As a rule of thumb, with the decline in Feed-in Tariffs and the advent of battery storage, a grid-connected commercial solar system is best sized with an annual output (kWh) approximately equal to the annual load of the business / property.
So if your load is 36,000kWh a year, and a kWp outputs 900kWh a year, size the system at 40 kWp.
What if your roof is too small to achieve this with standard panels? Which panel do you turn to?