look for "80+" certification.
this certification goes to power supplies that should be 80 percent, or more, efficient under load. basically, if it meets this level, it is a decent quality.
For all of the energy running through a power supply as you use the computer, it is not all totally used by the computer. Some is 'lost' as heat. It is basically laws of physics that the power supply can't be 100% efficient.
The 80+ kind of will ensure that the power supply is well designed and made with high quality components.
Another thing to consider is getting a power supply noted for begin "quiet." Like they review at silentpcreview and other sites.
A power supply gets quiet on one of two ways: 1. a quiet fan. If it has a fan selected to specially be quiet, that will be a great fan. Probably the main thing that kills a power supply after a couple years or even a couple months is a basic, nothing-special fan dying. Hopefully, if this happens to you, the noise will alert you and give you time to pull out the power supply, open it [WARNING DANGEROUS], monkey around inside {WARNING DANGEROUS}, and perhaps replace a dying or dead fan with another. Fans are inexpensive.
Inside the power supply are parts that can hold electricity even after being unplugged.
so why not just aviod all of this with a high quality power supply including a gret fan?
2. the other way to get quiet is to use a bigger heat sink. A heat sink is basically material that absorbs the generated/wasted heat, and lets it drift away. A power supply can get quieter by dissapating heat with more heat sink material thus less fan needed. Some power supplies have fans that are needed so little that the fan simply comes on when temp gets to a certain point. So, a rough guide is the weight-to-wattage ratio: the greater weight for a given wattage, the more heat-sink material, and so -rule-of-thumb - the better the power supply.--the only power supplies made this way will be quality power supplies.
these power supplies will have the 80+ logo when you shop. also, they may be called low noise, high efficiency, or something 'green' like low power, environmentally friendlier-than-the-next-power-supply, 'eco,' etc.
Lately, I like Seasonic.
