A method for the optimal sizing of a photovoltaic system is presented in this paper. The system studied is composed of photovoltaic array, power tracker, battery storage, inverter and load. The data used were the sunshine duration and solar radiation intensity for years 1990 to 2004 for eleven Nigerian stations: Calabar, Ibadan, Ilorin, Kaduna, Kano, Lagos, Lokoja, Maiduguri, Minna, Sokoto and Zaria obtained from the archives of the Nigeria Meteorological Agency. Appropriate programs were developed using MatlabR code to model the optimal size of a photovoltaic system. Input parameters which were estimated from the obtained heliophysical variables and used in the simulation were clearness index and total radiation on an inclined surface. The output parameters include utilizability, monthly-average fraction of the load covered by the photovoltaic system with battery storage, monthly-average fraction of the load covered by the photovoltaic system without battery storage, monthly-average of uncovered load fraction of the photovoltaic system, area of the panel, optimal area of the panel, total cost of the panel and the optimal total cost of the panel. Maximum incident solar radiation onto the photovoltaic array is obtainable in dry season and smaller sizes of photovoltaic system are used while minimum incident solar radiation
onto the photovoltaic array were witnessed during the wet season and larger sizes of photovoltaic system are used, this determines the optimal size of the photovoltaic system. This research also account for the cost of the optimized plant, capable of supplying 15kW, at N809,800. A comparison of this researched optimized cost with PHCN (Power Holding Company of Nigeria) current charge indicated that after one year and six months, the user of the photovoltaic plant will become a free user of
electricity. The optimized photovoltaic plant is short term cost effective and much cheaper than the non – optimized plant.