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How much power do you really need?

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September 4, 2009 4:32:22 AM

Well I just bought a nifty Kill-A-Watt power reader and hooked it up to my computer at the outlet. And after using it and monitoring the wattages in different computing environments and at different loads I have a question. As far as I know a 500W PSU is about the absolute bare minimum for any respectable quad core, mid to high range mobo/chipset, mid to high range GPU computer. So lets say (using my surrent system specs) you have an AMD 9950 125W CPU, a Gigabyte GA-MA790GP-UD4H, an EVGA nVidia 9600GT superclocked edition, w/ a 250GB 7200RPM HDD,a standard DVD recorder, and 4GB of DDR2 @ 1066Mhz at 2.0V, w/ 2 120mm fans and an 80mm. The highest wattage usage I have ever seen so far from only these components is a total of ~240W, and that was only momentarily while stress testing the 9950 at 3.0Ghz. But wouldn't say, a 300W PSU be too little for that configuration. At least that's what I thought before I had the machine to show me the actual power consumption. but nobody would run a configuration like that on a 300W PSU would they?? Could they even do it?!?

p.s. this really helped put into perspective how wasteful incandescent light bulbs are for me. I mean 60W will get you a bright bulb. Or it could get you a basic computer that allows you to explore all the knowledge in the world, compose documents, calculate pi to the couple hundreds and solve (most) mathematical questions you throw at it, that 60W could even bring HD content onto your TV (with the right configuration) and you could be watching your Netflix subscription through your comp, and you could even play many games at semi-decent quality. It really puts incandescent light bulbs to shame considering what a computer can do compared to a light bulb w/ the same amount of energy. IDK :\

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September 4, 2009 5:05:48 AM

That's about right for your system. The GPU is a good chunk of the power consumption in a modern system, and your 9600 is pretty energy efficient as GPUs go.

For comparison, on my i7+4870 quadfire setup, I have observed 350W idle and around 1000W at the wall peak. For a standard midrange setup though, a 400-500W good quality PSU is definitely adequate.
September 4, 2009 5:15:29 AM

Here's the kicker, techinically your computer is using even less. The PSU just needs to draw more from the wall to convert the required power to the computer. Most PSUs now are around 80% efficient. So technically your computer is using ~192 watts.

Though powering a computer isn't all about the total wattage. It's about the amps on specific rails. If there' not enough amperage available on the 12v rail, your Graphics Card/s and accessories will fail to work properly, or at all.

When my computer was running all stock speeds i checked my power usage at the wall and it was about ~280. Or ~220watts my PSU was producing in the form of DC current assuming 80% efficiency. This was running a 3DMark06 benchmark, so technically my system could draw more power.

I wonder what my system would pull running Folding@Home GPU Client, and Prime95...ew.
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a c 248 ) Power supply
September 4, 2009 5:54:06 AM

As cjl already mentioned the power draw at the wall outlet is just about right. One of the recurring problems we have are people who think they need a lot more power than they actually do. Most of them do not pay attention to the current (amps) on the 12 volt rail(s) either.

I liked the comparison to the light bulb. For my next build I am eriously considering building an energy efficiency pc that would qualify as a respectable mid-range system. Luckily I am not a hardcore gamer into serious overclocking about to go on a sucide mission to save the planet from alien invaders.
a b ) Power supply
September 4, 2009 6:35:20 AM

When playing most games, my peak is about 260W, but then if I run furmark it's 360W with both GPUs heating up pretty quick, or at least that's what Everest reports. Running fur mark and Prim 95 on the other core my max is 401W
a b ) Power supply
September 4, 2009 6:38:26 AM

I also use the Kill-A-Watt power reader by the way. Just keep in mind that the value it reports is what it's drawing out of the wall, and it does have some margin of error. Some of the power is simply lost as heat when it's converted by your PSU so the power the rest of your system is actually using is a bit less and thus a more efficient PSU would have the Kill A Watt reporting a slightly lower number.
a c 144 ) Power supply
September 4, 2009 7:39:12 AM

I also have a Kill-a-watt. However, I am not to surprised at the results. Several years ago when I built my first C2D system (eVGA 680i and E6600), I borrowed a clamp-on amp-probe from work and measured everything I could. I repeated when I built my first C2Q system (P35 and Q6600). BTW, a highly OC'd Q6600 pulls 9.5 amps.

The downside to all this is that it just is not a good idea to operate a PSU close to it's maximum capacity. Too many PSU's are rated at full power at 25 C. As a PSU heats, the available output drops as much as 2% per degree over the temperature rating. It's very likely a 500 watt PSU will turn into a 400 watt PSU at operating temperature.The really good PSU's are rated at 50 C.

I prefer to operate my PSU's at 50% - 60% of capacity. They run cooler. That is why I put a TX750 in the box with my OC'd Q9550 and single video card. And it didn't cost much more than the TX650. :) 

I also admit that I am never on that tight of a building budget. I can afford to over-spec good PSU's. I realize that many cannot. And for them especially, this is really valuable.
September 4, 2009 10:42:11 AM

For the 10.000th time: NO! You don't need a 500W PSU on a midrange computer, anything with 26-30A on the 12V line would handle such a system just fine for many years to come, even with overclocking, Corsair and OCZ both have a 400W PSU with that many amps.

