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[ The PC Guide | Systems and Components Reference Guide | Power | The Power Supply | Power Supply Output and Ratings ]

System Power Requirements

The goal behind analyzing the power supply's output rating and power distribution is to match it to the needs of your system to ensure it will provide you with the power you need. Unfortunately, this is usually much easier said than done. The key problem in this regard is trying to figure out exactly how much power your system uses. This is not an easy task, and the manufacturers of most systems don't make it any easier.

It's important to remember when picking a power supply that you need to leave room for expansion. Many people purchase or build systems using motherboards that they hope will allow them to upgrade to newer CPUs, or buy large cases with room for lots of drives and other peripherals. However, the power for these devices has to come from the power supply--something many people never consider. Newer processors in particular can be very demanding in terms of their power requirements, especially regarding the total of +3.3 V and +5 V power the supply can provide. If you want to increase your chances of success when upgrading in the future, leave "headroom" in your power supply.

Determining how much power your system needs can be either simple or difficult, depending on whether you want to make a crude estimation or a more exact calculation. Here are some methods you may find useful:

  • The "I Don't Want to Worry About It" Approach: This is what I use and recommend to most users. The idea here is simple: buy something really big, and then you don't have to worry about this issue. And not having to worry about your power supply's capacity can give real peace of mind. Rather than figuring out that your system requires 142.791 watts and then buying a 150 W supply for it, just get a 250 W supply and be done with the matter. For most regular desktop PCs, a 250 W power supply will provide enough power for most anything you can throw at it. For a typical tower PC, a 300 W supply is probably all you will ever need, and the difference in price between a 200 W and a 300 W supply of the same type and manufacture is often surprisingly small!
    On the other hand, if you are planning to build a server with four CPUs and 12 internal SCSI drives, this method is not likely to be sufficient for you. Even dual CPU systems can require prodigious amounts of +5 V and +3.3 V power. Obviously, the exact way that the power is distributed is important for special applications. Check out the different ratings before you buy. If your system has a lot of drives, pay particular attention to the +12 V rating. If it has more than one CPU, or one known to draw a lot of current, pay special attention to the +3.3 V and +5 V numbers.
  • Use an Approximation: Based on the general intended use of the machine and what you foresee requiring for future expansion, approximate the amount of power you will require. This can be difficult to do if you have not worked a great deal with PCs before, because it basically requires you to estimate power requirements based on your general knowledge of the components, and your past experience.
  • Calculate the Requirements: Calculate your requirements from the power use specifications of the components inside your machine. For each voltage level, determine how much current is required by each device, add them up, and get a power supply that can handle the load.This can also be difficult to do, because many devices do not come with complete specifications, and power use is not a spec that is commonly sought by most people. It's easier if you build your own PC, as then you usually get a manual of some sort with each device. However, even there, the manuals often don't say how much power the devices use. I personally don't recommend trying to do this unless you are fairly knowledgeable about PC components, and have a considerable amount of time on your hands.

Next: Peak vs. Continuous Power

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