Renewable energy plays an important role in the national energy policy especially in reducing greenhouse gas emissions. For a photovoltaic (PV) system, one important consideration is the cost of the system. One needs to select the best PV array from a range of selection, that is, the one which is the most efficient and with a best price. This article illustrates a method to compute the size and cost of a required PV array, and then after to compute the required battery for the case of a photovoltaic building in Malaysia. The computation is simulated using Matlab integrated with suitable mathematical equations. The generated current and power of the PV array are calculated for daily solar irradiation in Malaysia. The computation enables the user to quickly compute the initial cost needed to be spent if a given PV system is to be installed. A typical building requiring 12 kWh daily energy with 6 kW peak demand load was shown to need at least 114 solar modules at a cost of about RM53k. It is noted that the main cost of the whole PV system is mainly contributed by the cost of the chosen PV array. Hence, the right choice of a PV module is vital in achieving the minimum cost.