Why is there such a difference in price between various Power Supply Units (PSU) which purport to do the same thing?
The secret is known as "cost down".
What happens is that a design for an absolutely perfect PSU is created, then there is a round of eliminating components to save cost. If that works then another round of elimination is started and tested until there is a design which no longer works - one then goes back a step to the absolute bare minimum which did work.
So that's it? Not by a long shot. Now that there is a bare minimum of components which will work the next phase of "cost down" is put into effect. In this round the high quality components (such as capacitors) are replaced by lower quality ones. This is then tested and if it still works then those lower quality components will be replaced by even worse quality components. This continues until the PSU once again fails and then the process is taken one step back to the design which did still work.
So that's it then rock bottom has been reached?
Not quite; now that the design has been minimised and the components have been "cost downed" the focus is on the safety of the device itself. Do we really need all of them? Of course not, so various safety components are eliminated or replaced by cheaper alternatives until they can technically say that they are in compliance with safety standards without actually lying outright.
How can you tell a PSU from a potential IED (Improvised Explosive Device)? Simply by picking it up. If you pick up a PSU and it feels like all it needs is to be sealed and have some helium added to it to make it fly away then you are dealing with an "AL-Q Taliban Special" (AQTS). Another indicator would be the presence of a switch on the back of the PSU around the power switch where you can choose between 110 volts and 230 volts. If one looks at the label on the PSU and it has an entry for "-5V" on it then it is an IED candidate. If you look at the Amps with regard to the various voltages and the "+3.3V" and "+5V" are higher than the "+12V" then again you are dealing with an AQTS.
Last but not least there is of course the price. If a deal looks too good to be true then in the case of a PSU it most certainly is.
What are the typical "features" of an "Al-Q Taliban Special" IED? One of the main "features" is that this type of device will generally use your hardware as a fuse to defend itself against detonating. It is only logical (well in the minds of the designers) that the PSU has to be defended against damage by eliminating the cause of the threat, namely the motherboard and/or some other offending parts of your hardware.
If you are lucky then all that will happen is that the AQTS will just not turn on. Not so lucky if if it makes large "bang" noise and/or issues magic smoke. Unlucky if the "bang" noise and/or magic smoke results in the PSU catching on fire, and you can see the results of something like that here: http://www.theregister.co.uk/2012/11...vs_reg_reader/
Of course the worst luck is if it catches fire and you are not there (or asleep) to put it out before you lose your home and/or your own life and life of your loved ones.
Another "feature" of the AQTS is that although it is rated at a certain wattage it will never attain that performance in real use. Generally one of these devices will be "good" if they can actually put out half of the rated wattage. One thing you can be certain of however, if you buy a "700 Watt" IED then you will NEVER, EVER, be able to run 700 Watt worth of hardware from it - not even close.
One should not expect any kind of efficiency from an AQTS and the only "80+" energy rating is the writing on the box. What this means is that within a year (if it lasts that long) the AQTS will have ended up costing you more in wasted electricity than buying a proper PSU in the first place.
The most trivial of the "features" of the AQTS is the absolute dearth of connection cables issuing from it. Not only will there be a lack of those cables but the ones that are there will also be much too short.
The cheap price of the AQTS is also offset by its short lifespan. If it lasts for more than a year of normal use before it dies (and hopefully does nothing else) then the person who bought it can consider themselves lucky.
So what can you expect from a PSU that costs a bit more money? Safety features for one (for both the PSU and the attached hardware), they will also perform to their rated wattage, and energy usage, and also have a priority for the "+12V" rail over the other two. Really high quality PSUs (like the Corsair AX860i) will happily run ABOVE their rated wattage. For instance the AX860i has been tested and showed no signs of wear running at almost 1,000 Watts: http://www.kitguru.net/components/po...60i-review/11/ If anything goes wrong with a real PSU then it will quietly shut down, that is it will go out with a whisper rather than a bang. You can also expect the PSU to last and do its job for years.
In conclusion next to pouring a bucket of water over your running computer the PSU has the potential to do the most damage to your system. If you have a neighbour downstairs who is considering building his own computer and you see him eyeing advertisements for an "Al-Q Taliban Special", offer to give him an extra €30 (or £ or $) or so towards buying a decent PSU - it might just save your life.