Identify one of four calculable pathways, as preparation for calculating power. That way, you have more chance of doing a useful calculation.
The quantity of power in one pathway is often linked to the quantity of power in another pathway. That's because you can switch power from one pathway to another. Rather often you'll come across processes where this switching is quite deliberate, and designed in, for example:
A good name for the collection of things which switch between pathways is
Devices always switch power from one pathway to one or more pathways. A perfect device might switch all of the input power to a single pathway, but it's more common to get switching to two or three pathways.
Some devices are designed, some exist anyway. Here, for example, is an electrical torch (first device), a foggy atmosphere (second device) and an eye(third device), making up a
what happens description of a light-beam from a torch to an eye(devices and pathways). Also on the diagram(power in pathways) is a
how much description of calculated or measure powers.
You don't have to do every calculation, or even specify every pathway. Only describe things if the description is useful for you.