Look at my specs, I have an overclocked i7 920 + HD 4890 and it's running just fine on a Corsair HX520W, if I were to go around on the forums asking what kind of power supply I'd need for my system most people would probably tell me I'd need at least 650W, but 700W or 800W would be better. Anyone with a multimeter, or just general knowledge about TDP's and Voltage lines knows this is absolute rubbish, but some myths just won't die out...

P.S. PSU's are generally most efficient between 50-80% of their maximum load, so buying a PSU that's too big can actually increase your electricity bill!
a c 125 ) Power supply
September 4, 2009 12:23:44 PM

I agree with jsc.
gulli's last point was backwards; buying a larger PSU will often decrease your bill, if the bigger one is able to operate in a more efficient part of its range (e.g. 50%-60%), where a smaller one might be seeing some droppoff (e.g. at 90%).
The 80Plus certifications include testing at 100% load though, which is good news. It means that not only is the PSU actually capable of supplying its labeled output (many cheap PSUs simply are not), but it is able to do so efficiently.
Back to jsc's remark, there is no way my PC (Q9450 @3.2GHz, 1xHD4850) needs even 500W, but I put a SG-650 in my rig because I wanted the best, and happily this time I could afford it.
@JonnyLucky, my "Green Gamer" project PC tested this minimalist approach, and the results were favorable. Although I've replaced its CPU with a 720BE (the 4850e just wasn't quite enough), it is happily running on an Earthwatts 380W PSU; the GPU is a 4670, which handles all my games fine at 1440x900.
September 4, 2009 4:52:44 PM

For the build below, should I get a 650w or a 750w psu? I aim to overclock to 4 ghz if possible.

- Antec Three Hundred Black Steel ATX Mid Tower Computer Case - Retail
-Western Digital Caviar Blue WD3200AAJS 320GB 7200 RPM SATA 3.0Gb/s 3.5" Internal Hard Drive - OEM
-GIGABYTE GA-MA790XT-UD4P AM3 DDR3 AMD 790X ATX AMD Motherboard - Retail
- (2 for crossfire) SAPPHIRE Vapor-X Radeon HD 4870 1GB 256-bit GDDR5 PCI Express 2.0 x16 HDCP
-CORSAIR CMPSU-650TX 650W ATX12V / EPS12V SLI Ready CrossFire Ready 80 PLUS Certified Active
-G.SKILL 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1333 (PC3 10666) Dual Channel Kit Desktop Memory
-AMD Phenom II X2 550 Black Edition Callisto 3.1GHz Socket AM3 80W Dual-Core Processor Model
-ARCTIC COOLING Freezer 64 Pro 92mm CPU Cooler - Retail
-3 120mm fans
-possible fan controller

plus a dvdburner which I haven't yet decided on.

a b ) Power supply
September 4, 2009 5:10:45 PM

To mcattack. I look at it this way, it's a new build, you're going to be buying a new PSU with a 5 year warranty. The 650w Corsair has enough power to handle your above system, even when overclocking is taken into account. The 650w corsair runs $100 with a $20 rebate. The 750w runs $110 with a $20 rebate. To me the extra $10 is worth the long term (upgrade CPU/GPUs) advantages of buying the 750w unit.
a b ) Power supply
September 4, 2009 5:41:27 PM

Onus said:
buying a larger PSU will often decrease your bill, if the bigger one is able to operate in a more efficient part of its range (e.g. 50%-60%)
This is an excellent reason for buying an overspeced PSU. I chose the 525W version of the Enermax Pro 82+ specifically because it's efficiency graph shows that it peaks at around 40-60% of it's capacity.
September 5, 2009 12:23:42 PM

Onus said:
I agree with jsc.
gulli's last point was backwards; buying a larger PSU will often decrease your bill, if the bigger one is able to operate in a more efficient part of its range (e.g. 50%-60%), where a smaller one might be seeing some droppoff (e.g. at 90%).


You didn't get my point: I meant your electricity bill would increase if your PSU spends most of the time at say 30% load (idle, or light gaming), as would indeed be the case if you get a powerful 500Watt-ish PSU for a system with only a 9600gt.
September 5, 2009 1:00:40 PM

For very good reasons I never build systems with multi-cards, or even the potential to. Upgrading to the latest top-end when needed is always (no exceptions) more cost efficient in getting more FPS
For that reason I pick PSU that will be fully loaded to ~80% absolute max. and would idle above 25%.

