You should be able to calculate your PSU needs from the individual components.

You should be able to do this more or less exactly, if you are cognizant of the output specs of the individual PS connectors, and the requirements of the components that are drawing from those connections. Most people do not bother and just buy way more than they actually need.

I did exactly this when building my Office/Internet machine and was surprised by how low the actual requirements were.

Naturally I erred on the side of caution and bought a bigger one anyway - just in case!