They are rated in Watts, and typically an inflated figure is used, so if you need 250w, you'd have to buy a unit marked "400w." Very few companies have HONEST power ratings.
A vital factor is how well a Power Supply regulates itself. They are supposed to maintain a certain set of voltages and shut off if something goes wrong. Unfortunately, many P/S's have unstable voltages and/or keep running when something goes wrong! This can damage or destroy other parts such as Hard Drives and Mainboards. A quality Power Supply should be your #1 priority.
Skimp on other areas, but not this one.
The best ones that I've found have something called Active Power Factor Correction.(APFC)
This saves a great deal of energy, and promotes stable operation over a large range of voltages. APFC Power Supplies can be identified by the lack of a 110/220v switch, and they are usually labeled "100-250v - FULL RANGE".
Only two companies that I know of produce stable, HONESTLY rated Power Supplies that shut off quickly when trouble occurs AND have energy saving Power Factor Correction. Seasonic and Antec.
Seasonic uses the more desireable ACTIVE PFC,
Antec uses PFC on most units.
The only company I use is Seasonic, but Antec will also give you good results with somewhat lower efficiency.
My favorite Power Supply is the Seasonic S12 series, which uses a large, slow and VERY QUIET fan. Because of this fan and the APFC, this unit runs extremely cool and quiet. It keeps running through most "brownouts" too.
