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How can I calculate power consumption of my PC in Watt?

I'm living with my parents. My dad was complaining that because of my excessive computer usage , he had to pay huge electric bill. I think its not because of my PC. So, I want to calculate how much power exactly is my computer consuming.

I've read the label on my (450W) Power Supply. It reads:

| DC Output | +5V | +12V | +3.3V | SB +5V | -12V | +5V 3.3V |
| 450W      | 45A | 19A  | 28A   | 2A     | 0.8A | 240W max |

But it doesn't make any sense at all!

DC Output power is 450W that must be equal to 5*45 + 12*19 + 3.3*28 + 5*2 + !@$@#$

I don't understand what is SB there in the 4th column and what to do with the -12V and the information in the last column. Even if I don't consider them, its not coming out to be 450W. What exactly does these ratings mean? How much power is my PC consuming?

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marked as duplicate by Ƭᴇcʜιᴇ007, 8088, AndrejaKo, Diogo, Journeyman Geek Nov 9 '11 at 14:29

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Dont mix amps and watts there not the same beast, I dont know the answer to your question but I once asked an electrical engineer what the diffrence is and he said one makes you go "arrrrrrh" and the makes you go "eeeeek" –  Shutupsquare Nov 9 '11 at 12:57
Simply connect a multimeter between the electric outlet in the wall and the computer's power cable. –  Andreas Rejbrand Nov 9 '11 at 12:59
@Andreas Rejbrand Except there's no way to to that simply with most of the plugs used today in the world. Another point would be safety since a large number of multimeters can burn if connected to large currents or only be rated for several seconds which would be too short to get a meaningful measurement. Furthermore logging the data would be a problem unless the meter has it's own autologging feature. Another point which can be problematic is getting a reading from a representative use of the computer. –  AndrejaKo Nov 9 '11 at 13:12
@Andreas Rejbrand Then we also have power factor which needs to be taken into account. The reactive power may be charged separately (or maybe even free) and the meter will only show effective current on which the power factor (which may be unknown) would need to be applied. –  AndrejaKo Nov 9 '11 at 13:14
AndrejaKo: Did you click on my link? Perhaps the word 'multimeter' is not the appropriate one. What I mean is the thing on the image... –  Andreas Rejbrand Nov 9 '11 at 13:48

3 Answers 3

up vote 2 down vote accepted

The power supply has a max output of 450W. Each rail (+12V, +5V etc) shows the voltage and max current, it doesn't indicate that you are actually using that much.

It is unlikely to consume the full 450W, the peak is probably a bit lower. The real consumption is dependent on what you have plugged in inside the case, and what you are doing.

Surfing the web would probably consume the lowest amount of power. Playing video games would consume the highest.

The only way to be certain what it is drawing is to use something like a kill-a-watt meter that sits between the plug and the wall socket and reads the actual power consumed (approximately).

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+1 for measuring kill-a-watt. An external power meter is the only sure way to show the exact amount of power the computer is using. The nominal power the supply is providing is completely and utterly meaningless number from which nothing can be safely calculated. A "ballparl" figure could be obtained if exact efficiency and power factor curves are provided with the PSU and if there's a way to know at least approximately how much power computer is currently using, which there isn't. –  AndrejaKo Nov 9 '11 at 13:08
I agree! but why 5*45 + 12*19 + 3.3*28 + 5*2 + !@$@#$ > 450? –  claws Nov 9 '11 at 13:13
@claws because you can't just add those values. For example the 3.3 V is often derived from 5 V. So if you max out the 3.3 V, in the best case (which never happens in reality), you'd get 17 A for 5 V. It would be more like 10 A or so in real life. Also sometimes the 5 V standby isn't taken into account as it's completely different system form the rest of the PSU. –  AndrejaKo Nov 9 '11 at 13:22
@claws The 19 A * 12 V + 45 A * 5 V give close enough value to the nominal output (which is often made up figure or works only at 23 C or only for 1 second or only for several milliseconds etc.). –  AndrejaKo Nov 9 '11 at 13:23

The output and voltages mentioned are the different cables internally from the power supply to your devices in your PC (usually called 'rails').

The 450W is the maximum wattage it'll support, it will not be using that much. (else you would soon notice problems!)

What devices you have, number of them etc will 'draw' a certain wattage from the PSU, there are many online tools for working it out (some high-end system builders will check to make sure their PSU is powerful enough).

If you know your system specs then try running it through any of the following:


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Thanks! These links helped me. –  claws Nov 9 '11 at 13:30
Thanks! Vote me up! ;) –  HaydnWVN Nov 9 '11 at 14:09

In North America they sell and even rent/loan a device called a Kill-A-Watt meter. You simply plug th device into the wall and then plug the computer (or whatever) into the device. It displays the power consumption and other info. Several are listed here Power Monitors I used it on a desktop Sony I have and while the power supply is 200W it was normally consuming less than 100W

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