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Will a XXX watt power supply run this?

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larva

Inactive Moderator
Joined
Jul 12, 2002
I hear these type of questions asked over and over. I myself answer many of them. Since the answer is always the same, let me makes some comments that are vital to understanding power supply selection. I will attempt to provide a guide which might help you answer your own questions.

Knowing a PC power supply is 300W or 400W means absolutely nothing. The are no standards the manufacturers have to meet to call their supply a given wattage. The nominal wattage rating is just that, a rating. It is not a measurement of output. Just like audio amplifiers, it is a "maximum output", not the actual output. I'm sure you've seen some of those $25 dollar amplifiers on Ebay that claim to produce 800 watts. In reality they probably make 12.5 watts when actual output is measured by any reasonable standard. In the same manner less reputable power supply manufacturers exaggerate their wattage rating in an attempt to make their product look like it is more powerful than it really is. I have a hair dryer that produces 1200W... 1200W of heat. It is about as useful for powering a PC as a 550W supply that costs 35 dollars.

Once we understand that we have to look deeper into our prospective supply to know anything at all about it, there is a more meaningful spec. That is the 3.3+5V output which is on the side of any quality power supply. While not completely free of exaggeration this number usually has a much stronger correlation to actual output than the nominal total maximum output. Let's look at 300W supplies as an example. I have seen 300W supplies with 3.3+5V ratings as low as 125W. This is way too low for a quality supply. A 300W supply should have at least 175W of 3.3+5V output before it is of use for high performance PC's of modern porportions. Let's examine the 3.3+5V output of some popular choices-Antec, FSP/Sparkle (Fortron), and Enermax.

Until recently I was using an Antec Smartpower 300W supply, the PP-303X. This unit served me well in the P3 days, but was overwhelmed by the power consumption of my current P4 rig. It has a 3.3+5V output of 160W. These supplies have recently been replaced in Antec's linup by the Solution series, and the new Antec SL300s supply is rated at a 3.3+5V output of 220W. These are both 300W supplies, but one might run your proposed rig and the other probably wouldn't. In the past I have not been able to recommend 300W Antecs because of their low output and difficulty powering modern loads. The new version however looks to be a safe bet, and combined with a reasonable cost makes it far easier to recommend.

The FSP/Sparkle 300W supply has a 3.3+5V output of 200W. This is good, and in fact this supply will power all but the most powerful and expansive of PC's. Its actual total output has been measured at 390W, far in excess of its rating. FSP/Sparkle supplies are rated very conservatively, and as such compete with supplies from other makers that have larger numbers pasted on the side. For example the FSP/Sparkle 350W units have the same 220W 3.3+5V output as the Enermax 431W, and when tested produce more total output (454W).

The Enermax power supplies have a somewhat undeserved reputation. While not bad quality, they are lightweights when the output is compared to the best Antec and Fortron produce. The 300W Enermax has a 3.3+5V output of only 170W, and as such is only suitable for the lightest of loads. Even the 350W Enermax has only 185W of 3.3+ 5V output, making it far less capable than the SL350 Antec or FSP/Sparkle 350W supplies. While better than no-name or el-cheapo supplies, these units are outperformed by FSP/Sparkle's 10 times out of 10, and for less money. I see many more user reports of trouble with Enermax units than Antec or Sparkle, and as such can't recommend you spend the money that could have bought one of these on an Enermax.

Let me list some supplies of proven quality that are suitable for your consideration:

1) Antec Solution Series
2) Antec True Power series
3) Enlight
4) Fortron Source Power (FSP)/Sparkle (SPI)
5) Herolchi (HEC)
6) PC Power and Cooling
7) TTGI
8) Vantec

Once we deviate from quality supplies like these, the ratings are basically useless. Just like that 25 dollar 800W audio amplifier, the ratings are just not truthful. Considering how affordable the Antec Solution series and the FSP/Sparkle supplies are, there is no reason to use questionable supplies or ones known to be junk like Deer/Allied/LC power units. You can't save but a few dollars by using what comes to hand easiest or cheapest, but you can compromise the stability of your machine and/or damage the expensive components you hook to the supply.

