Wattage is a consideration based mainly on the needs of your graphics card. The GTX 1080 needs at least a 520 watt. The other consideration is the quality of the PSU itself. This has to do with the quality of the components used, the design, and how well it's put together. The EVGA B2 series are much better quality than the Corsair CX series. Most of the manufacturers will have good PSU's and some not so good, so the best thing to do is look for reviews on the exact model your considering.
http://www.tomshardware.com/reviews/evga-supernova-550-g2-power-supply,4244-10.html
http://www.realhardtechx.com/index_archivos/Page362.htm