Most power supplies for desktop PCs are advertised according to the wattage of the device. But this is a simplistic view of a complex issue, as the typical watts of a computer power supply matter less than the supply’s overall purpose.

The power supply converts the high voltage from the wall outlet into the lower voltages required to operate the computer circuitry. Irregular power signals can cause damage and system instability. It’s important to make sure you buy a power supply that meets the needs of your computer system.

Peak vs. Maximum Wattage Output

The peak output rating is the highest amount of power a unit can supply, but it only provides this amount for a short time. Units cannot continuously supply power at this level; if the unit attempts to do so, it may damage the hardware.

It’s more important and relevant to find the maximum continuous wattage rating of the power supply. This is the highest amount that the unit can stably supply to components.

Another thing to be aware of with the wattage output has to do with how it’s calculated. A power supply contains three primary voltage rails: +3.3V, +5V, and +12V. Each supplies power to the various components of the computer system. The combined total power output of all these lines makes up the total power output of the power supply. The formula used to do this is:

Make sure the maximum wattage rating is higher than you intend to use.

  • Wattage = Voltage x Amperage

When the power supply label shows that the +12V line supplies 18A of power, that voltage rail supplies a maximum of 216W of power. This may be only a small fraction of the 450W the power supply is rated at. The maximum output of the +5V and +3.3V rails are then calculated and added to the overall wattage rating.

+12V Rail

The most important voltage rail in a power supply is the +12V rail. This voltage rail supplies power to the most demanding components, including the processor, drives, cooling fans, and graphics cards. These items draw a lot of current. Make sure that you purchase a unit that supplies enough power to the +12V rail.

With the increasing demands on the 12V lines, many new power supplies have multiple 12V rails listed as +12V1, +12V2, and +12V3, depending on if it has two or three rails. When calculating the amps for the +12V line, it’s necessary to look at the total amps produced from all of the 12V rails. You may see a footnote that the combined maximum wattage will be less than the total rating of the rails. Just reverse the above formula to get the maximum combined amps:

  • Amperage = Wattage / Voltage

With this information about the +12V rails, you can use it to determine general power usage based on the system. Here are the recommendations for the minimum combined 12V rail amperages (and the relative PSU wattage rating) for various size computer systems:

  • Small Form Factor: 15A (250W)Mini-Tower: 25A (300-350W)Mid-Tower: 35A (400-500W)Full Tower: 40A (600-650W)Dual Video Card (SLI): 50A (750W+)

These are only recommendations. If you have components that require a large amount of power, check the power supply requirements with the manufacturer. Many high-end graphics cards can pull close to 200W under a full load. Running two of the cards can require a power supply that can sustain at least 750W or more of total power output.

Can My Computer Handle This?

Many high-end graphics cards have specific power requirements to operate properly. Many manufacturers now list some of this information. Most detail the recommended total wattage of the power supply, but some list the minimum number of amps required on the 12V line, which is more helpful.

Companies generally don’t list the PC’s power supply ratings in their specifications. Typically, you must open up the case and look for the power supply label to determine what the system supports. Most desktop PCs come with fairly low power supplies as cost-saving measures.

A typical desktop PC that didn’t come with a dedicated graphics card usually has a 300W to 350W unit with a 15A to 22A rating. This is fine for some budget graphics cards, but others have been increasing in power demands and won’t work as well.

Final Thoughts

Probably 99% of the time a computer is being used, it is not used to its maximum potential and, as a result, draws less power than its advertised capacity. The important thing is that the computer power supply needs enough headroom for those times that the system is taxed heavily. Examples are when playing graphics-intensive 3D games or doing video transcoding. These tasks tax the components and require additional power.

During average computing, a system may pull no more than 240W of power. This is below the rating of a power supply. However, if you play a 3D game for several hours, the power usage peaks upwards to around 400W of total power. Does this mean that a 400W power supply would be sufficient? Probably not, as you may have a large number of items that draw heavily on the 12V rail, such that a 400W supply could have voltage problems which result in system instability.

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