http://www.silentpcreview.com/article28-page2.html
POWER SHMOWER
or How PSU Power Ratings Mean Almost Nothing
A frustrating fact about PSUs is that there does not appear to be a stringent or regulated standard for reporting, advertising and labeling rated power. This is despite the existence of standards like ATX2.03 or Intel ATX12V.
There are well-established standards for measuring and rating HDD capacity, an engine's horsepower, or the heat generated by a furnace... but not one for how much power a PSU can deliver. There are so many cases of people with "450W" PSUs having power stability issues running a system that can't possoibly draw more than 150W. And "300W" units that keep running where the "450W" units are faltering.
It's not just about bad PSUs vs better ones. It's a dumb situation caused by uncontrolled marketing competition. Real regulation would bring PSUs out of snake oil territory and into a more sensible consumer-friendly terrain.
There are many ways PSU makers fudge to make their units seem more powerful.
1) Out and out lying. You add up the power on all the lines in many PSUs and they fall short of the rated power by 10, 20 30W or even more.
There are more sophisticated ways:
2) Limit the AC input voltage to a very narrow tolerance. The best PSUs are able to deliver their rated power given a decent range of AC input power, say 90~130V for a 120V unit. It's much more demanding to produce 300W w/90VAC input than with 120VAC, so what some PSU makers will detail in their tech specs (usually not in their consumer brochures) is to specify 115-120VAC for input power. A PSU specified this way will not deliver full power if the AC voltage sags, if there is a brown-out. Surely it causes instability more often than a PSU rated to deliver full power with 90-130VAC.
3) Specify a low operating temperature for rated output. This is quite common, but again not often seen in consumer brochures, but rather tech spec sheets provided usually only on demand by engineers or corp buyers. A typical PSU operating temp statement is somthing like this:
0ºC ~25ºC for full rating of load, decrease to zero Watts O/P at 70ºC
Examine what that says. Full power (let's say 400W) is available when the unit is at 0ºC ~25ºC. Hmmm. Think about this.
Have you ever felt air blown out of a PSU in a PC running absolutely full tilt (which it would have to do to get anywhere near 400W output) that felt cool to the fingers? 25ºC airflow would feel exactly that: Cool, given that normal body temperature is 37 °C.
So this PSU cannot deliver full rated power when its temperature goes over 25ºC. OK, what happens to the max power output capacity above that temp? It decreases gradually so that by the time the PSU temp reaches 70ºC, the PSU cannot deliver any power at all. So if you assume that this power drop as temp rises is linear, then max power capacity will drop by ~9W for every degree over 25ºC.
Now having examined as many PSUs as I have over the last 2~3 years, I have to say there's not a single PSU in ANY PC I have ever used or examined that would not measure at least 30~35ºC almost anywhere inside the PSU under almost any kind of load. And if/when it is pushed, 45ºC is nothing at all, especially for or near hot running components like voltage regulators.
So let's say 40ºC is a fairly typical temp inside a PSU. This 400W rated unit would actually be able to deliver a max of just 220W at that temp. Hmmm. Interesting, isn't it? At 50ºC, the available power would drop to just 130W. No wonder some PSUs have 3 fans each capable of 50 cfm!!
Here's a simple fact: Really high quality PSUs are actually rated for full power output at as high as 40ºC. The trick is get a hold of the spec sheets that tell such information so you can compare apples to apples. Or ask.
How Much Power is Enough?
Typical 300W models have replaced 230W and 250W models as baseline units since the introduction of the AMD Athlon. They feature a fan (or two) rated for 35~40 cubic feet per minute (CFM) airflow. Presumably, this level of airflow is required for adequate cooling at full power output to pass safety approvals under UL, CSA, CE and other regulations.
Our own experience shows idle AC power consumption of 60~90W, and about 110~130W at 100% CPU utilization in several fairly representative mid-line test systems. Given the typical 65~70% effiiciency of a PSU, the DC power delivered is 40~60W at idle and 80~90W maximum. In other words, the amount of power actually required by typical desktop systems is about 30% of the capacity of the typical 300W power supply. The highest DC power draw we have seen from any desktop PC is ~180W (System details: P4-3.2, 512MB RAM, dual SATA drives, ATI9800XT, Zalman 400 PSU). Although some headroom is always good to have, most of us are paying for power capacity that is never used. One of the nasty side effects is the fan noise of high airflow required to keep the PSU adequately cooled when delivering maximum power.
Why this state of affairs exists is a matter of marketing and technical obfuscation, probably more by accident than any massive conspiracy. With relatively low current requirements prior to the AMD Athlon processor, the aforementioned 230W and 250W were perfectly adequate for PC systems, even if the power supplies didn't deliver full rated performance. That changed with the Athlon and then the P4. PSU makers were quick to introduce higher rated models said to be required for the new power hungry processors. It was a good marketing opportunity. Rather than "Our 250W PSU is better than theirs," it is easier to sell the message "Our 300W PSU is better than their 250W PSU." Bigger is always better, isn't it? It also allowed higher prices to be charged.
A counterpoint is AMD's system builder's guide, which suggests higher numbers: up to ~180W DC for a typical system and ~250W DC for a high performance system, but these numbers are obtained by adding the maximum power rating for each component, then taking 20% off to account for real-world conditions. It is almost impossible for any application to demand 80% of maximum power draw from each component simultaneously. Intel's PSU recommendations are similar.
Suffice it to say that as manufacturers, both AMD and Intel are looking at worst-case secenarios. As custom builders, enthusiasts and system integrators can make choices based on real needs and applications.
Even so, Is Higher Power Better?
Without getting into technical details, the nature of a switching power supply is that it delivers as much power as is demanded by the components. This means that when installed in a PC whose components require 200W, a 400W PSU and a 250W PSU will each deliver 200W. Does this mean the 400W is coasting while the 250W is struggling? Not if they are both rated honestly and if they have the same efficiency. If one has lower efficiency than the other, then it will consume more AC to deliver the same power to the components, and in the process, generate more heat within itself. As long as there is adequate power, higher efficiency is the key to cooler, quieter PSU operation.
The main benefit of higher power PSUs is when the airflow in the PSU is deliberately set very low in order to minimize noise. This usually means the PSU components will run hotter. All other things being equal, a higher rated PSU will be a better choice in such an application because its parts are generally rated for higher current and heat than a lower rated model.
What are the Key Aspects to Good PSU Performance?
There is a great deal of fuzzy and unclear thinking about what constitutes a good power supply. The obsfucation caused by competitive marketing is certainly one cause of this confusion. Another is the proliferation of computer hardware web sites that publish so-called "reviews" of PSUs without the faintest notion of what should be examined or how.
1. Stable power delivery under load
2. High efficiency
3. Good cooling
4. Low noise operation
5. Long term reliability
The truth is that a computer power supply is a complex electronic device with a complex role that is little appreciated by most hardware reviewers. Most system integrators don't really appreciate it either, either. This is due partly to the assemble-and-sell nature of the PC industry, where manufacturers build components in accordance to an accepted standard specification for "universal" compatibility with other components. Such piecemeal component manufacturing does not nurture or reward system thinking, which has been much more the norm for Apple.