Every power supply has a wattage rating, which is simply the measure of how much power it puts out. Computer power supplies can range in power from less than 200 to over 1,000 watts. What you put in your own PC is dependent upon your hardware needs. For many people, the main driving factor behind a power supply upgrade is for powering a new video card. Most high end video cards requires a minimum 350-400 watt power supply in order to work properly, whereas most factory brand power supplies (like HP or Dell) are 250 watts or less.
Below you will find a very basic set of standards for determining power consumption in your PC. These wattage numbers are based on average ratings and can vary depending on the manufacturer, so double check before you settle on anything. This is mainly just to give you a rough idea of what you will need as far as the maximum wattage on your new power supply.
CPU: 125-150 watts
Motherboard: 50-150 watts, depending on built-in components
RAM: 15-20 watts per gigabyte of memory
Drives: @ 30 watts each
Cooling Fans: 3-5 watts each
PCI Cards: @ 10 watts each
Video Cards: Varies greatly depending on make and model – 100 to 400 watts
You can also use a PSU calculator to help you get a more specific estimate.
Unless your motherboard or any other components have specific voltage requirements, they aren’t that big of a concern when choosing a power supply. Video cards use power at 12 Volts, so if you are using high-end or multiple cards make sure your PSU can deliver enough power at that voltage. The CPU also draws at 12 Volts, so most modern power supplies (ATX 12V 2.x, described on the next page) are set up to deliver most of their total wattage at 12 Volts. Be sure to check your hardware to make sure of any special requirements, especially with your motherboard, CPU, and video card.