Computer Power Supply Wattage

Understand PC PSU wattage ratings to be sure you have enough power

Businessman sitting at desk behind computer
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Most power supplies on the market for desktop PCs are advertised solely on its wattage. Unfortunately, this is a simplistic view of a very complex issue. The power supply is there to convert 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 the unit can supply, but it can only provide this amount for a very brief time. Units cannot continuously supply power at this level, and if it 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.

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

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 make up the total power output of the power supply. The formula used to do this is:

  • Wattage = Voltage * Amperage

So, if you look at a power supply label and it shows that the +12V line supplies 18A of power, that voltage rail can supply 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 would then be 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. All of 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 produces 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, one can use it against a general power usage based on the system of the system. Here are the recommendations for the minimum combined 12V rail amperages (and their 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+)

Remember that these are only a recommendation. If you have power-hungry components, check the power supply requirements with the manufacturer. Many high-end graphics cards can pull near 200W on their own under full load. Running two of the cards can easily 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 requirements for power in order to operate properly. Thankfully this has improved with manufacturers now listing some information. Most will just list the recommended total wattage of the power supply but the best is when they list the minimum number of amps required on the 12V line. Previously, they never published any power supply requirements.

Companies generally don't list the PC's power supply ratings in their specifications. Typically the user will have to open up the case and look for the power supply label to determine what exactly the system can support. Unfortunately, most desktop PCs will come with fairly low power supplies as cost-saving measures.

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

Conclusions

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

During average computing, your system may pull no more than 240W of power. This is well below the rating of a power supply. However, if you then 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 would result in system instability.