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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.

  • 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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    If it drew 500W, what would you expect it to do with the extra 400W or so? Turn itself into a heater? – developerbmw May 9 '15 at 7:21
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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
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    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 Is Fiddling By Firelight 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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+50

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 Is Fiddling By Firelight Feb 9 '10 at 21:25
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    @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 Is Fiddling By Firelight Feb 9 '10 at 21:29
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    @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 Is Fiddling By Firelight Feb 12 '10 at 15:47
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    @Dan, no, we don't get charged for poor power factor. Our domestic customers, like yours, are metered by meters that compensate for power factor and only charge for actual power used. Commercial customers can be charged by PF but I'm not one so I don't know for sure. – pipTheGeek Feb 12 '10 at 19:03
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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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A power supply is just a transformer or a transducer.

A power supply converts energy that comes in as alternate current with a "high" voltage (230 V and 50 Hz in Europe, 120 V and 60 Hz in North America) into the same amount of energy in a direct current mode (i.e 0 Hz) with a very low voltage (3.3 V, 5 V, 12 V) but a higher amperage. (scroll down for explanation of units)

You can compare electricity flowing through wires and cables with a liquid flowing through pipes. To make water flow through a pipe, you need to have different amounts of pressure at both ends of the pipe. When you have the same pressure ar both ends, no water will flow. This difference of pressure is the voltage in electricity.

The amount of water flowing through a pipe within a second corresponds to the amperage.

A machine that "consumes" energy (like a CPU, a light pulp or an electric engine) corresponds to a mill that is driven by flowing water. It's the product of voltage (difference of pressure before and behind the mill) and the amperage (amount of water flowing through the mill) that gives the power the machine needs to do its job.

The power supply in this picture is just a kind of turbine that uses one stream of water that has too high pressure but is flowing slowly to produce a second stream with less pressure but flowing faster. Both streams carry the same amount of energy per second, and both carry just as much as the "mills" (CPU etc.) will consume.

So, simplified: The power supply will draw only as much energy from its source as needed.

But this is only half the truth.

A power supply itself also consumes energy. This is exactly the energy that is emitted as heat from the power supply. A hypothetical power supply that really draws only that amount of energy from its source that is provided to the "real" consumers, would stay cold (at room temperature). But due to certain laws of physics this ideal power supply is impossible to be built.

The higher the maximum power is that a power supply can provide, the higher is the amount of energy it takes to heat up itself. (This strongly depends on how well crafted the power supply is.)

What does this mean?

It means: Find out how much power your system will need (take the maximum that is possible) and add some extra margin to be on the save side. This is the power your power supply should be able to provide. A more powerful power supply will not damage your system. It will just draw some extra energy to heat up itself that a less powerfull power supply wouldn't need.

But a too weak power supply can have negative effects. Your system might crash if you need to more power than your power supply can provide.

But all in all you should know, that the power supply draws more energy from the net when you need a high amount of power, and it will draw less energy when your system is idle.


Here are some physical units:

  • Hz = Hertz = number of waves per second
  • V = Volt = unit for voltage
  • A = Ampere = unit for strength of current (aka amperage)
  • W = Watt = unit for power = product of Volt and Ampere (the amount of Energy flowing through the device per second)
  • J = Joule = unit for energy
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I realize that this has already been answered (correctly) by multiple people but I figured I'd give you the simplest answer. Yes. Why? Electric current is like a rope. It can be pulled, but it can't be pushed. The motherboard and other components of the system will each only pull as much current as they need from the power supply, and the power supply will only pull as much current from the wall as it needs to supply (and convert) the load placed on it by the motherboard and other components.

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  • +1 for the rope analogy. – Hashim Nov 19 '19 at 23:58
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I want to put the focus on a certain aspect when it comes to price. Therefore I start with a small "correction": a "500W power supply" doesn't refere to the power supply "drawing" 500W from the outlet but that it's able to "provide" up to 500W to the device connected to the power supply.

That being said the OPs goal was to build use inexpensive power supply in terms of initial cost (the price) and running costs (energy consumption). As a general rule of thumb the power supply should provide much more power then the device needs. Simply speaking the higher the watt output the higher the loss due to "overhead". This especially applies to cheaply built power supplies.

So as a conclusion: there is a trade off between cheap and expensive power supplies. The cheap ones are usually more expensive in operation especially if they are oversized.

For the technical details I would refer you to the other answers, they are quite through.

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