I'm in the same camp as Gulli.
a c 125 ) Power supply
September 5, 2009 1:17:12 PM

But that is not correct, unless the load is insignificant. Many 80+ PSUs are actually not very efficient around 10% load. No one is suggesting you overspec your PSU by such an extent that you're operating in that range; we're describing the opposite end of the usage spectrum.
Here's a common example:
A system pulls 250W under load. Its owner used an old 450W Chokemax PSU, which is only 71% efficient at that load (56% of its label). That means it is pulling 250/0.71=352W from the wall.
Someone wises him up and he buys an Antec Earthwatts. He goes overboard and buys the 650W model (when even the 380 would do). His system will only use 250/650=38% of that PSU's capacity, but it will be 84% efficient at that load. It will only draw 250/.84=298W from the wall. Much bigger PSU; 54W less power used, so a lower bill.
Had the original been a Seasonic 250W 80+ PSU, although running at 100% of its rated capacity, it would still be at least 80% efficient, so the difference would not be as great.
The situation you're describing would happen if the person started out with, in this example, an Earthwatts 380 (e.g. 83% efficient) and replaced it with something like a Zilchmaz 750W PSU that is only 79% efficient at that load. The Zilchmax, though bigger, is less efficient so it would draw a few more watts from the wall, and result in a higher bill.

September 5, 2009 1:18:53 PM

While I agree with those numbers as being the most efficient, it also depends on usage (amount of time used) and how long you plan to keep it, as the caps age, and those numbers change.
Being that if you want your psu to last 2+ years, to keep within those numbers towards the end of your psus lifetime, maybe 75% max may be a bit better
September 5, 2009 1:25:00 PM

Onus said:
But that is not correct, unless the load is insignificant. Many 80+ PSUs are actually not very efficient around 10% load. No one is suggesting you overspec your PSU by such an extent that you're operating in that range; we're describing the opposite end of the usage spectrum.
Here's a common example:
A system pulls 250W under load. Its owner used an old 450W Chokemax PSU, which is only 71% efficient at that load (56% of its label). That means it is pulling 250/0.71=352W from the wall.
Someone wises him up and he buys an Antec Earthwatts. He goes overboard and buys the 650W model (when even the 380 would do). His system will only use 250/650=38% of that PSU's capacity, but it will be 84% efficient at that load. It will only draw 250/.84=298W from the wall. Much bigger PSU; 54W less power used, so a lower bill.
Had the original been a Seasonic 250W 80+ PSU, although running at 100% of its rated capacity, it would still be at least 80% efficient, so the difference would not be as great.
The situation you're describing would happen if the person started out with, in this example, an Earthwatts 380 (e.g. 83% efficient) and replaced it with something like a Zilchmaz 750W PSU that is only 79% efficient at that load. The Zilchmax, though bigger, is less efficient so it would draw a few more watts from the wall, and result in a higher bill.


You are assuming here that an 80plus PSU is as efficient at 30% use as it is at 80% use, if that's true than yes, it doesn't matter how big your psu is (except the initial purchase would be more expensive), now if an 80plus PSU is say 80% efficient at 80-100% load, 85% at 50-80% load and 75% at 0-50% load, then size does matter.
a c 125 ) Power supply
September 5, 2009 1:32:40 PM

80+ tests at 20%, 50%, and 100%. It's down in the 10% range where a lot of PSUs suffer. A few sites are now testing for efficiency at that level, including jonnyguru.
September 5, 2009 1:41:18 PM

Onus said:
80+ tests at 20%, 50%, and 100%. It's down in the 10% range where a lot of PSUs suffer. A few sites are now testing for efficiency at that level, including jonnyguru.


With PSU's like that it doesn't matter what you have, it doesn't matter if you use 40-60% of capacity or 60-80%, you'd get the same electricity bill, so just get the cheapest (probably a small one.)
September 5, 2009 1:41:44 PM

I didnt know theyd gotten that solid thruout their range, this is good news indeed.
September 5, 2009 2:05:38 PM

Gulli said:
With PSU's like that it doesn't matter what you have, it doesn't matter if you use 40-60% of capacity or 60-80%, you'd get the same electricity bill, so just get the cheapest (probably a small one.)


Precisely my point. And in my case, already knowing what the build will ever need even when upgraded because I stick with single-card gfx.
Even when doing benching with DICE on CPU + GPU it's not very easy to push over 550W. (On LN2 tho I'd go over) Translate that to most air-cooled overclocking not even 400W is needed so 500-550W 80Plus PSU are more than adequate for single gfx overclocked system.

Again, exactly in line with what Gulli said. Most people just waste $ to buy over-wattage PSU and risk running below 20% load @idle which drops efficiency below 80% for 80Plus certified PSU.
a c 125 ) Power supply
September 5, 2009 2:27:15 PM

One of the really good things about the 80+ testing is that it is done at 100%. Chokemax PSUs that are only good for 60%-70% of their label will blow chunks at that load. Formerly, I think a lot of people bought oversized PSUs because there was no guarantee that a "500W" PSU was actually good for 500W. With an 80+ logo on it, people will realize that it had to be good for its label, so they won't need to do that any more.
I don't see running multiple GPUs either, unless it was for GPGPU stuff (unlikely). I expect my SG-650 to be good for at least five years, hopefully ten.
September 6, 2009 12:29:27 AM

mm k good. Cause just got a 500W OCZ PSU and I love it to death compared to the cheap thing I had before. I also LOVE the modular design... saved on sooooo many cables. And it's 80+ certified so thts reassuring.
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