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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.

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closed as off-topic by kasperd, Chris S Nov 8 at 19:03



"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 at 17:36

6 Answers 6

up vote 32 down vote accepted

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.


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.

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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. –  Brad Gilbert May 5 '09 at 3:51
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. –  Kevin A. Naudé Apr 24 '13 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..

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+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 '09 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 –  Thomaschaaf May 4 '09 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?



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

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

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...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. –  Nathan DeWitt May 9 at 18:24
