Many custom PC builders have a hard time determining what size, i.e. number of watts, to use in their rig. Some just go with the thought that bigger is better, and that isn’t necessarily the case. I’ve been searching around trying to solve this problem, and it looks like the folks over at ThermalTake have a page that simplifies this out greatly! Just enter in some of your basic specs, like CPU, GPU, attached devices, etc, basically anything that is drawing power, and it will calculate what you should need.