A design aid expert system for solar photovoltaic (PV) power supplies corresponding to Indian region is developed. The knowledge base is evolved from the climatological data of 14 stations in Indian region. The approach involves combining both the site and array characteristics in a single parameter referred to as equivalent unit array output and expressing the composite parameter as a function of geographical co-ordinates. The resultant expert system is envisioned to provide general users the power of an expert, in the matter of design considerations. It will also be easily adaptable to a change of design conditions. It considers two modes of system operation; with battery storage and without battery storage. It enables the determination of size of PV array and battery bank, which meet the demand of load throughout the year allowing the battery to be discharged up to a minimum acceptable level. (c) 2006 Elsevier B.V. All rights reserved.