You build an energy description assuming that energy is conserved, but also that it's often a tricky quantity to calculate.
Be guided by the reason for building your description. What's the question you want to answer? As energy only tells you what cannot happen, this limits the kind of questions you can answer.
Energy descriptions are good for figuring out four kinds of things.
Possible or not? The description sets up calculations so you can decide.
The description sets up a calculation so you can figure out the value of a quantity.
The description sets up calculations so you can explore trade-offs.
The description sets up calculations to find how much of the energy ends up where you intended. That's called the efficiency.