The Cost of Running a PC 24 Hours a Day



I saw an interesting question over at slashdot on the topic of how many companies actually had their employees power down their PC’s overnight. The site in question had about 8000 PCs about half of which stayed powered on overnight. There’s a lot of talk these days about “going green”. I’ve always been interested in the idea of conservation because it just makes sense to not be wasteful where it’s possible. So, as many of you know already I have somewhere around as many hobbies as there have been Presidential debates this last year…. At one point in time I’ve spent a lot of time working with off-grid power ideas (built a small somewhat portable emergency power system with solar/car recharge capabilities.) And if you’ve ever seriously looked at alternative energy you know that you need to learn about power consumption…. so based on what I learned here are some things to think about…


Anyway, several years back I used one of those “kill-a-watt” meters to make a household wattage inventory… i.e. how much power does everything I can measure in the house actually use. I know devices have power ratings written on them, but for most devices the actual consumption is not as high as the rating. For instance, my PC with a 400 Watt power supply has the capacity to use 400 Watts, in reality it consumed around 110 watts while up (it likely varies over time, boot up would be more.)

So, let’s see 110 watts times 4000 (that’s about how many PC’s they claimed stayed on overnight at their business…) That’s 440kwatts per hour or 440 kwh… so assuming “overnight” is 6PM through 6AM that’s a grand total of 5280kwh. Now around here our power is around 9.1 or 9.2 cents per kwh (Why don’t we round to 10 cents for fun….) So, if I’ve done the math right, we get $528 of power usage from those machines overnight EACH NIGHT. I just calculated the monthly rate and it’s a bit too scary for me to post. For Pete’s sake…. cut of your pcs overnight if your not there to work on it! You are hemorrhaging money if you don’t. Even if we round down to 9 cents it’s a scary amount… $475.2 per night.

Now I heard some reasons given why some places don’t do that, but encourage people to leave their PCs on overnight. mostly, security update, but I also heard some complain about slow boot times making it take around 15 minutes in the morning before the PC’s were of any use. Hibernate features were mentioned, but I’ve seen flakiness with those (and support desks hear things like… “my computer screen has gone black and doesn’t come on when I move the mouse.”) Maybe we just need to start an education campaign on power features?

I must admit my PC stays on almost constantly, but I also have a home office and a tendency to sit down and start using it at almost any hour of the day. (Yes literally… 3AM-6AM hasn’t seen as much traffic lately, but ….) So, in other words, if you have no chance of using the thing for several hours overnight do everyone a favor and plan to switch off. (Especially business environments… (I wonder how much money could be saved on cooling in the summer this way too?)

By the way if your curious, that means that a single PC consuming 110 Watts of power per hour for 12 hours at a rate of 10 cents a kilowatt hour uses about 13.2 cents worth of power. (And for 24 hours a day, that totals 26.4 cents per day per PC.) No you know why virtual machine solutions and the concept of server consolidation is SUCH a big deal for big data centers. (Different power consumption on rack mount hardware, but ANYTHING running 24 hours a day adds up VERY quickly.)

Try and cut waste where you can and it’s amazing how much some places could save (in REAL dollars.) It could be worthwhile for businesses to do power consumption audits on a regular basis to see if they could take opportunities as simple as cutting things off overnight to save a bit of money.

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