Panel count is the last decision, not the first. Start with consumption, roof geometry and what you actually want the system to do.
Most homeowners start by asking how many panels they need. That is the wrong end of the problem. The right starting point is twelve months of electricity bills, because seasonal swing matters more than the annual average.
Next comes the roof: orientation, tilt, shading from neighbouring buildings and available unobstructed area. A south-facing surface at roughly 30 degrees is ideal in most of Iran, but east–west layouts often produce a flatter, more useful curve across the day.
Finally, decide the goal. Bill reduction, outage resilience and full independence lead to three very different designs — and only the last one truly requires a large battery bank.