Start with annual kWh, not square footage
Twelve months of utility usage is usually a better starting point than home size because two houses with the same floor area can have very different HVAC, appliance and vehicle loads.
Translate annual consumption into solar kW
A simple planning formula is annual target solar kWh divided by expected annual kWh produced per installed kW. The production factor must be local: climate, orientation, tilt and shading all matter.
Choose the energy offset intentionally
A 100% annual offset is not automatically optimal. Where exported electricity is paid less than imported electricity, oversizing beyond daytime self-consumption can have weaker economics unless a battery, EV or other flexible load uses the excess.
Include future loads
If an EV, heat pump, pool, electric water heating or other major load is planned, include it before system design. Otherwise a system sized from historical bills may undershoot future electricity use.
Then check the roof
After calculating an energy-based system target, confirm whether the roof can physically accommodate the panels with acceptable orientation, setbacks and shading. Use the Home Solar Atlas Roof Solar Calculator for a manual area check or the address-level Roof Analyzer where building data are available.