In the past, a small group of people knew the actual power consumption of computers; while people are mad-buying 500W PSUs just for that Pentium 4 and power-hungry (at that time) Radeon 9800Pro I ran it smoothly on a 300W FSP PSU.
At that time, there were PSU calculators that showed much higher wattage numbers (probably dropped from the sky) and encouraged people to buy oversized PSUs.
Now, the blatant lying in PSU wattage ratings has been exposed, thanks to sites (notably SPCR) that stress-test the PSUs to their rated limit and measure actual system and component power consumption. The truth is clear: a decent PSU of modest ratings can handle lots of medium-to-high-power systems. A system with a mid-end graphics card + cpu never takes more than 200W.
The move was good; previously manufacturors could overrate their products because the user would believe that the product failed due to their systems indeed drawing too much power, now they are forced to produce PSUs that do withstand the torture.
So all is well and cheery? Well not to everyone.
I have seen a few cases of people buying underpowered PSUs and then selling them due to being unable to sustain their rigs. And more cases of people thinking that an underpowered PSU would be able to support a system on forums.
Why the reverse?
When people read somewhere that the system only measure 300W, then they recommend a 300W PSU. Or maybe 400W. But it will never be adequate.
1) The more problematic mistake would be not looking at the seperate power ratings on the different voltage rails, most importantly, +12V. Because bulk of high-power draw comes from here. The +3.3V and +5V, I can never imagine more than 50W coming from it.
So lets take 50W from +3.3V and +5V rails combined. That leaves 250W from the +12V. Now, find me a 300W PSU that has 250W from the +12V rail.
Manufacturors knew they were getting busted on the false ratings, so they needed to have real ratings, yet have a nice big number on the packaging. So they have lots of wattage on the +3.3V and +5V (which, btw, is most usually true), and very little on the +12V. Any failure due to an underpowered PSU will be blamed on the user not reading the fine print carefully.
2) So now we have PSUs that are both solid and have lots of amperage on the +12V, and I have a 400W PSU that has 300W on +12V. So now what's the problem here?
Well, firstly, you would not want to run anything at or near its limits for long, not especially for a product that is known to deteriorate over time. Plus, any sudden huge power draw (like spinning up a harddisk - this eats a lot of power - while gaming) is going to force a shutdown.
Running a PSU near its limit has another side effect - your system is going to have a tornado inside. The fan is going to spin at near its limit, too.
Secondly, though less importantly, is cross-loading. Cross-loading is about having too much load on one rail while not having much on the rest (quite strangely for the name of the term). The result is voltage going out of spec, either more or less. Most PSUs today can handle this quite well in normal circumstances, but not all can do it perfect at the max rating of any rail. I myself have seen a PSU that die and got replaced multiple times due to too little power draw on the +12V.
It may be a small problem, but everything adds up.
My guideline is to have the system draw around half of the PSU's max rating, personally I prefer less but some may like more due to the better efficiency it gives on certain lines of PSU. (However, I have noticed that more and more higher-power PSUs are having their optimal efficiency at loads much less than half of maximum rating, suggesting that they are building a PSU that is optimized to lower power loads, but it is also solid enough to withstand more power. This is a good move for most people with low-power-draw system, but bad for the hardcores and does not reflect good on the product)
Hence, for a system that draws 300W, I'd give it around 600W on +12V, which gives us...
A 700W-800W PSU.
Aren't we back to where we first started off?
Only except this time, we can be darn sure that our PSU is not going to blow up anytime soon.
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