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Understanding Desktop PC Power Supply Wattage

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Pretty much every power supply on the market for a desktop PC computer is 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. If this is not done properly, the irregular power signals that are sent to the components can cause damage and system instability. Because of this, it is important to make sure you buy a power supply that meets the needs of your computer system.

Peak vs. Maximum Wattage Output

This is the first real big gotcha when it comes to looking at power supply specifications. The peak output rating is the highest amount of power the unit can supply, but this is only for a very brief time.

Units cannot continuously supply power at this level and if it attempts to do so will cause damage. You want to find the maximum continuous wattage rating of the power supply. This is the highest amount that the unit can supply stably to the components. Even with this, you want to 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 is calculated. There are three primary voltage rails inside of the power supply: +3.3V, +5V and +12V. Each of these supplies power to the various components of the computer system. It is the combined total power output of all these lines that make up the total power output of the system. 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 say 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 and as a result you want to 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 that will be listed as +12V1, +12V2 and +12V3 depending on if it has two or three rails. When calculating the amps for the +12V line, it is necessary to look at the total amps produces from all of the 12V rails. Often times there might be a footnote that the combine 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 for various size computer systems:

  • Small Form Factor - 15A
  • Mini-Tower - 20A
  • Mid-Tower - 24A
  • Full Tower - 30A
  • Dual Video Card (SLI) - 36A

Remember that these are only a recommendation. If you have specific power hungry components, check the power supply requirements with the manufacturer. An example of this might be with the new GeForce 8800 series graphics cards. These cards can easily draw over 200W of power. Running two of the cards in an SLI configuration can easily require a power supply that can sustain at least 750W of total power output.

Can My Computer Handle This?

We frequently get questions from people who are looking to upgrade their graphics card in their desktop computer system. Many high-end graphics cards have very specific requirements for power in order to operate properly. Thankfully this has improved with manufacturers now listing the minimum number of amps required on the 12V line in order for the cards to work. Previously they never published any power supply requirements.

Now, in terms of most desktop computers, the companies generally do not 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 savings measures. A typical desktop PC that didn't come with a dedicated graphics card will usually have between a 350 to 400W unit with around 15 to 18A rating. This will be fine for most budget graphics cards, but high end cards typically require more. Therefore, to use a high end card in such a desktop PC would require that the power supply also be upgraded which can be quite difficult.

Conclusions

Remember that everything we have been talking about involves the maximum limits of the computer power supply. 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 graphic intensive 3D games or doing video transcoding. These things heavily tax the components and need additional power.

As a case in point, I put a power usage meter between the power supply and the wall outlet on "my computer" as a test. During average computing, my system was pulling no more than 240W of power. This is well below the rating of my power supply. However, if I 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 I have a large number of items that draw heavily on the 12V rail such that a 400W could have voltage problems.

 
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