Things to consider before buying a PSU:-
AT vs. ATX: Some PSUs are made for AT-type motherboards, others for ATX-type, and some for both. Chances are, you’re using a standard ATX motherboard and will need a standard ATX PSU, but you may want to double check your motherboard’s details just in case.
Voltage Stability: If you’re ticking all of the above boxes, then voltage stability won’t be an issue. This basically refers to the PSU’s ability to keep supplying power at 12V without dropping.
Power rating : Most PSU come with a power rating usually between 300W-1000W. Make sure you use a PSU with power output sufficient enough for the PC. If you are using dual high end discrete graphics card like a Crossfire or SLI, high end multicore processor with high wattage (usually less than 100W) and multiple HDD system you will probably need a high power PSU. One rule is to add the power rating of all components and the PSU o/p power should not be less than that.
Quality : Some PSU may claim a power output of 500W but deliver 300W, always go for high quality tested PSU. Also, the only way to be absolutely sure that you are getting the genuine article is to check the validity of any certification claims by going to the following website.
Efficiency : Check PSU certifications, 80+ certification or ISI certification, before buying a PSU. An 80 plus certified PSU is more likely to be efficient than non 80 plus PSU.