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

  • walk2k - Tuesday, November 18, 2008 - link

    If you're curious to find out what your system actually uses, for about you can buy a P3 "Kill-a-Watt" power meter from Amazon for about $24. This is a doo-hicky that plugs into the power outlet, and then you plug whatever you want to measure into that. I've personally found it very enlightening.

    My current PC uses about 120 watts idle in XP, up to about 150 watts when actually doing stuff (cpu slightly loaded, HDD churning). Then in games with the GPU loaded it uses about 200 watts. I only managed to get it to 233 watts when running both a cpu stress test (Orthos) and GPU benchmark (3dmark06) at the same time.

    I'm in CA and pay about 13 cents/Kilowatt/hour so theoretically if I left my system on 24/7 (I don't) at idle it would cost me about $11 per month. In reality I use my computer about 5 hours a day, with about half games (200 watts) and half just surfing, etc (120 watts) so say average 160 watts x 8 hours x $.13 x 30 days = $5 per month.
  • glynor - Tuesday, November 18, 2008 - link

    The prices listed on the EIA website that you used to generate all these numbers (for the US) are based only on the price residential customers pay for energy generation, and do not include the cents per KWH charge that all customers must pay for transmission. Even though I simply take the standard offer from my utility company (like most people), my actual price is roughly DOUBLE what the EIA lists as the average for my state.

    Look at your bill. Make sure to add the totals for both Generation (Supply) and Transmission. These are billed separately.

    With that in mind, most of the example totals reported in this article, and certainly all the comparisons to "Europeans paying more" are completely bogus!
  • Maiyr - Tuesday, November 18, 2008 - link

    Why bother with all of this just to figure out how much it costs to run your system ? Surely someone must have thought of just plugging in a Kill-A-Watt Electricity Usage Monitor....

  • gochichi - Tuesday, November 18, 2008 - link

    I would look for better efficiency rating for the following reasons (in this order):
    1) I speculate the product is a higher quality product.
    2) Inefficiencies turn into waste heat, cooler is more stable, more pleasant, and longer lasting (speculations)
    3) I guess maybe saving a couple of bucks on my bill, but it's definitely not as important to me as just knowing that a have a "pimp" power supply.

    What this article does make me want to do is try to figure out what sleep modes best suit my desktop computers (particularly my seldom used older desktop). Cold booting is inconvenient and I have been careless about finding a reliable sleep mode. I think the trick is to go S1, I just switched my main computer from S3 to S1 and it really helps stability and responsiveness.

    I wonder how running laptops does in terms of power efficiency. There's got to be a ton of inefficiency caused by charging and discharging the batteries all the time. The design circling around a portable battery also makes it pretty power thrifty at the same time. Just curious as to how it actually pans out. Laptops have typically been more robust in sleep mode too though.

    Random comment: Is it just me or have software updates gotten beyond the ridiculous point? Seems like they are not only often, but they are in your face... like it's OK to interrupt what you're up to a few times a week to get the update "right now"... I mean forcefully.

  • bob11d50 - Sunday, November 16, 2008 - link

    I just wanted to justify my purchases of energy efficient computer components.

    The reason for this was about 3 years ago I had 9 people living in my house and the power company was on a teared system. I got up to the fourth tear where power was up to $0.85 KWH. This was in San Jose California with PG&E as the power company.

    I got my server down to 77 WATS with a EE Athlon at idle from about 230 with my dual Opteron. All measurements were using the Kill-A-Watt.

  • sheh - Saturday, November 15, 2008 - link

    I was always under the impression your total power readings were at the outlet, and so included the efficiency loss of the PSU. If that wasn't the case, how do you measure total power draw from within the computer?
  • Christoph Katzer - Sunday, November 16, 2008 - link

    Motherboard, system, graphics-reviews got readings from the wall. power supplies are loaded with a programmable load, so we can easily calculate the losses. You can know the power draw from mobos, gfx and so on when measuring the power distribution inside of the case which isn't difficult if you know which cable powers which part of which component ;) Reply
  • bigsnyder - Saturday, November 15, 2008 - link

    What kind of case is that in the picture? Reply
  • Christoph Katzer - Sunday, November 16, 2008 - link

    Silverstone TJ10 Reply
  • GimpyOne - Saturday, November 15, 2008 - link

    Not sure about Europe, but here in the US, the cost of "electricity" is only about half the bill. Once they tack on things like Fuel Cost charges, Energy improvement riders, resource adjustments, etc. you double the actual bill (literally).

    So more appropriate cost saving could actually be closer to 2x what is here.

Log in

Don't have an account? Sign up now