You want as large a rooftop solar PV system as you can fit on your roof to reduce the amount of time you need to rely on your backup generator that may be required for periods during winter periods or times of low solar radiation (i.e. multiple cloudy days).
We generally recommend a minimum 10kW solar PV system, but the final size will depend on your anticipated load/demand. We can help you work this out by using our load profile sheet as part of our comprehensive quoting process.