What Patriot said.
Plus, faster components have no direct correlation to the amount of power needed. Just because a CPU has a higher clock rate, for example, doesn't mean it needs more power. Die size is one determining factor and that's why CPU manufacturers work to shrink the process...to get more processing power without increased energy requirements (and heat). Look at NVidia and AMD video cards as another example. Similar performing cards from each company will have different power requirements, so another factor besides die size is the chip architecture.
The biggest factor affecting PSU longevity has more to do with the design and quality of the PSU components than anything else. Also, the capacity of the power supply will determine it as well. Running a PSU that just barely meets requirements won't last as long as one with some overhead. Think of it like having a smaller engine in your car...the engine can certainly move the car at the speed limit, but if it has to run hard and at higher RPM constantly it won't' last as long as one with more power that just cruises easily. With that said, you don't need a 1200 watt PSU to run a 400 watt system. A 600-650 watt would do just as well and still have plenty of overhead for some future expansion. Extreme amounts of overhead doesn't necessarily mean increased longevity. One of the hardest environments for a PSU is when you first power up the system. The voltage in-rush through the components produces the most stress and is typically when a PSU will "pop".
I have lost a computer to a failed PSU only once in all the years of building and using PC's. When the PSU failed it took out the motherboard, HDD, RAM, CPU and video card. I assume the failure was some sort of short in the PSU that zapped everything with a massive voltage and current spike. On the other hand, I've had a couple cheaper PSU's die without affecting anything else. It depends on which component(s) fail and how.
Another factor with the cheapo PSU's besides cheap components and poor design is they have a tendency to way over rate the output. Technically, they aren't outright lying about it, but the methods they use to derive that output don't necessarily correlate to the way the end user needs it. You can see this less than honest type of rating frequently when looking at equipment or devices that use electric motors. Look at the HP rating of say, a shop vac for example. You'll see ridiculous claims of 5 or 6HP on some models. The reality is, those little 120v motors are nowhere near 5 or 6 HP output. We can show this through the basic math: Watts = Amps x Volts. HP=Watts x 0.00134, so in order to output 6 HP on 120v house current, that thing would have to be pulling almost 37.5 amps! That exceeds most standard 20 amp house circuits buy a factor of almost 2 and would require some pretty hefty gauge wiring to carry that kind of amperage. So, how can they say that motor is 6 HP? Simple, they cheat. When you stall an electric motor the amperage draw ramps up significantly, so these manufacturers will stall test the little motor and measure the amperage draw in that state and claim the HP rating based on the amperage it is trying to draw while stalled. The lie is in the fact that in a stalled state, there is no HP since HP is a calculated measure of the ability to do work. In other words, there has to be motion because without motion there is no work. Cheap PSU's have a rating that is based on tests that measure a momentary surge of output that doesn't simply fry the PSU. It has nothing to do with it's actual, sustained output capability, which is the measure an end user needs to see.
You can also get a rough idea if the power supply actually outputs its rating by its physical weight. Certainly not an exact measurement, but the higher the true output is, the heavier the PSU will be due to the size and weight of the components (primarily transformers) if it is truly built to output that power. If you compare a cheap 500w PSU to a good quality one, you will absolutely notice the good one is a fair bit heavier.
One other area you will see these various, less than honest ratings is with audio equipment, especially amplifiers. Many of the crappy, low-end junk manufacturers make all sorts of wild claims about wattage output that is less than true. Unfortunately, what you see there is the manufacturers playing on ignorance in 2 ways. Many people assume more amplifier wattage equals louder sound and they use the less than honest methods of measuring the output. There's a reason good quality high output amplifiers are expensive, large and heavy.
Edited by Ram4x4, 04 March 2016 - 09:21 PM.