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I find it slightly more difficult to build a computer when I do not know how much power is needed for a component. When selecting a power supply for a computer, it is difficult to know how large of one to get. You don't want to go too large for cost reasons and circuit reasons, but you don't want to go too low and not be able to properly use every component.

For instance, a graphics card might say "Minimum of a 500 Watt power supply. (Minimum recommended power supply with +12 Volt current rating of 30 Amps.)" But it really needs 360W (12V * 30A). So why don't they just say "Uses 360W max and xxxW peak"? Processors, I have noticed are good at reporting their power usage, but aside from processors and sometimes graphics cards, power usage is easily found.

What is the power consumed by the Blu-ray / DVD drives? By the HDDs/SSDs? By the Mobo? etc.

Why are these questions not easily answered when building a machine?

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closed as not constructive by Dave M, 8088, Nicole Hamilton, BBlake, ChrisF Dec 13 '12 at 22:45

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Part of it is because it depends on usage. E.g. a MB with no expansion cards in use will use less power then one with a dozen PCIe cards plugged in (and that is ignoring the power used by the cards). -- As for HDD, 5-10 Watt at idle is normal, more during spin-up. -- SSds: see reviews -- optical drives: who cares, noone uses those anymore -- Floppy drives: ditto -- Processors: TPD comes close to peak power used. --- RAM: 2-4 Watt per DIMM --- And lets not forgets fans: 3-5 Watt each (so 4 fans would be up to 20 Watt). –  Hennes Dec 13 '12 at 16:27
    
Here's a pretty solid calculator: PSU Calculator Lite –  Darth Android Dec 13 '12 at 16:33
    
@Hennes I realize the mobo usage is variable based on the PCIe cards. If you are given the wattage used by each PCIe card, you could figure out the wattage needed for mobo operations. On optical drives, plenty of people still use them.... –  Drew Dec 13 '12 at 16:50
    
@DarthAndroid Thanks! Great site! –  Drew Dec 13 '12 at 16:54
    
With PCIe cards I meant this: MB (power use: X Watt). PCIe card (say 20 watt). Mobo plus PCIe card: X + 20 + extra power used by an the PCIe bus. Darth's link seems to place that at 30 Watt per x8 link. I am not sure how accurate that is, but it is non-zero. –  Hennes Dec 13 '12 at 17:10
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1 Answer

It would be misleading to simply add up all of the power loads and size your power supply based on that number. As @Hennes points out, power load varies based on configuration, cpu load, drive activity, etc. When you start your computer you will see a spike in current requirements due to drives spinning up. The more drives you have the higher the peak current. If you're running a graphics intensive game with a high end GPU the card's power requirements may be an order of magnitude greater than it's 'rest' requirements. The list goes on.

All of this means that you need a supply with enough 'headroom' to absorb the peak system requirements (computers don't like their voltages to sag, mmm-mmm not at all). That's why the power supply guidance is so conservative.

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In my graphics card example, I do mention my ideal of Base and Peak ratings. I realize the differences, I think it would be useful to know the base power consumption and the peak levels to ensure a selected PS fill fit the bill. –  Drew Dec 13 '12 at 17:21
    
With that said, I am wondering why those values are not posted for better PS selection. –  Drew Dec 13 '12 at 17:21
    
But what defines the peak level? Temperature shutdown? Good case design can keep cards cooler and allow higher maximum current. Are you overclocking? Higher maximum current. –  BobT Dec 13 '12 at 17:26
    
I know this is more of an opinion here, but I would say peak power used without overclocking. I guess I didn't realize that a change in temp causes a significant change in current. I know that a change in current causes a change in Temp though. –  Drew Dec 13 '12 at 17:31
    
Limiting a change in temp (via cooling) allows an increase in current (via overclocking) –  BobT Dec 13 '12 at 17:54
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