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PC Energy Use


Calculating PC Power Use.



Let's face it, with the extensive use of oil based plastics and a carbon footprint the equivalent of the airline industry, IT has a long way to go to even qualify as "not particularly offensive" in the environmental stakes. Sure, HP will sell us highly efficient blade servers with reduced power and cooling requirements, but they're probably still made in a highly polluting factory in China somewhere, the chips produced using some of the nastiest chemicals available, then they need to get shipped, and of course we still have to dispose of our old kit in an environmentally friendly way (something which is getting a whole lot easier these days, thankfully).

But does your company care about any of this? If they're anything like mine then it is pretty irrelevant, all a business knows how to do is carry on. Sure we make reductions in power consumption, and take advantage of government schemes to install new, more efficient plant, but at the end of the day it is energy use and therefore money which drives the ROI on these investments. Sure we'll trot these initiatives out as evidence of our green credentials in marketing literature, but at the end of the day unless your business is highly sensitive to consumer opinion, or governments actually start legislating to reduce carbon emissions, the only argument a business is going to appreciate is cash.

That might sound cynical, but it is also a highly effective method of getting a business to reduce energy use, it also makes good sense whatever side of the climate change argument you happen to be on.

Available Resources

As an IT Manager (and sometimes DBA), I have been asked by accounts to estimate the amount of energy our department uses. With the amount of green initiatives the UK government is churning out you'd think this was easy, right? Wrong. What I needed was a spreadsheet into which I can type the number of PC's and monitors on site, the amount of money I spend per KwH on power, and then get an answer as to how much I'm spending. Maybe somewhere on the web there is a simple spreadsheet which we can all use to calculate our energy usage, but with the number of companies trying to sell us the new "green" thing you'll probably find it around page 2000 of any given Google search. I couldn't find it. I found it interesting, though, that we can save the planet by buying more stuff. I didn't think it worked like that.

I got very frustrated that even the UK Government's Carbon Trust, a body set up specifically to help businesses reduce carbon emissions, couldn't help me do something which was so obviously useful.

So I complained. In detail.

And they sent me this spreadsheet, or something similar, which was a suprise. I corrected some minor errors and added a few columns and now I have a pretty good ballpark calculator of my energy use, which I thought might be useful for some of you guys, no point duplicating work is there?

So What Does it Do?

You can calculate the energy use of any appliance by looking at the sticker at the back at the number of Watts it is using. This figure represents how many Watts are burned in an hour of constant operation (in theory). Electricity is billed in kilowatt hours, so essentially if you have a 300W PC you take your charge per kilowatt hour (in my case £0.074), and multiply it by (WATTAGE/1000), a PC with a 300W power supply would cost (300/1000) * 0.074 to run for an hour (£0.022). Only it isn't quite that simple, because a PC doesn't drain all that power all the time, in fact it very rarely does, which is where the Carbon Trust have helped me by giving me an average rate of 74 Watts. I haven't questioned this, I've just accepted it. If you know better I'd love to hear about it.

Ok, so I have many PCs on site, but some of them run all the time, and some of them run 9-5, 5 days a week. Therefore I had to build in the hours of operation per day, and the days of operation per week to enable me to calculate the hours of operation per year. This is why I've had to use separate lines for 24/7 PCs and 9-5 PCs.

You can simply calculate CO2 emissions in kilograms by multiplying your kWh by 0.43, and then divide the answer by 1000 to get tonnes.

In the last column you have cost, which is the total cost for all units with those hours of operation. This can be calculated for your site by putting your own KwH cost of electricity at the bottom of the sheet.

So there you have it - I have 100 PCs running 24/7 and 100 PCs running 9-5 and various other peripherals and the IT departments contribution to electricity costs (excluding the server room - I'll get to that) is £17,584.21 a year. We also produce just over 100 tonnes of CO2 a year, just from the desktops. That's pretty bad.

And How Does That Help?

But once we have these figures we can play with them a little. For example - a monitor running a screen saver burns the same power as a monitor running office. I can see my 24/7 PCs run for 8736 hours in a year, but these kinds of PCs are really just lightweight servers or production management machines. This means they are running their screen savers most of the time, say 60%. What a waste of money. 60% of £6464.64, in fact. £3878.78.

Which of the following initiatives would be most likely to succeed in your company?

a) We want to ban screen savers because they are not energy efficient and they are destroying the planet.
b) If we ban screen savers we will save the company £3878.78 and reduce our carbon footprint by 22 tonnes a year.

Which I hope makes my point.

Random fact I learned last week, I'm reminded because the savings there come out at 3878.78. Did you know the official height of Everest was originally 29,002 feet, but the actual height of Everest is EXACTLY 29,000 feet? The guy who measured it didn't think anyone would believe him so he added 2 feet. I think we all have to do that kind of thing from time to time...;)

What About Servers?

OK, so server rooms are better catered for out on the web, if you're using HP servers I should direct you to :


Which is a pretty useful resource.

Air conditioning is also well served on the web, look at the UK governments Carbon Trust site for details here, their publications ECG019 and CTL004 are pretty useful (but you have to subscribe to get them).


Any Other Ideas?

Now none of this is exact science, and I'm hoping someone out there has done this job better than me, and has advice or opinions or web links to things like average office usage patterns (eg how long does power save mode go into operation in an average office if it is set to 5 minutes, or 10 minutes etc). Maybe you have a better idea of calculating the power profile of a PC, or I'm missing something. One reason I've written this is because there seems to be a scarcity of resources out there, and I'm very glad Steve sees it as worth publishing despite the fact I haven't mentioned databases even once. Except just then.







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