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