In theory a PV installation can be sized and the cost calculated afterwards, much as engineers might size a heating system which is indispensable for a building. However, because PVs are an option and in the case of buildingintegrated, grid-connected systems, the grid supply is always available (and currently at a lower unit price), sizing and costing in practice proceed iteratively. Thus, before looking at determining the area of PV array we will examine some basic costs.