As long as the supply in question is one of the types listed above, and has adequate 3.3+5V output, it will likely do fine for your rig. By adequate I mean 200W for P4 systems and 220W for AMD rigs. Increase these recommendations to 220W for P4's and 230W for AMD's if hard core overclocking is the goal. Bear in mind that nearly everyone here has multiple optical and hard drives, this does not make your machine a special case. If you wish to use a larger supply feel free, but notice the 3.3+5V output doesn't increase much beyond the 220-230W of a good quality 350W model, regardless of the total wattage. And feel free to use the search function on this fine forum, as the actual experiences of users like yourself is the most reliable indicator of power supply quality and suitablility for your application.

Dealing with a vendor that takes value seriously is all that is required along with the information presented here to get a great supply at a reasonable price. Always check Newegg and Directron first, as these are reputable dealers that will sell you quality products at a reasonable price. I find the 350 and 400W Fortron units sold by Directron to be the best value available, followed closely by the Solution series Antecs as priced by Newegg.

www.newegg.com
www.directron.com

In closing let me say I have personally constructed in excess of 7000 PC's, and have never built one that the 350W FSP/Sparkle would not power. On the other hand I wouldn't even try to run a string of Christmas tree lights off a 350W Deer/Allied unit. Knowing just the wattage class simply doesn't tell you anything at all. Quality is vital where power supplies are concerned, and you would be shocked (no pun intended :)) just how much you can run off even a modestly rated supply from one of the premier makers.

*edit*

One year later, some things have changed. Nowdays P4's draw enough 12V current to overwhelm the 350 Fortron. The 530 Fortron or the Antec True Power 430 should be seen as practical minimums for P4's running in excess of 3GHz. I have seen little evidence that a Fortron 350 is not still adequate for AthlonXPs though. But the time is coming where they too will exceed the 12V output of this supply, as they have moved on the powering the CPU off the 12V line as P4s have always done.

Ratings are still a sticky matter. It's still hard to put much stock in single line ratings, and this is most inconvenient as knowing the 12V output is now the critical parameter. But quite simply, ratings vary from manufacturer to manufacturer (and some simply lie), so it's had to put complete faith in the 12V line's rating in most cases.

We would like to have 18-20A on the 12V line as a practical minumum for P4s. AthlonXPs are still well served by 15A. But if we just blindly accept the 12V rating for fact, we come to the (incorrect) conclusion that Enermax supplies are the most powerful. It is still a good idea to look at the 3.3+5V rating as an indication of general output and quality, but we must recogonize that the 12V output will be the functional limit in the vast majority of cases.

Power supply selection really isn't that hard these days though. If you are strapped for cash, the Fortron FSP350-60PN with the 120mm fan is the best value, but will prove limiting for ambitious P4s. As we get more aggressive with the OC, smart choices are the 530 Fortron or one of the Antec True Power units of 430W or above. If you want to get just as good a supply for 2-3 times the money the PC Power Cooling units are certainly good, but offer essentially no functional improvement over the best that Fortron and Antec have to offer. I'm sure the owners of PC Power and Cooling units would choose to differ on this point, but we are all entitled to an opinion, and that is mine.
 
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mbentley

Gloriously Lead, Overclockix Chief Architect
Joined
Sep 26, 2002
Location
Indianapolis, IN
inspectorhammer said:
cool good info. Dude make this one a sticky!

*cough* sticky *cough*

:D

great info. i am sure that'll answer a lot of questions...

the real way to save bandwidth is to make this a sticky :D
 

ArBiTaL 24

There is no spoon
Joined
Aug 17, 2002
mbentley said:


*cough* sticky *cough*

:D

great info. i am sure that'll answer a lot of questions...

the real way to save bandwidth is to make this a sticky :D

HINT- HINT :p

BTW, Antec 480W Power Supply TRUE480 or SPARKLE 400 WATTS ATX which do you reckon is best? (these are good make AND good power)
 

bwass24

New Member
Joined
Nov 5, 2002
Location
New York
While I agree with a lot of what you said here, my experience measuring the actual current loads of systems under stress indicates some things that might clear up even more misunderstanding.

First, I've never been able to draw more than ~110 watts TOTAL out of a power supply in a loaded P4 system under test. Even the crappiest power supply can deliver this amount of power. A system simply doesn't consume the amount of power that people are speculating it does under real-world use.

Second, the actual measured current draw suggests that the 12 volt rail is the most important and the one delivering the greatest amount of power. Next is the 3.3volt rail, and then the 5 volt rail. If you have more than one USB device then the 5vSB line is also very important.

