Unfortunately power supplies are one of the last things many people pay attention to. What separates a good PSU from a bad PSU is reliabiity.
The basic general rules are as follows:
1. The PSU should cost at least $40. Less if it's on sale. More for higher wattage.
2. Is it heavy for it's size? Good PSUs tends to be heavy because they use bigger components like a larger heatsink.
3. Do not buy a PSU for that flashy blue LCD fan.
Those are the rules you should follow if someone just dumped a bunch of unmarked PSUs on your head.
How can you find out how reliable a PSU is? Reviews. Not just any reviews, but reviews from hardware site. Don't take people's opinion at face value, even mine, and don't base it on user reviews from Newegg. User reviews are good to find out what the general consensus is, but that far from actual hardware insight.
Hardware review sites:
SilentPCReview
PCStats.com
ExtremeOverclocking.com
Systemcooling.com
A good primer is this article:
Power Supply Fundamentals & Recommendations
Okay, anything else other than reliability that makes a PSU good? Yep, efficiency is one. Cheaper PSU tends to have lower efficiency (65%) while more expensive ones will have higher efficiency (80%+). What does that mean? Let’s say all the components in your PC draws 200 watts by measuring the power coming out of your PSU. That will be lower than the amount of power being drawn by the PSU. It is less as a result of conversion from AC to DC. This result is heat that gets dumped into the case making the PC technically harder to cool. A 65% efficient PSU delivering 200 watts of power will be drawing 307.7 watts from the AC outlet. An 80% efficient PSU will draw 250 watts from the AC outlet. There are some PSUs that can peak at 85% efficiency which drops the draw down to 235.3 watts.
Another aspect of PSU people tend to ignore is it’s acoustic characteristics. A PSU has a 120mm to many that means they are all going to be loud. Right? Not exactly. Cheaper PSU have a greater tendency to be louder than a more expensive one because of cost cutting. But even expensive PSUs can be loud. So which PSU are quiet? Read the reviews.
Other factor is if the PSU is modular. That means instead of the molex wires being permanently attached to the PSU, modular PSUs have removable wiring so if you have a couple of molex wires you are not using just disconnect them and store them away. That’s great for wire management. But I read a short blurb that modular PSUs have certain issue. What are those issues? I don’t know, I didn’t read the review / article.
Aren’t you glad you asked?
In my opinion the one of the best PSU on the market is the Seasonic S12 series. They are efficient, quiet, and reliable. Don’t trust me? Then read the reviews.