Calculating Power Requirements and Costs

To find out now how much your PC actually costs to run, you will first need to know your power consumption. For this article, we will use three sample systems representing differing levels of hardware and performance. The specifications for the sample systems can be found in our previous article on power supply units. Power consumption is as follows:

System Power Consumption (Watts)
  Idle Load
System 1 90 140
System 2 160 350
System 3 310 550

Electricity providers report power use in kilowatts hours, since the power consumption of your entire house is going to be large compared to a single PC. Every light bulb, TV, microwave, refrigerator, vacuum cleaner, etc. requires power. Unless you are running a lot of computers, it may not even be necessary to think much about how much your computer uses without addressing those other areas first. Still, there's a large difference between an entry-level PC with EIST/Cool & Quiet sitting at the desktop and a high-end PC running the latest 3D game.

For our comparisons, we will look at two states in the US (North Carolina and California) and Germany will represent Europe. We used an exchange rate of $1.30 per Euro. Power use is calculated by the above chart, factoring in the efficiency of the power supply. For simplicity's sake, we will start by assuming 82% efficiency on all systems and loads. Divide the power consumption by the power supply efficiency and you end up with the actual power use in Watts. Converting Watts into kWh requires a bit more math: take the power draw in Watts and multiply that by the number of hours a device is running, and then divide that number by 1000. The results are as follows:

System 1 Power Costs
  Cost/kWh Outlet Power 1 Hour 8 Hrs 24 Hrs 1 year
(8 hrs/day)
1 year
(24 hrs/day)
Idle - NC $0.075 110 $0.008 $0.066 $0.198 $24.09 $72.27
Idle - CA $0.128 110 $0.014 $0.113 $0.338 $41.11 $123.34
Idle - GER € 0.220 110 €0.024
Load - NC $0.075 170 $0.013 $0.102 $0.306 $37.23 $111.69
Load - CA $0.128 170 $0.022 $0.174 $0.522 $63.54 $190.62
Load - GER € 0.220 170 €0.037

System 2 Power Costs
  Cost/kWh Outlet Power 1 Hour 8 Hrs 24 Hrs 1 year
(8 hrs/day)
1 year
(24 hrs/day)
Idle - NC $0.075 195 $0.015 $0.117 $0.351 $42.71 $128.12
Idle - CA $0.128 195 $0.025 $0.200 $0.599 $72.88 $218.65
Idle - GER € 0.220 195 €0.043
Load - NC $0.075 427 $0.032 $0.256 $0.769 $93.51 $280.54
Load - CA $0.128 427 $0.055 $0.437 $1.312 $159.60 $478.79
Load - GER € 0.220 427 €0.094

System 3 Power Costs
  Cost/kWh Outlet Power 1 Hour 8 Hrs 24 Hrs 1 year
(8 hrs/day)
1 year
(24 hrs/day)
Idle - NC $0.075 378 $0.028 $0.227 $0.680 $82.78 $248.35
Idle - CA $0.128 378 $0.048 $0.387 $1.161 $141.28 $423.84
Idle - GER € 0.220 378 €0.083
Load - NC $0.075 671 $0.050 $0.403 $1.208 $146.95 $440.85
Load - CA $0.128 671 $0.086 $0.687 $2.061 $250.79 $752.38
Load - GER € 0.220 671 €0.148

If you've ever wondered why Europe seems to be pushing for higher efficiency devices than the US, the above charts should provide an easy answer. Sure, very few systems actually consume 400W or more continually, but plenty of businesses run hundreds of 100W-200W PCs 24/7. Of course, other business expenses generally far outweigh power costs if you have that many PCs -- for example, the hundreds of employees sitting in front of those PCs likely cost 100 times as much per year, give or take. Still, the cost of leaving a high-end system running even eight hours a day at your house is not trivial, with idle power consumption costs ranging from around $100 to $300 per year. So let's delve a little deeper.

