This paper presents a development process of a forecast and monitoring system for a photovoltaic (PV) solar plant. PV solar system is one of sustainable resource of energy. So, Korean government encourages businessmen to build a PV solar plant. Renewable Portfolio Standard (RPS) system is one of encouraging policies. Most RPS businessmen use monitoring system for a PV solar plant and they need an accurate forecast of power generation for business purpose. Therefore we propose an estimating algorithm of power for a PV solar plant using weather forecast. Proposed algorithm is implemented in a forecast and monitoring system and it works better than existing estimating methods.