Describe a physical process
Quantify, then perhaps develop an explanatory mechanism
Generate an energy account
The third question(
By what mechanism?) is not an energy description - it belongs to a different semiotic register - although it may be useful in identifying calculations that might be insightful, and so denote fruitful qualitative descriptions in identifying stores and pathways. That's the last you'll see of mechanisms here.
(Because power is about what's happening, it's easier to connect to the lived-in world, and perhaps move from thinking about power into discussions of mechanisms. Power and energy might be co-developed, as opportunities for their natural use occur, rather than developing at first one, and then the other.)