Originally posted by: Bobthelost
Using his you get: 20.9A (ish and including the 10% comfort blanket and the highest P4 draw he lists).
Real world test you get: 16.9A (continuous draw, not peak)
http://www.silentpcreview.com/article265-page4.html
Your numbers appear very padded for continuous draw as that system draws around 17A from the 12V rail.
Yes. I did base my numbers off of peak numbers, but I'd rather be OVER then under.
Like I said, his numbers are broken down a lot more and there's nothing wrong with that. Like differentiating a Venice from a Clawhammer... that's cool. But if I did that, I'd be in here every weekend updating the numbers and have an Inbox full of people saying things like, "Well I have an Pentium 4 this and that and that's not on your list, so what number should I use?" So I used the highest number possible so I wouldn't have to constatly update the post and so nodbody could blame me if someone runs out and buys an underpowered power supply.
Using your example: I don't think 25A is unreasonable. It's not like I'm telling someone with a 3.8GHz P4 to run out and buy a 600W with 34A on the 12V rail. But I would definitely feel better suggesting a power supply with 25A on the 12V rail (which you can get in some 400W power supplies) then one that has only 18A on the 12V rail (something that you can find in a 480W power supply!)
So regardless of who's math you use, the message is still very clear: WATTS DON'T MEAN SQUAT!!!
Originally posted by: Bobthelost
Since most good PSUs are rated for continuous load and have higher peak tolerances continuos load is a better value to look at imo.
That's a loaded statement in itself that needs to be addressed on it's own. EVEN IF it's a "good" power supply that's rated at continuous load, at what temp? What's the de-rating curve?
And if we take a look at the user's build; we don't know the ambient temps or what other cooling apparatuses are in place elsewhere in the chassis.
So the actual capabiity of the power supply may not be label spec in every person's situation. I'm not willing to give MOST power supplies that much credit.
Again, judge on the high side and you won't end up screwing yourself over in the long run.
Here's a prime example.....
I know a system builder that was using a particular case because it came with a QUALITY power supply. They were "modest" builds, so if you used a "conservative" calculation, you'd find he only needed a PSU with 18A on the 12V rail. A decent 300W can do that for you. The cases came with 350W PSU's with 10A/15A (22A combined) on the 12V rails. The PSU was cooled with a pair of 80's, front and back. The rear fan was thermostatically controlled.
Because the builder only used a front 80MM in the case and nothing in the rear, the PC would heat up and lock. The air coming out of the PSU was ice cold. It wasn't helping with the chassis cooling. So he switched it out with a PSU with a 120MM that had 14A/14A (24A combined) on the 12V rails. The PSU didn't lock up because the CPU or GPU were overheating, but now the air coming out of the PSU was very hot. He essentially dropped that PSU's "continuous wattage" capability by 50W because it was being used as the system's exhaust fan.
Yes, the ultimate solution is to just put a 120MM fan in the back, but that costs money, time and adds noise. Small things to me, but everyone's different.
Speaking of rear fans....
Now let's talk about "quiet" PSU's that have very low CFM 120's... Different person, similar problem. They started with a particular 120MM fan PSU, and they also had a 120MM fan in the back of the case. The PC would lock up under heavy loads. CPU and GPU temps were good, but the PSU was VERY hot to the touch. Apparently, the PSU was getting so hot that it simply could not continue to put out the power demanded of it.
On a whim, he put a piece of tissue paper on the back of the PSU and it seemed like the PSU was sucking in outside air! Assuming the fan was installed incorrectly at the factory, he RMA'd the PSU. The second one did the same thing!!!!
He pulled the PSU out of the case and bench tested it. Low and behold, on the bench the tissue was sucked in at the fan and blown out of the back grill the way it's supposed to. A different PSU, but still a quiet model, was purchased and installed and it had THE SAME PROBLEM! The air wasn't moving from the inside out!
It turns out that the rear fan of the chassis was causing a vacuum. A venturi effect, if you will. The fan was moving so much air that it was sucking air away from the PSU. The solution was to add a fan to the front so the chassis would have positive pressure, but of course if you do that; you add noise! Can't win? You can always go with water cooling.
Bottom line, you can not take into account every possible combination of components in every possible environment. It's impossible unless you work with each issue on a one on one basis. If you want to help the masses, you use generalizations. That simple.