As a rule of thumb, with the decline in Feed-in Tariffs and the advent of battery storage, a grid-connected solar system is best sized with an annual output (kWh) approximately equal to the annual usage of the property. Battery storage capacity in kWh is typically set at 1 to 2 time the system size.
So if your load is 5,400kWh a year, and a kWp outputs 900kWh a year, size the system at 6 kWp and install 6kWh - 12 kWh of battery storage.
What if your roof is too small to achieve this with standard panels? Which panel do you turn to?