Start with the inputs
A calculator can use postcode, electricity demand, roof direction and reported shade to model a plausible system. Small changes to those inputs can change array size and expected generation, so check them before focusing on the headline result.
Read energy flows separately
Generation, solar used at home and electricity exported are different figures. Their financial value depends on when the household uses energy and on the import and export tariffs assumed by the model.
Know what remains unknown
An online estimate cannot confirm roof dimensions or condition, structural suitability, electrical upgrades, cable routes, access, equipment compatibility or grid requirements. Those belong in the installer survey and final design.