Index Actual System Power Costs


View All Comments

  • Kyanzes - Friday, November 14, 2008 - link

    I've kind of anticipated a calculator but still a nice read. Reply
  • JarredWalton - Saturday, November 15, 2008 - link

    Isn't that what the spreadsheet is? Reply
  • vandaliser - Friday, November 14, 2008 - link

    All you had to do is buy a Watt Meter which is kind of like a surge protector (but with a digital reader) where you connects your PC's power plug to the meter, then the meter to the main. (just go to ebay search Energy Meter and you will know what I'm on about)

    Take the reading in watts, divide it by 1000 to gives you the number of kwph. Finally, multiply it by the cost of one kwph on your electricity bill and numbers of hours you want to run it for.

    I'm not sure about their expected cost of running, but it actually surprises many people that their PC uses a lot less power then what they expects.
  • Griswold - Friday, November 14, 2008 - link

    Just that truly el-cheapo equipment will give you horribly wrong readings (cos-phi anyone?). Not saying a "watt-meter" must be expensive to give you acurate readings for home use, but there is way too much junk on the shelves out there. Reply
  • Souka - Friday, November 14, 2008 - link

    Buy a Kill-a-watt meter of eBay.... I did years ago, still using it today.

    It'll show real time Amps, Volts, Watt load, KWhr used, and time.">

    I just pulled it out for a co-worker to try at her home. :)
  • DeepThought86 - Friday, November 14, 2008 - link

    Given how little power even beefy systems consume, why is it that Anandtech continually reviews rediculously overpowered PSUs? What % of the market is made up of those 600W-1000W monsters? How about comprehensive reviews of the 300-500W market Reply
  • anartik - Tuesday, November 18, 2008 - link

    I would have to say that is a common misconception... There are reasons to buy more power than you "need". I bought "extra" for future upgrades and headroom. The problem with the calculator is most people plug in and come to the conclusion they need some fixed amount of power. All power supplies degrade in output over time with the cheaper ones faster (or use misleading claims as to output in the first place). If the calc says you need 400 and you buy 400 you’re in for trouble as the output deteriorates even quicker from running it at full capacity. The more you strain the PS the hotter its going to run and the louder its fans get. Plus you decide to run out and buy the latest power sucking hardware and voila you need a new power supply.

    I have a 4.3ghz E8500/X48 (SB w/bay,2 sticks DDR2, 2x drives,1 dvd burner and 3x120, 3x140) system and according to the calc I only need 462 with my current OC'd 8800 GTX. My old 550 Antec couldn't hang, screeched harmonics and was replaced with a Corsair HX1000. If I did a worse case upgrade... OC'd Q9550, more HD's, bluray burner and either a single 4870 X2 or possibly two and that power jumps to the range of 650-850 on paper. Factor in overages for peaks, efficiency, deterioration, percent utilization and it ranges from in the ball park to pushing it.
  • nilepez - Wednesday, November 19, 2008 - link

    With all due respect, sites have been pushing large PSUs for years. As I posted earlier, people were trying to convince me I need 600w 3 or 4 years ago, when I built an Athlon 64 Rig with an X800XL: a rig that couldn't not possibly have used 300w, even when overclocked, from the wall, much less from the PSU.

    As for the idea of what you'll need down the road, by the time you need more PSU (esp due age), you could just buy a new quieter, more efficient PSU, with more bells and whistles of equal or higher quality with the money you saved.

    Besides, in 20 years of computing, I've never had a PSU die. The worst thing that happened was a fan died. Bought a new fan and it worked like a champ, and that was some POS PSU that came with my Inwin Case (I think I still use that PSU, 10 years later!).

    buying a quality PSU makes sense. Buying 750w+ PSUs only makes sense for someone running Tri or quad SLI, which means almost nobody. I've seen developers at work return 600W PSUs, because they feared that they'd need more to run to 8800GTs.

    Those 2 cards pull at most 160w...add in a Core2 CPU, and you're looking a rig that is unlikely to pull 300W while playing far cry with Super Pi (just in case there's an idle cycle) running in the background.

    It's almost all marketing hype.
  • Griswold - Friday, November 14, 2008 - link

    I've been saying this since Chris' first (excellent!) review here at AT. I really wish he would push those insane power monsters with extra bling off his workbench and start reviewing those PSUs the majority actually buys. Reply
  • Christoph Katzer - Friday, November 14, 2008 - link

    Next one up will be most probably the Thermaltake TR2 QFan series with 300, 350, 400, and 450W. Everyone cheer up! ;) Reply

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