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I recently built a small mini-itx Intel ATOM-based Ubuntu home server. The case I choose is a small case but only a full size ATX power supply would fit it. I choose a mid-grade OCZ 500 watt modular power supply for it and it works great. I could not find a modular ATX power supply out there that was less than 450W.

So my question is, does my 500W power supply draw 500W just because thats what it is? Or does the power supply only draw as much power as is needed to power the computer components?

One mini-itx ATOM board + two SATA HDDs = less than 100W I figure. My goal was to build a low cost, low power consumption server, so hopefully the 500W power supply isn't drawing 500W.

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also remember that HDDs typically need lots of power when spinning up, but general operation uses much less. the specs of your drive model should specify both power levels. –  quack quixote Feb 9 '10 at 20:12
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3 Answers

up vote 18 down vote accepted

When you buy a 500W power supply that means this PSU can deliver a maximum of 500W ! So if yours motherboard + HDDs consume 100W then your power supply will get 100W (+ a negligible cost of transformation ~10%) from the power plug !

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So your conclusion is: Yes it only draws as much power as it needs ;-) Your edit deserves you a +1! –  Ivo Flipse Feb 9 '10 at 19:16
Only the best PSUs on the market are able to approach 90% efficiency. Most good PSUs today do somewhere in the low to mid 80's. Cheaper PSU's tend to be nearer to 70%. –  Dan Neely Feb 9 '10 at 21:27
Other than the minor efficiency discrepancy, the information is a theoretically accurate and definitely helpful! +1 –  IAbstract Feb 10 '10 at 3:01
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It varies according to the power efficiency rating of the PSU, in short - yes, it only draws the current its asked for by the computer, but the efficiency of converting the power from the wall socket to something the computer can use is roughly 80% (ie your PC uses 80W, it'll draw 100W from the socket)

Look for APF (active power factor) for more details. This efficiency rating varies also according to how much power is drawn and the 'size' of the PSU, eg a 1000W PSU will not be very efficient when supplying 50W.

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+1 for mentioning that bigger supplies are in-efficient at light loads. Although I'm not sure what APFC has to do with anything. –  pipTheGeek Feb 9 '10 at 19:48
While true in a quality PSU the difference between max efficiency and efficiency at very low/peak load is only a few percent. Running at near max load also has the fan spinning faster and louder. IF you want to keep the fan idle you need to be a few hundred watts under max power. This is because the amount of cooling from a fan at an acceptable noise level is fixed, and the rest has to be done with big heatsinks. Corsair publishes efficiency and fan speed curves for it's PSUs. Here's one (charts on resources tab): corsair.com/products/hx650/default.aspx –  Dan Neely Feb 9 '10 at 21:25
@pipTheGeek APFC doesn't have anything to do with efficiency directly but in general for the US market is only implemented on better quality models; so it can be used as a filter when efficiency data isn't published. IIRC due to different regulatory/billing requirements it was common in the EU much sooner than in the US with the result that there are low efficiency APFC PSUs in the EU market. –  Dan Neely Feb 9 '10 at 21:29
@Dan - Ah. I didn't realise that. Yes, here in the UK all PSUs offer APFC. Including the ones that you wouldn't trust to power a bettery charger. –  pipTheGeek Feb 12 '10 at 12:33
@pipTheGeek the short version is that your power meters know about power factor and factor it into how much you get charged. US residential power meters don't so we don't get penalized for devices with bad power factors. (AFAIK US commercial/industrial meters do adjust for power factor.) –  Dan Neely Feb 12 '10 at 15:47
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Yes, a power supply draws power relative to how much is being used. So if your PC hardware is only using 200W, your 500W power supply won't draw 500W. How much it draws will vary from power supply to power supply.

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-1? Well I'll be. :-/ –  Dan McGrath Feb 9 '10 at 19:21
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