IMO, the most important characteristic of a good power supply is it's ability to deliver current at the slew rate that the system needs it. Since my measurements indicate that we are really consuming relatively little power compared to a power supply's (overstated or actual) ratings, the reason a cheapy power supply doesn't work well in a powerful system has much less to do with maximum power ratings than the TIMING of current delivery. If the power supply can deliver the needed power, but it can't do it in time for the components, it's gonna make the system really unstable.

Slew rate testing is really hard and requires very expensive and complicated programmable electronic loads and current sensing probes. Most PSU manufacturers never even test this very important spec.
 

james.miller

Member
Joined
Jun 8, 2002
Location
Dunstable, uk
well done larva. Its about time these answers were all written down in one post. And, you were just the man to do it.

There no way on earth this wont be a sticky. Good job!
 

ArBiTaL 24

There is no spoon
Joined
Aug 17, 2002
ArBiTaL 24 said:


BTW, Antec 480W Power Supply TRUE480 or SPARKLE 400 WATTS ATX which do you reckon is best? (these are good make AND good power)

BTW, has anyone got a reply for this Q? saves making a thread JUST for this one Q ;)
 

mbentley

Gloriously Lead, Overclockix Chief Architect
Joined
Sep 26, 2002
Location
Indianapolis, IN
ArBiTaL 24 said:


BTW, has anyone got a reply for this Q? saves making a thread JUST for this one Q ;)

Originally posted by Hoot


How good is your particular PSU?
I see a lot of posts from people wanting to know is their particular make and model of PSU is any good. I have a simple "Gold Standard" for assessing a PSU. It involves looking at the specification sheet that came with it, is printed on the side, or is at the manufacturers Web Site. The specification that tells the most about your PSU is the Combined +3.3V & +5V maximum output power. First off, if your unit does not have that specification, it is instantly suspect. This is the equivalent to buying a car that does not have a specification of the Brake Horsepower for the engine.

So here's the magic numbers to compare to.

If your unit claims to be:

300W, the Combined 3.3 & 5V output should be 150-160W
350W, the Combined 3.3 & 5V output should be 180-185W
400W, the Combined 3.3 & 5V output should be 220-245W
500W, the Combined 3.3 & 5V output should be 270W

If your PSU falls short on this specification, you need to question what other specifications does it falls short on.

73, Hoot

check the 1st sticky ;)
 

ArBiTaL 24

There is no spoon
Joined
Aug 17, 2002
mbentley said:

check the 1st sticky ;)

THX:), but that all seems a little complex. Anyway, i was just asking people their opinions between 2 specific PSU's.
don't worry, i'll make a new theread.
 
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OP
larva

larva

Inactive Moderator
Joined
Jul 12, 2002
bwass24 said:

First, I've never been able to draw more than ~110 watts TOTAL out of a power supply in a loaded P4 system under test. Even the crappiest power supply can deliver this amount of power. A system simply doesn't consume the amount of power that people are speculating it does under real-world use.

I never stated how much power a PC actually draws, P4 or otherwise. But since you bring up the P4 as an example, lets look at the ramifications of the electrical quidelines Intel publishes for the Northwood CPU, specifically the 3.06GHz version. The maximum current it can draw is rated at 70A. Since this is 70A at 1.525V, this equates to 106.75W. And of course power consumption can readily exceed this level as we increase core voltage and overclock the processor. While it is true that power consumption lies at a lower level than people generally conceive, It is ramping up rapidly with clock rate. The 3.06 pushed power consumption to a level that requires re-design of the motherboard's power supply, which is why the older 845 and 850 series motherboards cannot claim support for the 3.06.

I mentioned the fact that none of the PC's I've built required any more power than the 350W Sparkle produces. This was intended to demonstrate that power supply size beyond a certain level serves more to satisfy some people's desire for big numbers rather than meet any particular power demand. If you read the responses I post when people ask about power supply sizes for their system, I continually state that the 350W Sparkle is more than enough for their application, and is my first choice and recommendation at all times. I do point out that a 400W Sparkle is not much more if they simply feel they have to have a giant supply in spite of my assurance that the 350W model is sufficient.

In my closing paragraph I point out that quality is most important. I continued to point out that you can run a LOT more equipment of an even modestly sized power supply from one of the premier makers. I agree with what you are saying, and this fact is reflected in the advice above.

bwass24 said:

Second, the actual measured current draw suggests that the 12 volt rail is the most important and the one delivering the greatest amount of power. Next is the 3.3volt rail, and then the 5 volt rail. If you have more than one USB device then the 5vSB line is also very important.

