The basic calculation is to find out how many kilowatt hours of electricity you use on a monthly basis. WKh will be shown on your monthly bill. Add the number of KWh for the whole year and divide by 12. This is the number kilowatts you need your system to provide. Yes, there are a ton of other factors such as the number of sun hours your home receives, but this is the basic calculation. It is best to always round up a little to be safe, of course. If you are off grid this is particularly important as you will not have grid power to supplement you in higher usage months. In you are grid assisted you can add a number of additional panels if possible and actually sell your additional power to the electric company. They don't pay a ton, but it doesn't cost you anything to generate this monthly check other than the original additional cost of adding extra panels/equipment.
You can never be too prepared