Why Estimate When You Can Measure?

Don't see any moving parts on your gameboy do you? Or your PDA for that matter. If desktop computers were made of APPROPRIATE parts instead of the "my computer has to be faster than yours" parts we'd have silent desktops that run in under 20 Watts of power that cost 150$ and run whatever OS you choose.

This is a companion discussion topic for the original blog entry at: http://www.codinghorror.com/blog/2005/08/why-estimate-when-you-can-measure.html

Just to play Devil’s advocate to my own post-- let’s say I did want to burn 450w under load. How could I do it?

The CPU and video are by far the biggest power consumers in a modern PC. Nothing else even comes close. So let’s start there.

First, I’d definitely need a SLI (two video card) gaming rig with two high end video cards in it (say, 6800 Ultra or 7800gtx). That’s +60w per card (when gaming) for a total of 120w on video.


Second, I want a Pentium 4 “Hot n’ Prescott”. AMD chips use about half the power of a P4. And to make it as bad as possible I want the Pentium 4 dual core chip. For a high end dual core P4, you can easily burn 100w on the processor alone!


That puts us at around 300w for the entire system, with a generous fudge factor for motherboard and other misc. peripherals. Adding dual 10,000RPM raptor hard drives might add another 50w at best, although I’m doubtful it would be that high.

So the best I can do is 350w. Bring your wallet.

Two dual-core chips would definitely be enough to push us over the 450w barrier, but I am not aware of any desktop motherboards or even CPUs that would make this feasible. It’s an entirely different chip family (Opteron, Xeon) and I don’t think those mobos support dual video card SLI, either.

We’re more likely to see quad-core CPUs on the same die before that happens. A quad-core CPU would be hot, but far more efficient than two physical dual-core CPUs.

“…stop estimating and start measuring.”

Or researching (google, this post) :slight_smile:

I had an Athlon XP 2500+, 5 harddrives, two optical drives, and a 300watt power supply.

Windows would randomly blue screen every day.

I put a 350watt power supply in it, and the blue screens stopped.

Why would that have been, I doubt my machine would have been drawing more than 150watt total

I put a 350watt power supply in it, and the blue screens stopped.

This is likely an issue of power supply QUALITY rather than absolute watt rating.

But yes, PSU quality is tremendously important!

You could generalize and say that “generic 500w PSU will probably be higher quality than generic 300w PSU”. But that’s not really getting to the actual problem…

buying a bigger one is the simplest way to ensure that all outputs get sufficient power.

Not necessarily; generic ultra-cheap “500w” power supply may not be any more reliable than a 300w from a quality brand (say, Zalman or Fortron).

what are the screens: kind (LCD/CRT) and size?

It’s 3 Samsung 19" 1280x1024 panels. Nothing that special nowadays.

I tried various powersupplies, I’m not convinced the quality had anyting to do with it

How many did you try? Quality is definitely a big factor. There are a lot of crappy no-name PSUs out there:


“However, not every model delivered the performance that was promised. Some devices also showed voltages falling outside the specified ranges, which can result in a system crash. Usually these crashes cannot be reproduced, and everything seems to run normally again after restart. Only in the rarest of cases is the power supply diagnosed as the source of system problem. In our lab studies, however, we have often had the experience of seeing inexplicable crashing that resolved itself after exchanging the power supply.”

I wonder if the kill-a-watt device properly accounts for reactive loads, like power supplies. For non-reactive loads like incandescent lamps, it’s probably pretty accurate. But reactive loads need more analysis.

I went to the P3 international website, but their user-manual is not online. Their site doesn’t mention the difference between resistive loads and reactive loads.

This is also known as power factor correction.

What does kill-a-watt say about that?

Typical computer power supplies list their capacities in VA (volt-amps), not watts. And because these are reactive loads, I do not think there is a direct conversion from VA to watts due to the phase difference…

Power supplies have multiple outputs with different voltage ratings and different maximum loads. The nominal maximum load is not spread evenly or dynamically across all outputs – each voltage group has its own transformation circuit that determines its output characteristics.

Your system will crash if even a single output comes close enough to its limit to cause fluctuations in the voltage level. Short of having detailed data on a PSU’s individual outputs (and who has that?), buying a bigger one is the simplest way to ensure that all outputs get sufficient power.

