What should I know to select a power supply?

Solution 1:

http://www.firingsquad.com/guides/power_supply/

http://forums.techarena.in/guides-tutorials/1093105.htm

Here are a few guides to help you understand better.

Get a decent estimate of what your computer power requirements are because you want a power supply with a higher wattage rating then your needs.
Make sure to get the most efficient power supply you can afford. One with greater then 80% efficiency. This will save you on your power bill and your power supply will produce less heat.
You want a power supply with a min wattage rating at least 20% higher than your actual need.
Make sure the power supply you purchase has all the connectors you will need. If you are running SLI (dual video cards) or have many hard drives, you will need to make sure you will be able to hook it all up. Also consider future expansion options.
Not all 500watt (or any rating) ps are the same. (efficiency, rails, and fan noise can vary greatly)
Make sure the rails of the power supply can power your graphics card min. amperage needs.
Keep in mind if you want a quiet computer to look at the sound level ratings of the fans. They even make silent power supplies with large heat sinks and no fans for those who want total silence.
Try not to cheap out on the power supply if you build a relatively expensive computer. It is one of the most ignored components of new system builds. You want good clean power.
Another reason to buy a larger one would if you plan to upgrade the machine significantly later on.

Here are some reviews of power supplies available. Newegg.com is also a great place to look.

http://enthusiast.hardocp.com/reviews/psu_power_supplies/

Other good points brought up by other users:

@GalacticCowboy Motherboard/PS compatibility, definantly read about a motherboard purchase to make sure this will not be an issue.

@hanleyp Weight can be a rough estimate on a quality vs lower quality power supply. More weight commonly can possibly mean better beefier components and heat distribution.

Solution 2:

Here are the various considerations I look at when I'm building a PC:

How much power do you need? For example, is there some device that will draw a lot of power (video cards are probably the worst, especially if you're running SLI, because you have two cards...) A plain vanilla machine may run happily with 350W, but a gaming machine will probably need at least 450-500.
There's not a huge price difference on wattage steps, but there is a significant brand difference. With a PSU, you (generally) get what you pay for. A high quality PSU will set you back a few more bucks, but it will provide stable power. Your computer can get very flaky if the PSU isn't stable.
I've seen a couple rare circumstances where a PSU and motherboard aren't compatible and lead to system instability. Like I said, it's rare, but read your motherboard manufacturer's site and documentation to see if there are any known issues. For example, some Asus boards won't work well with a "high-efficiency" PSU.

Solution 3:

Having a watt rating much higher than what you actually need is not desirable, because efficiency get much worse when you're operating below capacity. i.e. if you need 250W and buy a 500W unit then it may well operate at 60% efficiency even though it's declared to be 90%. And remember: more power used (or wasted) does not only mean a higher electricity bill, but also more heat, which in turn means more noise.

Most systems are never significantly upgraded, so you should buy a power unit that supplies just a bit more than your maximum need.

Solution 4:

The main thing you need to consider when selecting a power supply is the power rating. make sure it has enough capacity to run all of the devices you plan to power with it.

If you are starting from scratch then all you need do is keep a running total of power usage for all of the components you plan to populate the system with. If you add up all of the power usage values you should arrive at your gross power usage which should always be less than the capacity of your power supply. If your total power is more than your power supply is rated for, then it would be a good idea to select a unit which has a greater maximum capacity.