Does a 500 watt power supply always pull 500 watts? Or does it depend on the load being placed on the computer?

It's a n00b hardware question. I'm trying to figure out how much it costs to run my compuer without buying a meter that actually measures power usage.

  • "Power" can be everything (for marketing reasons): maximum deliverable at output, maximum at input. In addition with AC, there are a lot of definitions of "power": peak power, instantaneous, reactive, complex... all of that usually expressed in watts. But usually the power consumed at input is proportional to the load connected at output (and its resistive, capacitive or inductive nature). For a computer, most of the power is drained by the display... or is evacuated in the form of heat by the fans. Heat is not lost... at least in winter.
    – mins
    May 8, 2014 at 17:36
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    I'm sorry, but closing this topic for the cited reason was simply absurd. Feb 17, 2017 at 23:47

6 Answers 6


No. A 500 Watt Power Supply can DELIVER 500 Watts, but it will ever use only as much as the components in your PC need (and of course that depends on Load and Activity, if Energy Savings Mechanisms like AMD's Cool'n'Quiet or Intel's SpeedStep is enabled etc.).

In Theory, with a 100% efficiency rating, which is impossible.

The usual Efficiency rating lies around 80%, but it can vary greatly between low quality and proper power supplies.

So with 80% efficiency, your power supply will use as much power as your components need and then about 20% extra.

Another caveat: Optimal efficiency is only reached at a "proper" load. If you have a 500 Watt Power Supply but then a super-low-consumption PC that only consumes 80 Watt, you're not going to reach 80% efficiency and could easily use ~120 Watt (~50% efficiency).

Due to the ~80% efficiency, you can also not use 500 Watt out of a 500 Watt Power Supply.

Those numbers are all estimates, as PSEs vary greatly, but a rule of thumb is that you should get a PSU with at least 80% Efficiency and get one that is not too big (but not too small either) for your PC.

  • 6
    Actually if the 500 watts is the actual amount of energy you can get from it to run your computer, then at times it could actually use more than 500 watts from the power line. May 5, 2009 at 3:51
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    Small correction: if efficiency is 80%, the extra energy lost in heat is 25%, since 20/80=0.25. So if you need 100W delivered, an 80% efficient supply will burn 125W in order to do so. Apr 24, 2013 at 14:10

No it doesn't. It just uses the power it needs plus a little overhead so the voltage doesn't go down if you plug something in there...

Just buy a power meter.. they cost 20 bucks and are always good to have around.. or borrow one from your neighbor, work, parents, friends etc..

  • 2
    +1 for the Power Meter. Just keep in mind that Power Meters also have a certain tolerance, but with the high consumption that PCs usually have, it should be "close enough" with pretty much any of them.
    – Michael Stum
    May 4, 2009 at 19:18
  • yep Michael you are right but since the computer will work a bit more and a bit less it shouldn't matter much I'm guessing it's just for a rough estimate we did the same at our company.. what fun things you do if you are bored :D May 4, 2009 at 19:22

No the power supply meets the load up to its rated wattage plus some overhead.

Energy Star computers power supplies must meet the following standard: Internal power supplies: 80% minimum efficiency at 20%, 50%, and 100% of rated output and minimum Power Factor 0.9

If you are purchasing a separate power supply and looking for a more efficient one look for the 80 Plus logo on the power supply.

What is the 80 PLUS specification?

The 80 PLUS performance specification requires multi-output power supplies in computers and servers to be 80% or greater energy efficient at 20%, 50% and 100% of rated load with a true power factor of 0.9 or greater. This makes an 80 PLUS certified power supply substantially more efficient than typical power supplies and creates a unique market differentiation opportunity for power supply and computer manufacturers.



Nah, I have a 750 W power supply, and I've never seen my wattage go above 200.

  • 3
    ...thereby proving that a power supply doesn't constantly draw its max load. Not sure why I was downvoted for providing anecdotal evidence directly answering the question. May 9, 2014 at 18:24
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    ^^^^No worries Nathan. I upvoted you to compensate for the malcontent that downvoted you. Answers like yours are helpful, especially in a corporate environment. Feb 17, 2017 at 23:48

Absolutely not. See http://www.codinghorror.com/blog/archives/000353.html For good details.


No, that's just the maximum rated supply. The amount of power drawn is actually variable. You can observe this by plugging the machine into this device and watching it in real-time. I know you don't want to buy it, but believe me, after you use it, it'll pay for itself in less than a year.

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