This handbook explains the design optimisation process for on-grid rooftop solar photovoltaic (PV) systems. It sheds light on the fundamentals of solar PV systems and presents detailed examples and scenarios to illustrate the optimal design of such a system. The handbook takes a practical approach to system design, focusing on the customer’s needs and aspirations. This handbook specifically focuses on solar PV grid-connected systems with rooftop installation. It does not include ground-mounted installation practices, storage-type solar PV systems and other solar PV applications such as off-grid solar PV, solar water pumping, or solar mini-grids. Similarly, it does not include solar on-grid PV with battery inverter(s) or diesel generator(s) synchronization. The handbook incorporates inputs from industry practitioners and the experience of the authors.