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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Kyanzes - Friday, November 14, 2008 - link
I've kind of anticipated a calculator but still a nice read.JarredWalton - Saturday, November 15, 2008 - link
Isn't that what the spreadsheet is?vandaliser - Friday, November 14, 2008 - link
All you had to do is buy a Watt Meter which is kind of like a surge protector (but with a digital reader) where you connects your PC's power plug to the meter, then the meter to the main. (just go to ebay search Energy Meter and you will know what I'm on about)Take the reading in watts, divide it by 1000 to gives you the number of kwph. Finally, multiply it by the cost of one kwph on your electricity bill and numbers of hours you want to run it for.
I'm not sure about their expected cost of running, but it actually surprises many people that their PC uses a lot less power then what they expects.
Griswold - Friday, November 14, 2008 - link
Just that truly el-cheapo equipment will give you horribly wrong readings (cos-phi anyone?). Not saying a "watt-meter" must be expensive to give you acurate readings for home use, but there is way too much junk on the shelves out there.Souka - Friday, November 14, 2008 - link
Buy a Kill-a-watt meter of eBay.... I did years ago, still using it today.It'll show real time Amps, Volts, Watt load, KWhr used, and time.
http://energyseeds.com/2007/10/11/go-solar-and-kil...">http://energyseeds.com/2007/10/11/go-solar-and-kil...
I just pulled it out for a co-worker to try at her home. :)
DeepThought86 - Friday, November 14, 2008 - link
Given how little power even beefy systems consume, why is it that Anandtech continually reviews rediculously overpowered PSUs? What % of the market is made up of those 600W-1000W monsters? How about comprehensive reviews of the 300-500W marketanartik - Tuesday, November 18, 2008 - link
I would have to say that is a common misconception... There are reasons to buy more power than you "need". I bought "extra" for future upgrades and headroom. The problem with the calculator is most people plug in and come to the conclusion they need some fixed amount of power. All power supplies degrade in output over time with the cheaper ones faster (or use misleading claims as to output in the first place). If the calc says you need 400 and you buy 400 you’re in for trouble as the output deteriorates even quicker from running it at full capacity. The more you strain the PS the hotter its going to run and the louder its fans get. Plus you decide to run out and buy the latest power sucking hardware and voila you need a new power supply.I have a 4.3ghz E8500/X48 (SB w/bay,2 sticks DDR2, 2x drives,1 dvd burner and 3x120, 3x140) system and according to the calc I only need 462 with my current OC'd 8800 GTX. My old 550 Antec couldn't hang, screeched harmonics and was replaced with a Corsair HX1000. If I did a worse case upgrade... OC'd Q9550, more HD's, bluray burner and either a single 4870 X2 or possibly two and that power jumps to the range of 650-850 on paper. Factor in overages for peaks, efficiency, deterioration, percent utilization and it ranges from in the ball park to pushing it.
nilepez - Wednesday, November 19, 2008 - link
With all due respect, sites have been pushing large PSUs for years. As I posted earlier, people were trying to convince me I need 600w 3 or 4 years ago, when I built an Athlon 64 Rig with an X800XL: a rig that couldn't not possibly have used 300w, even when overclocked, from the wall, much less from the PSU.As for the idea of what you'll need down the road, by the time you need more PSU (esp due age), you could just buy a new quieter, more efficient PSU, with more bells and whistles of equal or higher quality with the money you saved.
Besides, in 20 years of computing, I've never had a PSU die. The worst thing that happened was a fan died. Bought a new fan and it worked like a champ, and that was some POS PSU that came with my Inwin Case (I think I still use that PSU, 10 years later!).
buying a quality PSU makes sense. Buying 750w+ PSUs only makes sense for someone running Tri or quad SLI, which means almost nobody. I've seen developers at work return 600W PSUs, because they feared that they'd need more to run to 8800GTs.
Those 2 cards pull at most 160w...add in a Core2 CPU, and you're looking a rig that is unlikely to pull 300W while playing far cry with Super Pi (just in case there's an idle cycle) running in the background.
It's almost all marketing hype.
Griswold - Friday, November 14, 2008 - link
I've been saying this since Chris' first (excellent!) review here at AT. I really wish he would push those insane power monsters with extra bling off his workbench and start reviewing those PSUs the majority actually buys.Christoph Katzer - Friday, November 14, 2008 - link
Next one up will be most probably the Thermaltake TR2 QFan series with 300, 350, 400, and 450W. Everyone cheer up! ;)