The Cost of Running Your PC
by Christoph Katzer on November 14, 2008 3:00 AM EST- Posted in
- Cases/Cooling/PSUs
Actual System Power Costs
On the previous page, we estimated the efficiency for each system as being 82%. Obviously, that's not a true way of calculating power requirements or efficiency. Now we're going to shift to the real world and see what the three sample systems end up costing on an hourly basis.
Before we get to the tables, it's important to remember that just because a power supply advertises 90% efficiency doesn't mean you'll always reach that level. You can look at any of our power supply reviews -- or just read Debunking Power Supply Myths -- to understand this better. The short summary is that all power supplies have an efficiency curve, which depends on the load you place on the power supply.
At lower loads and maximum load, efficiency is lower than if you run at a medium load (relative to the PSU's rated output). If you're just surfing the Internet, writing a document, or viewing pictures your system will largely sit idle. Playing a game, doing 3D rendering, encoding a video, or other complex calculations will place a higher load on your PSU. The following tables use actual efficiency with a real power supply to calculate power costs.
System 1
Our entry-level system, System 1, will utilize the Thermaltake TR2 QFan 300W power supply we recommended in our last article. System 1 consumes 90W to 140W of power, depending on load -- those are best-case/worst-case figures. We haven't posted our review of the QFan yet, but it achieves 82% efficiency at 90W load and 84% efficiency at 140W load. The hourly power costs are:
| System 1 Power Costs with Real Efficiency | |||||
| Load | Efficiency | Outlet Power | Cost/hr NC | Cost/hr CA | Cost/hr GER |
| 90W | 82% | 110 | $0.008 | $0.014 | €0.024 ($0.031) |
| 140W | 84% | 167 | $0.013 | $0.021 | €0.037 ($0.048) |
System 2
System 2, our midrange system, will use the OCZ ModXStream Pro. This system requires between 160W and 350W of power. The OCZ power supply runs at 84% efficiency for 160W and 85% efficiency for 350W. That gives the following power costs:
| System 2 Power Costs with Real Efficiency | |||||
| Load | Efficiency | Outlet Power | Cost/hr NC | Cost/hr CA | Cost/hr GER |
| 160W | 84% | 190 | $0.014 | $0.024 | €0.042 ($0.054) |
| 350W | 85% | 412 | $0.031 | $0.053 | €0.091 ($0.118) |
System 3
Lastly, our high-end system is running two graphics cards for maximum performance. This time we selected the OCZ EliteXStream 800W PSU. Note that even this beefy system still only requires 550W at maximum load, whereas it idles at 310W. In this case, efficiency is 84% idle and 83% at full load.
| System 3 Power Costs with Real Efficiency | |||||
| Load | Efficiency | Outlet Power | Cost/hr NC | Cost/hr CA | Cost/hr GER |
| 310W | 84% | 369 | $0.028 | $0.047 | €0.081 ($0.106) |
| 550W | 83% | 663 | $0.050 | $0.085 | €0.146 ($0.190) |
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The0ne - Friday, November 14, 2008 - link
I would have to agree. If only power requirements were more accurate or rather stated for general usage some of us wouldn't have to go out and buy these 700-1000W PS for a system that draws half of that.All in all though, I have to put things in perspective. I waste more time and thus money playing games on my PC; Heroes of M$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?