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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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