Yes — with the right roof, tank, and filtration, your home can run entirely on rainwater. See the math, the science, and where it's already the norm.

Short answer: if it's designed well, not much. A good rainwater system is built around the dry spells, not in spite of them. Your tank level draws down, you pay a little more attention to how much you're using, and when the rain returns the system refills. The whole point of storage is to carry you from one rain to the next — and a properly sized system is sized for the longest gap your region has ever seen, not the average one.