The Cost of Running Your PC
by Christoph Katzer on November 14, 2008 3:00 AM EST- Posted in
- Cases/Cooling/PSUs
The Difference a Few Percent Makes
Hopefully we've made it clear that upgrading an existing power supply to a higher efficiency model purely for the power savings doesn't make sense. However, there are times when you need to buy a new power supply, so we will wrap things up with a closer examination of how efficiency impacts power costs. Should you really care about the difference between 85%, 87%, or 90% efficiency?
This time, we don't need to worry about specific systems, but instead we will focus on efficiency and monetary savings at various power loads. The following table is again a best-case scenario for saving money -- i.e. you are running the system 24/7. Efficiency 1 is the base value and we compare the savings you would gain by selecting a power supply that achieves Efficiency 2. Efficiency ratings at the various loads represent what you might realistically find in various high-end power supplies currently on the market -- so getting 90% efficiency with a load of only 50W isn't going to happen.
| Savings from Incrementally Higher Efficiency - 24/7 Yearly Usage | |||||
| Output - Watts | Efficiency 1 | Efficiency 2 | Savings NC | Savings CA | Savings GER |
| 50 | 78% | 79% | $0.53 | $0.91 | €1.56 ($2.03) |
| 80% | $1.05 | $1.80 | €3.09 ($4.01) |
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| 81% | $1.56 | $2.66 | €4.58 ($5.95) |
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| 82% | $2.05 | $3.51 | €6.03 ($7.83) |
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| 200 | 80% | 81% | $2.03 | $3.46 | €5.95 ($7.73) |
| 83% | $5.94 | $10.13 | €17.41 ($22.64) |
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| 85% | $9.66 | $16.49 | €28.34 ($36.84) |
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| 87% | $13.22 | $22.55 | €38.77 ($50.40) |
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| 400 | 85% | 86% | $3.60 | $6.14 | €33.61 ($43.70) |
| 87% | $7.11 | $12.13 | €10.55 ($13.71) |
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| 88% | $10.54 | $17.99 | €20.85 ($27.10) |
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| 89% | $13.90 | $23.72 | €30.92 ($40.19) |
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| 700 | 85% | 86% | $6.29 | $10.74 | €18.45 ($23.99) |
| 87% | $12.44 | $21.23 | €36.49 ($47.43) |
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| 88% | $18.45 | $31.48 | €54.11 ($70.34) |
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| 89% | $24.32 | $41.50 | €71.33 ($92.73) |
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Obviously, the higher the load the better your savings, since a difference of 1W hardly matters. Your best course of action would be to select a power supply that offers the best efficiency at the load you will use most frequently. So for example, if you only play games on your computer and otherwise have it shut off, you might seriously consider a power supply with optimal efficiency at the 500W-600W range. On the other hand, if you typically just surf the Internet you'll probably be more interested in the efficiency at 100W-200W.
At the maximum load of 700W, and going with German power costs, the difference between an 85% and 89% efficiency power supply could be as much as €71. That's enough to get a significantly better power supply, but of course that sort of savings is unrealistic since it will be extremely difficult to achieve a 700W load all the time. The 400W load represents a more realistic maximum, as something like an overclocked quad-core system running Folding@Home could actually draw that much power around the clock. In that case, your savings could still be a pretty significant €30 per year, so over three years you could save almost €100. If you only run the system eight hours per day, however, the difference in cost drops off quickly.
Obviously, spending $20 more just to increase efficiency by 1% isn't necessary. You'll probably use a power supply for at least three years, so all other things being equal higher efficiency is good. That "all other things" is the problem, however, since rarely are the other areas the same. Pay attention to the other features like noise levels, voltage regulation, and the number and type of connector as well. Also keep in mind that we still have changing ATX standards, and sometimes new connectors, so spending a small fortune on a top quality PSU that might be outdated in a year or two might not be the best course of action either.
The bottom line ends up being a simple case of common sense: don't buy more power supply than you actually need, and don't spend a lot of money for a small increase in efficiency. Figure out how much power your system will normally use, and then choose a power supply appropriate for that sort of workload. If you routinely stress your system (i.e. workstation loads or intense gaming), an extra $100 for a high-end power supply might be a good idea. For most users, however, moderation will be the better course of action.
Finally, we spent quite a bit of time putting together the spreadsheet that we used to generate the tables in this article. We selected a few different markets for our power costs, and then we selected several different systems. Obviously, we couldn't cover everything, but for those who are interested in running their own calculations we thought you might appreciate our spreadsheet. Feel free to insert your own KWh costs, efficiency, and system power requirements to see how things change. (The highlighted fields should be the only areas you need to modify.)
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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.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??Christoph Katzer - Friday, November 14, 2008 - link
That's why the article calls "The Cost of Running Your PC"........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.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.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.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.