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Let's say my computer is downloading a huge update, or maybe I want to perform a slow virus-scan that's likely to take hours. The question is, my machine is gonna be up all night, sleep/hibernate is not an option, but it's not gonna need much processing power either: what can I do to minimize power consumption?

Will killing every single unnecessary process help? (both answers regarding windows and Linux are welcome)

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up vote 7 down vote accepted

Turn off your monitor and necessarily kill any process that isn't needed. There's not much more you can do.

However, you need to take into consideration the following:

Computer Power Consumption is rated in the Watts/hour level. Let's assume that your Power Supply unit can go up to 600 Watts and that for some unfathomed reason you would be using up all that power during the night (you won't even be using half of it):

  • To calculate the kWh: (Watts x Used Hours) / 1000

So, if you leave it running for 8 hours during the night: (600 * 8) / 1000 = ~5 kWh

If you pay $0.20 USD per kiloWatt, that night cost you 1 dollar.

And because I went to the extreme and assumed full power consumption, you can see how this is pretty much irrelevant. Any minor changes you do will not have a big impact on the cost over such a small period of time. If you add them over an year, well then... that's where the differences start to be more meaningful.

In any case let's do a more approximate calculation. With monitor turned off, speakers turned off, and just the machine working in the background along with your modem (which usually consumes around 15 watts. See note at the end), I estimate you will be using somewhere around 270 watts.

(270 * 8) / 100 = 1.656 kWh

Multiply that for your local kWh cost. If it's $0.20 USD, the night will cost you ~33 cents :)

Despite all this, I share with you a certain... how should I put it?... irrational feeling towards savings. So, one thing I do is keep a window partially opened and detached the side panel from my computer. This increases cooling and makes the fans do less work, which results only in marginal power savings (because fans don't really use that much). But hey!

Note: To find power consumption of a device without a watt or kilowatt indicator, multiply Amps by Volts. That will give you watts/hour.

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What an excellent answer. I love these "you actually don't want to do what you want to do"-type answers. – Josh Hunt Oct 5 '09 at 5:41
Indeed, I've seen the math being done before, and it's more of an environment/clean-conscience kind of thing than actual monetary needs (and yes, an irrational feeling does seem to fit the description). But the beginning and the end of your answer are pretty much the kind of stuff I was looking for. – Malabarba Oct 5 '09 at 7:00
+1 Very good answer! – alex Oct 5 '09 at 7:26
600W constant draw? You'd need a tri SLI/XFire actively doing gaming to draw that much. My quad core/8800Ultra only takes 180W gaming and 100W at idel. Dial your calculation down to 100W and you'll see how much you are really saving (pennies). – kmarsh Oct 7 '09 at 12:06
erm... didn't you noticed that was being used purely for illustrative purposes? – A Dwarf Oct 7 '09 at 13:07


Your motherboard will try to conserve power if the processor isn't doing anything. So it might help if you boot into "recovery" or low-graphics mode so you don't load any Desktop Environment (Gnome, KDE etc...) stuff and then use wget (command line downloader) to download.

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+ This seems like the most effective option so far, any idea how much of a difference it might make? – Malabarba Oct 5 '09 at 18:08

Both Intel and AMD (SpeedStep and Cool n' Quiet, respectively) have technologies that automatically slow down your processor to reduce power use and reduce the need for cooling. For those to function effectively, it helps to eliminate processes that poll for updates or otherwise needlessly use the processor.

Most motherboards also support Q-Fan, which adjusts the fan speeds depending on temperature. Processor fans usually have the third wire which is used for the fan speed, but your case fans might not. Also, make sure that the case fans are connected to the motherboard and not the power supply.

For Windows 7, there's two handy commands to optimize power use and ensure that your computer goes to sleep/hibernates properly: powercfg -requests shows which factors are preventing your computer from entering sleep mode, and powercfg -energy gives you an overall report on power-wasting peripherals and processes.

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Works on Windows 7, but not on Windows Vista. – Brian Knoblauch Oct 5 '09 at 11:47
+ nice commands, though now I'm wondering why my USB aren't suspending. – Malabarba Oct 5 '09 at 18:06
Thanks, I've edited my response to note that those two are Windows 7-only. – Hirvox Oct 6 '09 at 4:53

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