I find it slightly more difficult to build a computer when I do not know how much power is needed for a component. When selecting a power supply for a computer, it is difficult to know how large of one to get. You don't want to go too large for cost reasons and circuit reasons, but you don't want to go too low and not be able to properly use every component.

For instance, a graphics card might say "Minimum of a 500 Watt power supply. (Minimum recommended power supply with +12 Volt current rating of 30 Amps.)" But it really needs 360W (12V * 30A). So why don't they just say "Uses 360W max and xxxW peak"? Processors, I have noticed are good at reporting their power usage, but aside from processors and sometimes graphics cards, power usage is easily found.

What is the power consumed by the Blu-ray / DVD drives? By the HDDs/SSDs? By the Mobo? etc.

Why are these questions not easily answered when building a machine?