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

  • bob4432 - Tuesday, November 18, 2008 - link

    exactly, and if you are not afraid of flashing your video bios, then you can really tweak the power setups for the lower end power "2d" mode to much less than what ati put in as a default. i think ati rushed the 4850 and sounds like 4870 bios when they first came out (extreme heat, lack of real power savings, etc) but hopefully now all that is fixed.

    fwiw - i do consider the amount of $$$ in cooling when getting a real measure on how much it cost to run the pc. i am in phx, az and ac is a must, so in my computer room/office it usually gets about 5-8F warmer than the rest of the condo and therefore that difference needs to be taken into account.
  • BitBodger - Friday, November 14, 2008 - link

    One thing not considered here is the effect of the heat from the computer. That computer sitting beside your desk is also an electric heater constantly warming your home. Live in a cold climate and this is not entirely a bad thing since it takes some of the load from your main heating system. But if you live in a hot climate and depend on air conditioning don't forget that the heater never stops meaning that your AC works that much harder and consumes that much more energy getting rid of the extra heat. And given the inefficiency of AC technology, it costs more watts in the cooling process than are emitted by the heater. Reply
  • Hammarby - Friday, November 14, 2008 - link

    In all these calculations shouldn't you also factor in how much extra it will cost to cool your house when you have a 100-500 watt space heater running for 8 hours/day?? Reply
  • Christoph Katzer - Friday, November 14, 2008 - link

    That's why the article calls "The Cost of Running Your PC"........ Reply
  • Lifted - Saturday, November 15, 2008 - link

    If "running your PC" increases the demand on your home cooling, then that is a cost directly resulting from running your PC. It is out of the scope of the article since there are too many variables to consider than just the cost of electricity. Reply
  • JarredWalton - Saturday, November 15, 2008 - link

    But if it's cold and running your PC reduces the amount of time you run your heater, then running your PC would cost less in the winter. Besides which, plenty of people don't have AC, so even in the summer there's no added expense. Thus, we chose to limit the discussion specifically to how much your PC costs to run, and how PSU efficiency can play a role in those costs. Reply
  • 7Enigma - Monday, November 17, 2008 - link

    While electricity is 100% efficient (or near enough that we don't need to quibble), it is also true that very few people use it to heat their homes due to $/BTU. Until there comes a time when it is as cheap or cheaper to heat your home electrically I don't think your comment holds true.

    I also agree with the original poster that while not exactly in the scope of the article it is closely related and should have been mentioned. The common position of why spend more money on a more efficient PSU or any other component when the electricity costs show a ROI much greater than the usable life of that product is not accurate when cooling costs are not taken into account. While heating electrically is near 100% efficient, cooling is definitely not.
  • nilepez - Wednesday, November 19, 2008 - link

    Lots of houses are heated with electricity. I've never lived in an Apartment with anything but electric Heat.

    Nevertheless, the heat from a PSU is negligible in the winter (even in the south) and an efficient PSU, by definition, produces less heat, which is especially true when the PC is idle.
  • ZoZo - Friday, November 14, 2008 - link

    Or you can factor in how much less it costs to heat your home when you have a 150W space heater running for 8h/day. Reply
  • Staples - Friday, November 14, 2008 - link

    I have not even read the article yet but I am surprised that was not even mentioned. That often adds 50% more to the cost if you live in a hot climate. Same thing with light bulbs. There is a double savings with CF bulbs. Reply

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