How Many Watts Does a Desktop Computer Need?
Due to using faster, higher-power parts, desktop computers use significantly more power than laptops, which generally use fewer than 100 watts. Desktop computers can vary significantly, but a typical computer under average load uses around 65 to 250 watts. For a PC with a high-end video card, add another 150 to 300 watts while rendering graphics. Note that the number of watts listed on your computer's power supply represent its maximum output, not how much power it uses continuously.
1 Power Use Variables
The number of watts your computer draws depends on two main components: the parts in your computer and how you use your computer. Higher-end parts generally use more power. For example, discrete video cards require far more electricity than integrated graphics processors. Your computer also uses more power while processing more complex information, so tasks such as encoding audio or video or playing games will increase power use compared to typing a document.
2 Picking a Power Supply
When buying a power supply unit for a desktop computer, choosing one that only meets average power requirements may not be enough. Computers use more power while under heavy use, and your PSU needs to have power to spare for these peak times. To pick a PSU, add together the maximum potential power use of all the components in your computer, then multiply by 1.5 to compensate for inefficiency and uneven use of the 12-volt rail. You can use an online calculator to help find the requirements for your particular setup (see links in Resources).