Power supply energi usage

Neither really. Closer to the first though.

The power supply has a wattage rating. This is the maximum power that can be drawn from it without it blowing up. The voltages are constant but the current varies. The current depends on the computer's needs. The computer will draw the amount of current it needs from the different voltage supplies (3V, 5V and 12V). Since voltage multiplied by current gives wattage and wattage is power, the computer will only draw as much power as it needs.

I say neither because the power the computer needs is not constant. It varies with load and other factors such as power saving features. The calculated power you have is the max power it could ever draw under extreme circumstances. But idling it will draw less than half that.
Components power usage will depend on load (more current drawn when being used) and on power saving features (reduced voltage when not in use).