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A couple years ago I was building a new Core 2 Quad system and after reading all the reviews was convinced that I would need at least a 400 watt power supply unit (PSU).

I bought a 500W Antec EarthWatts

However, last year I bought a Kill-A-Watt power meter to test some things around our house and found that my PC was only using 80W of power while idle! (C2Q, 4GB RAM, SATA HD, & DVD burner)

Well, here I am building another computer with a 65watt Core 2 CPU in it and I'm wondering if I can skimp out this time and get a 300watt or so unit since my usage doesn't seem to be what everyone claims it is.

I'm sure that the people in the reviews who exhausted 500watt PSU weren't lying - so what is it that uses all that? The high-end dual SLI video cards? Lots of SATA drives? Overclocking?

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Andd how much does your computer use under full load? –  AndrejaKo Jan 8 '11 at 1:52
    
Including the 19" widescreen LCD - it uses 138 watts at 100% load. –  Xeoncross Jan 8 '11 at 2:02
    
Getting a 300W unit doesn't mean you'll have lower power consumption. Let your PSU remain as it is, it'll help in the future –  Sathya Jan 8 '11 at 2:25
    
Higher wattage power supplies are generally more energy efficient. Also what happens if you are burning a DVD from the hard drive, and your computer processor hits a peak. –  AttackingHobo Jan 8 '11 at 8:05
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2 Answers

up vote 4 down vote accepted

Typically your big power users are:

  • Processor
  • Graphics adapter
  • Hard disk drive

Processors (including graphics processors) use less power when they're under less load, BUT you should always spec a power supply based on the maximum load of these devices. You could go with a smaller power supply and have your computer run fine when idling and under basic usage, but if you start something graphics intensive or do something processor intensive then power usage increases and the system will become unstable due to too little power.

Don't fall in to the idea that only video games will put a demand on your system. Although this is generally true for your graphics card (although there are programs you might not think of, like Google Earth, that will take advantage of a high-end graphics card), it is not true for your processor. When you decompress files, for example, the system knows you're waiting, so full processor capacity will be used.

Symptoms of power-starvation include freezing, unexpected hard reboots, graphics artifacting, and other general stability problems.

Antec provides a nifty tool that will help you determine the power supply capacity you should have available: http://www.antec.outervision.com/

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And there's Flash now and other programs which use GPUs for general computing too! –  AndrejaKo Jan 8 '11 at 2:34
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The power rating on a PSU is the peak power it can put out. The power demands of your hardware will change depending on system load. With that said, making sure your PSU can handle the current demands of your hardware is at least as important as the overall power requirement. Your video cards will use quite a bit of power, and often the documentation will tell you the recommended current rating of your PSU (I've seen modern cards recommend as much as 25A, which is a great deal of current). Most of these high-end components take 12V, so you should look at the current rating on the 12V rails and connect your components appropriately.

In summary, if you're building a reasonably high-end system, you really shouldn't skimp on the PSU.

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There's also the wear factor. If a PSU is under full load often, it will age more rapidly. For that reason people often take a bit stronger power supplies than they need. –  AndrejaKo Jan 8 '11 at 2:36
    
This is an interesting point. I've heard a lot about capacitor aging and so forth, but I've never had it happen to me (well, I've had "catastrophic" capacitor failure - bulging/leaking - which might be caused by this, but it was a really cheap 200W Shuttle PSU). Another reason to use high-quality parts, I guess. –  user55325 Jan 8 '11 at 3:00
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