GeForce 5200 PCI video (for 3rd display)

Oh man, I’ve barely forgotten that you use Athlon X2’s, now you have to rub my nose with the third display?! ;))) [I have only two 18" LCDs]
OK, to complete the picture (pun intended ;)) tell us - what are the screens: kind (LCD/CRT) and size?

Id like to get one (several) of those meters to see where my power $ going. I had a couple of machines (one win adv serv 2003 and one linux (also hosting vmware with win adv server 2003)) which eventually evolved into hyper threading P4’s. Underload (i.e. either of them doing ‘heavy’ sql processing) or rendering 3d they’ both lockup, shutdown or reset as they saw fit. I had to upgrade the powersupplies in both to over 400 Watts to solve respective problems. I tried various powersupplies, I’m not convinced the quality had anyting to do with it. Perhaps there are peaks/spikes of power required that the meters sample rate is to slow to ‘see’?

Power supplies are rated in exaggerated terms (like cheap stereos). The wattage is usually advertised as peak, not continuous.

Interestingly, 500W Peak to peak = 357W RMS (true V*A power - divide by square root of 2 or about 1.41).
So get a no-name advertised as 500W may well be the same as a 350W quality branded PSU. lol

You wouldn’t happen to live in the midwest area and want to let me bum that Kill-A-Watt for a day or two, would you?

You can find out where I live easily enough by visiting the “About Me” link on the main page.

I bought a killAWatt shortly after you mentioned it, and I’ve seen it up to 356watts while running BattleField2 in network mode.

Here’re the specs of the machine:
p4 3.4g
gForce 7800gtx
2 gigs mem
raptop 10k rpm hdd, caviar 400gig, misc 100 gig
20" dell lcd
pioneer elite receiver- for sound
2 cube speakers from Cambridge

I’m suspecting that the graphics is adding a good amount of the load. With CPU maxed without the 3d graphics, it’ll normally be around 320. With the CPU at idle and the LCD on, it’s normally around 260-280 watts.

LCD off, largely idle, it’s 220 or so.

Everything off, it’s about 4-6 watts.

graphics is adding a good amount of the load

The CPU, particularly an Intel CPU, will always be the biggest single power draw in your system.

The graphics card is a close second though. And in extreme cases (low end, recent generation CPU combined with very latest video card) you could have the video card pull more power than the CPU – at least while gaming.

I like the idea of getting as little watts in a power supply as you need. If you want the best idea of what you’ll need, with the most precise guestimates as to how much power your system will use, I don’t think you can do much better than here:
a href="http://www.extreme.outervision.com/index.jsp"http://www.extreme.outervision.com/index.jsp/a

Every component, broken down according to watts used, and it will even let you know how much more you are using at various degrees of overclocking and at peak cpu/graphics usuage.

Save yourself the 30 dollars and try the extreme PSU calculator instead. (not affilated with this non-commercial site in any way, I just have found it very useful in putting together higher end systems without the unneeded PS megawattage)

The calculator is OK, but it’s telling me my recommended PSU wattage is 248w for the system described in the post (see top of this page). That’s way, WAY over the actual maximum measured consumption of 133w with two instances of Prime95 running.

The calculator is fine for estimating if you bear in mind it’s giving you about 1.5 times the size of the PSU you really need. Thirty bucks is worth it to actually be able to MEASURE what is happening-- instead of passively taking someone else’s word for it.

There’s a fun thread here at SilentPCReview documenting the maximum amount of “stuff” people have run in a PC using a standard 300 watt power supply. You’d be surprised:

How much will a 300w power supply run?

I have a question. Would my pc (according to the calculator needs a 525 watt psu) use more power with a powersupply closer to what it uses with a small headroom, or would a really big powersupply use more. You have to take in to account that ex. a 200 watt psu running at 150 watts would make more heat than a 400 watt psu running at 150 watts. you would loose power there. My situation is that i am rigging up multiple(6) pc’s and i need to use the minimal ammount of power because i am hooking them through 2 power cables. so i need to use the least ammount of power. note these pc’s range from a 486, p1-4 and my dual xeonl. I am planning to get the 200 watts for all but the xeon which i will get a larger one to take the overclocking.

You have to take in to account that ex. a 200 watt psu running at 150 watts would make more heat than a 400 watt psu running at 150 watts. you would loose power there

What you’re asking about is efficiency. Most power supplies are less efficient at LOWER loads, so you would want a PSU that runs closer to its actual limit.

I highly recommend reading through the PSU reviews at http://www.silentpcreview.com ; they determine the efficiency graph for each PSU reviewed.