If you’re building a new machine and can’t decide what Power Supply Unit (PSU) you shoud use, check out OutterVision’s Power Supply Calculator.
Just describe your new setup: the CPU you are planning to use, what GPU, how many disk, etc. and it will estimate the total amount of power needed by this setup. It will also break down the total amount along each tension (+3.3/+5/+12V), very convenient when building exotic systems.
The expert mode is very exhaustive and the database is really up to date: even the XEON E3-12xx v5, released end of 2015, are available.