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) |
||
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 $100 for a high-end power supply might be a good idea. For most users, however, moderation will be the better course of action.
Finally, we spent quite a bit of time putting together the spreadsheet that we used to generate the tables in this article. We selected a few different markets for our power costs, and then we selected several different systems. Obviously, we couldn't cover everything, but for those who are interested in running their own calculations we thought you might appreciate our spreadsheet. Feel free to insert your own KWh costs, efficiency, and system power requirements to see how things change. (The highlighted fields should be the only areas you need to modify.)
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The0ne - Friday, November 14, 2008 - link
I would have to agree. If only power requirements were more accurate or rather stated for general usage some of us wouldn't have to go out and buy these 700-1000W PS for a system that draws half of that.All in all though, I have to put things in perspective. I waste more time and thus money playing games on my PC; Heroes of M$M 3 and FFXI. So while I can save a little by turning off the PC once in a while and getting more efficient parts, I'll save even more if I just cancel my FFXI account :)
Mr Perfect - Friday, November 14, 2008 - link
Seconded.It's especially important to have reviews of reasonably size PSUs when you take a look at efficiency curves on PSUs. PSUs achieve their best efficiency at higher loads, which is why 80+ testing only requires 80% efficiency at 20%, 50% and 100% output to qualify. So a 80+ certified 1000watt PSU will be at least 80% efficient if you're pulling over 200Watts, but if your system draws less then that, efficiency can tumble down into the 70s or 60s without breaking any rules. On the other hand, if you have a 500 watt 80+ PSU, you'd have to draw less then 100watts before you get into the low end of the efficiency curve. For people with HTPCs or budget boxes that really do draw under 100watts, they'll probably want something even smaller, like 300watts.
Clauzii - Friday, November 14, 2008 - link
Agree! Most systems use under 300W total, so a bit more of those would be nice.nilepez - Tuesday, November 18, 2008 - link
I also agree. I have a Core2 CPU and GTX260, and at idle it's pulling around 120w from the wall. I don't recall what it was pulling at 100% CPU/GPU, but I believe it was roughly 220-240.A few years ago, I was talked into buying a 500W PSU, because I needed
that to power a Athlon 64 and an X800XL.....of course it idled between 70-90w (from the wall) and never hit 200w....ever.
I did replace it with another 500ish PSU, but in this case, I bought it because the Corsair is very quiet and has modular cables. Power wise, I would have been fine with a smaller psu.
mpjesse - Friday, November 14, 2008 - link
This is a great article. What would be even cooler is if ya'll made some sort of web calculator that could compute the total cost of running your system based on a few known variables (CPU type, GPU type, # of hard drives, time spent idling, etc) and maybe even each U.S. state's electricity rate. That'd probably be a lot of work, but I'd certainly use it everytime I start a new build.Clauzii - Friday, November 14, 2008 - link
You can try this:http://extreme.outervision.com/psucalculatorlite.j...">http://extreme.outervision.com/psucalculatorlite.j...
TennesseeTony - Friday, November 14, 2008 - link
From what I've seen and read, my ancient power supply is at best 65% efficient. Judging by the many comments I've read here on this site, many many others are still using their ancient P4 3.06Ghz systems on a daily, often 24/7 situation, as well.I for one don't consider the efficiency rating to be marketing hype, and am very glad to see these better designs.
Fortunately for me, I held off on the Conroe, saved my pennies, and next week (hopefully) I get to place the order for the final component in my new build...a Nehalem Core i7 920. (Got the Asus p6t ordered from zip....fly last night.)
Just a few more days and I get to fire up my new 85plus power supply...Woohoo! At idle, with the increased efficiency, perhaps my power costs will remain the same? I could pinch those pennies really tight and reus my old PS, but the new one will pay for itself in short order in my situation.
joseps75 - Sunday, November 1, 2009 - link
My PSU for my 5 computers varies from 3 to 5 yrs old. Mostly I keepupgrading my MB AND CPU'S. Now all my 5 boxes are running quad cores processors. Since I runn them 16hrs daily, I hook up a KILL A WATT EZ to each box to check how much power each box consume. Here are my data for each box tagged by MB NAME: 1) #1 P5K-E $0.0164/HR, 2) #2 P5K-E, $0.0144/hr, 3) P5K-V, $0.0125/hr, 4) #4 P5Q-SE, $0.0156 and 5) #5 M3N78-VM, $0.0105.I use my computer hobby to run volunteer research on medical cure for human diseases at Rosetta@home and World Community Grid. They are worthy non profit research to find cure for human diseases HIV, AIDS, ALSHEIMER, CANCER ETC.
joseps75:-)
whatthehey - Friday, November 14, 2008 - link
I don't think anyone is suggesting that a six year old power supply shouldn't be retired. I looked at the spreadsheet they gave on the last page - damn sweet, I must say! Anyway, I was poking around with some numbers to see what it says about lesser PSUs. If you have a 60-65% efficient PSU with your old system and the PC drew 125W to 250W (for the components, not at the wall), and you run it 8 hours per day with half the time at full load and half at idle, you can get a result for your savings per year.Assuming the spreadsheet is correct and I put things in the proper spot, you're looking at a yearly power savings of around $15 to $30 using the above scenario. If you run all the time, your savings would be anywhere from $40 to $100 per year. That's all going with equal power requirements and 60% idle/65% load efficiency to 84% idle/85% load efficiency. The new power supply might pay for itself in a year or two, but for power requirements your PC would take much longer to pay off. But then, more performance is its own reward, right?