There’s websites to calculate it based upon satellite photos of your roof alone (what the installer and system designers did for me along with permitting that would have taken me far too much time / effort). Your panel type, inverter, etc. all matter as well. A meter to determine your electricity usage throughout the year is good to have as well although I didn’t do that before I installed my system and expected to try to size a battery backup system only once I had sufficient data. But basically so far my house is pretty much a dud in terms of ROI for any more modernization efforts without tearing it down basically and this is what I’ve been thinking about a lot more than the usual trend of renewables and such. Old homes in the US are really, really, really hard to retrofit remotely economically and are burdens upon the energy grid and infrastructure throughout the lifetime of the structures. The costs fall disproportionately upon our population that can least afford it as well and subsidies are laughably minuscule and mostly taken by more affluent homeowners in practice.