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

  • 7Enigma - Monday, November 17, 2008 - link

    I think you mean 1/2. Most of these charges are regardless of actual power used. Things like transmission charges, local/state taxes, "improvement" fees all will be the same each month whether you draw 100kW or 10. Where the savings *can* become more than stated is if your local utility uses a stepped price plan. That is to say you pay $X up to 500kWh's, then a higher rate after that level. This is how my bill is done unfortunately. I was under the impression there was some sort of peak hour price and another price for off-peak (as many places have). This can be taken advantage of by doing high energy tasks like the washer/dryer/dishwasher/ect. at night or early in the morning where you may be paying anywhere from 20-50% less for the same amount of power. With flat rate stepped plan you cannot benefit from using off-peak, and in general wind up paying more for your energy since they don't care if you used that 1000wh at 3am or 5pm.

    Everyone should check their bill statement and look to see how they are being charged. If you have a stepped plan like mine you may benefit more from being more energy conscious than if you have typical peak/off-peak pricing.
  • raWill - Saturday, November 15, 2008 - link

    Infact I have thought about it - I'm so glad I got rid of my 8800GTS, what a pointless consumer of power when all I do is surf the net 90% of the my computer is on.

    Even worse is people that leave thier sli systems on whilst downloading torrents, etc.

    By managing my standby power sources (turning them off every night before bed) and only downloading torrents and such whilst I am using the computer I saved about $20 a month in electricity. I live on my own too!
  • mongo lloyd - Friday, November 14, 2008 - link

    PSU efficiency will always be higher in Europe than the US due to 240VAC. I didn't see you adjusting for this, but granted, I only skimmed the article because my electricity bill is baked into my rent. Reply
  • ggordonliddy - Friday, November 14, 2008 - link

    It's okay, Osama will pay for it. I mean Hussein. I mean Obama. Reply
  • atm - Friday, November 14, 2008 - link

    Thanks for posting this article. Without the monitoring equipment at home, I was in the dark about true system power draw. Reply
  • ytsejam02 - Friday, November 14, 2008 - link

    I am all for saving energy. I have 3 computers in my house (laptop, desktop, and htpc), and both the htpc and desktop are running 45W cpu's with the Western Digital Green Power drives, and using the onboard graphics and sound. I run all the programs I need with that, so I hope I'm doing something right with these configurations, and that they are low power consumption.

    Now my problem. How much energy would be required to recycle a constant PSU turnover? I'm sure it wouldn't be constant, but I've no idea what it would cost, so I'm thinking in generalities at the moment.

    Either way, I'm guessing that would eat into a large part of the overall global energy problem.
  • IcePickFreak - Friday, November 14, 2008 - link

    I'm waiting for PC Power & Cooling to release their 10MW fusion reactor PSU. No longer will I be tethered to a power outlet or subjected to power outages.

    As a bonus, think of the m4D 9AM1nG 5ki11z I'll have when I sprout a third arm!
  • Carnildo - Monday, November 17, 2008 - link

    I'm afraid you'll be disappointed, then. Fusion reactors don't cause mutations. Reply
  • chenedwa - Friday, November 14, 2008 - link

    Could anyone estimate the cost of a typical laptop setup? Many will keep the laptop's adapter plugged-in 24/7 regardless of whether the computer is attached. Reply
  • strikeback03 - Friday, November 14, 2008 - link

    Using a Kill-A-Watt for measurement, my T43 with 15" S-IPS screen, Pentium M 1.86, ATi X300 graphics, 2GB RAM, and 100GB 7200RPM HDD uses 11W at idle with the lid closed, 18-21W at idle with the screen on (depending on brightness) and about 40W under load. Other laptop reviews on the site here offer power consumption numbers as well. Reply

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