No doubt about that one for P4 systems. Since P4's have gone to running the CPU's power supply off the new 12V auxiliary connector, the 12V consumption is fierce. I believe AMD's still power the CPU from the 5V feed though, changing this characteristic markedly. I'm not saying that the 12V output is unimportant, merely that the 3.3+5V spec is the most telling indicator of the actual brawn of the supply in question. If the 3.3+5V output is below the levels recommended above, you tend to run into problems regardless of the 12V situation. Again these numbers are simply ratings, not indicators of actual power consumption.

bwass24 said:

IMO, the most important characteristic of a good power supply is it's ability to deliver current at the slew rate that the system needs it. Since my measurements indicate that we are really consuming relatively little power compared to a power supply's (overstated or actual) ratings, the reason a cheapy power supply doesn't work well in a powerful system has much less to do with maximum power ratings than the TIMING of current delivery. If the power supply can deliver the needed power, but it can't do it in time for the components, it's gonna make the system really unstable.

Undoubtably the slew rate is extremely important. Having worked in the audio amplifier industry I am well aware of the impact and importance of adequate slew rate. There are two ways to insure adequate slew rate though, buy a quality power supply and make sure it is sufficiently large. This was rather the point of my post. It was intended to give real world guidance for power supply selection, not ovewhelm the users with technical aspects of the load and the supply. The chief shortcoming with poor quality supplies is that their actual output capability falls short of the demands modern systems place on them. Slew rate is greatly degraded in the case of overload. And I agree whole heartedly that system stability evaporates under these conditions.

bwass24 said:

Slew rate testing is really hard and requires very expensive and complicated programmable electronic loads and current sensing probes. Most PSU manufacturers never even test this very important spec. .

Exactly. This is why I didn't include a slew rate dissertation in my post. People have no way to ascertain what the slew rate is. As I mentioned above good slew behavior results when you buy an adequalty sized supply from a quality manufacturer.
 
OP
larva

larva

Inactive Moderator
Joined
Jul 12, 2002
Heh, patience, patience. I don't think there is a 24 hour a day sticky squad yet :)

I appreciate all your kind comments, they mean a great deal to me. I hope this thread does serve to help all of us further our knowledge of the subject. I feel helping people is its own reward, but it is always nice to know the material has value to the audience it was created for. Thanks again :D
 

bwass24

New Member
Joined
Nov 5, 2002
Location
New York
Hey Iarva...

I think that we agree about the size of PSU's but we are drawing the same conclusion from different points of view. Either way, we're saying much the same thing. "Buying a quality PSU is much more important than buying total power rating."
 
OP
larva

larva

Inactive Moderator
Joined
Jul 12, 2002
bwass,

I agree totally. Quality is priority one. I appreciate your technical input and don't disagree with what you are saying a bit. I was trying to keep the content light but at the same time give a useful foundation for anyone's decision making. Hopefully the balance chosen was the correct one.

And I hear you on the output issue. Unless you have a large rack of SCSI drives and other server-grade features you don't need a 500W supply. The best 300W models are fine for the vast majority of users, the best 350W models cope with any demand they are likely to see, and good 400W models could run your PC and the one next to it :)
 

Dangerous Dave

Registered
Joined
Dec 6, 2002
Larva...

You are not only sexie but smart two. If you have bilt over 7000 masheens, you are defnatly smart...

I took the stance when I built my newest machine that I wanted a top quality power supply. Think of all of the elements that make up a computer. Clean ample power is a must. I may go as far as running my computer via DC with AC as a backup. Sort of like an UPS in reverse. I'm in the middle of a cross country move now so I don't have time to adequately research it all but I am considering this as a clean power source. Maybe my thinking is off though I'm not sure. My power supply is supposed to have some sort of line conditioning built in as most supplies probably do but to what extent I am not sure. Sorry to get off topic there.

If my system had a 300 watt requirement, I would not buy a 300 watt supply. A decent supply will exceed it's rating but why push your supply. All things being equal, an oversized supply unit should run cooler and last longer. Maybe your car will do 120mph but how long will it last at that speed? The same analogy somewhat applies to the power supply. Bigger is better but quality is important also. Like my dream girl Larva said about the amplifiers. Ratings are important but some of them are not worth the paper they are written on. Don't risk your expensive components with a cheap $25 dollar, no name supply. Remember, you get what you pay for.

Hugs and kisses to you Larva... XOXOXOXO