Panel count starts with energy, not the number of bedrooms
The most useful starting point is annual household electricity consumption in kWh. Home size can be a fallback, but actual usage captures HVAC, appliances, occupancy and lifestyle much better.
Step 1: estimate the solar system size
Divide the annual solar energy you want to produce by the expected annual production per installed kW at your location. If the home uses 12,000 kWh per year and the local planning yield is 1,500 kWh per kW, a 100% annual-offset target is roughly 8 kW before design adjustments.
Step 2: convert kW to panel count
Multiply system kW by 1,000 and divide by the selected module wattage, then round up to a whole module. An 8 kW target with 440 W panels requires roughly nineteen modules because 8,000 divided by 440 is a little over eighteen.
Roof space can become the limiting factor
The energy calculation may call for more modules than the usable roof can hold. Chimneys, vents, setbacks, shade and poor orientations can reduce usable area even when the total roof looks large.
Future electricity use changes the answer
An EV, heat pump, electric water heater or pool equipment can materially raise annual kWh. Include expected future loads before finalizing panel count so the design is not based only on historical bills.