The Cost of Running Your PC
by Christoph Katzer on November 14, 2008 3:00 AM EST- Posted in
- Cases/Cooling/PSUs
Using a Higher Efficiency PSU to Reduce Costs
These days manufacturers are all promoting their high efficiency power supplies, and we have organizations and certifications like 80 Plus encouraging even small boosts in efficiency. Not surprisingly, plenty of users have been sucked in by the marketing and are now convinced that they need to purchase a new power supply in order to save money each year. Does it really make that much of a difference? The answer as usual depends on how you use your system. The previous page provided a baseline measurement, but now let's look at how much money you can save if you go out and purchase a new 80 Plus Bronze or Silver certified power supply as an upgrade to a slightly older ~80% efficiency PSU.
Our sample power supplies on the previous page are all relatively high-end choices for the specific market. Many (most) systems don't have power supplies anywhere near that nice, relatively speaking. So what happens when we switch to an older ATX 1.3 PSU -- something that would have been more or less state-of-the-art three years ago? Will a newer power supply really help you save the planet? Will it at least reduce your power costs and save you money? Let's find out, this time looking at power costs over the course of a full year: 24 hours a day, seven days a week.
For reference, we looked at some PSU efficiency results stashed away in our files and estimated ATX1.3 PSU efficiency at 75% idle and 78% load. That represents a high-end ATX1.3 PSU, and in some cases the discussion is hypothetical as it wouldn't be possible to find an older PSU with the necessary output rating. (That applies specifically to the high-end system.)
| System 1 24/7 Yearly Costs ATX1.3 versus ATX2.2 |
||||||
| Outlet Power ATX v2.2 |
Outlet Power ATX v1.3 |
Wattage Difference |
Savings NC | Savings CA | Savings GER | |
| Idle | 110 | 120 | 10 | $6.57 | $11.21 | €19.27 ($25.05) |
| Load | 167 | 179 | 12 | $7.88 | $13.46 | €23.13 ($30.06) |
| System 2 24/7 Yearly Costs ATX1.3 versus ATX2.2 |
||||||
| Outlet Power ATX v2.2 |
Outlet Power ATX v1.3 |
Wattage Difference |
Savings NC | Savings CA | Savings GER | |
| Idle | 190 | 213 | 23 | $15.11 | $25.79 | €44.33 ($57.62) |
| Load | 412 | 449 | 37 | $24.31 | $41.49 | €71.31 ($92.70) |
| System 3 24/7 Yearly Costs ATX1.3 versus ATX2.2 |
||||||
| Outlet Power ATX v2.2 |
Outlet Power ATX v1.3 |
Wattage Difference |
Savings NC | Savings CA | Savings GER | |
| Idle | 369 | 413 | 44 | $28.91 | $49.34 | €84.80 ($110.24) |
| Load | 663 | 705 | 42 | $27.59 | $47.09 | €80.94 ($105.23) |
Now we can see exactly how much money you might save during the course of a year by purchasing a new high efficiency power supply. Obviously, the more power your computer uses, the better your monetary savings. Looking at these tables, you might begin to think there's actually a point in upgrading power supplies -- and there is, provided you're running your computer a large portion of the time.
What happens if we change our usage model to something more realistic for most families? Instead of looking at 24/7 usage, let's change it to three hours of use per day on average, with two hours at idle and one hour at load.
| Yearly Power Savings for 3 Hrs/Day | |||
| Savings NC | Savings CA | Savings GER | |
| System 1 | $0.88 | $1.50 | €2.57 ($3.34) |
| System 2 | $2.27 | $3.88 | €6.66 ($8.66) |
| System 3 | $3.56 | $6.07 | €10.44 ($13.57) |
The need to upgrade power supplies suddenly doesn't seem as dire once we switch to a more realistic usage model. Particularly on low-end systems that only use 100W of power give or take, even an extremely inefficient PSU probably doesn't matter too much if the system isn't on more than a few hours per day. Even with power costs that are up to three times higher in some parts of Europe compared to areas in the US, the savings don't make sense.
If you happen to be the type of user that leaves your system on all the time, certainly you can save a fair amount of money by purchasing a better power supply. An easier solution would simply be to turn off your computer when it's not in use, unless you have a really good reason to leave it running overnight. Similarly, if your current PSU happens to fail, it might be worthwhile to spend a little bit more money to get a higher efficiency, better quality power supply. If you figure on a moderate amount of use and a five-year lifespan, you might want to spend as much as $50-$100 extra. Otherwise, there's very little incentive to go out and spend $150 on a top quality power supply just so you can save $10-$15 per year (or less).
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The0ne - Friday, November 14, 2008 - link
I would have to agree. If only power requirements were more accurate or rather stated for general usage some of us wouldn't have to go out and buy these 700-1000W PS for a system that draws half of that.All in all though, I have to put things in perspective. I waste more time and thus money playing games on my PC; Heroes of M$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?