at least 350 watt of power. We recommend installing a power supply that provides more power than what the computer requires. For this example, we recommend you replace the 300 watt power supply with a 400 or 450 watt power supply.