How do I model complex whole-building energy systems?

You can model most buildings now, but the complex, whole-building ones still defeat you, the ones where every system touches every other and the model seems to sprawl beyond your grip. That difficulty is not a sign you have hit your ceiling. It is the next level asking to be learned. A whole-building model feels impossible when you try to build it all at once. The people who handle these do not hold it all in their heads. They build it in layers, get the envelope right, then the systems, then the loads, then the interactions, checking each layer before adding the next. Complexity becomes manageable when it becomes sequential. With the next hard model, resist doing everything together. Build one layer, sanity-check it against what you know, then add the next. When a result looks strange, trace it back one layer at a time. And lean on others who model these regularly. The complex ones are conquered by patience and order, not by being clever enough to see it all at once.