Fuel cell technology based on natural gas is a kind of promising and cleaning energy conversion technology under development. The generation mechanism of fuel cell is different from the traditional power generation device, and the efficiency of fuel cell is not restricted to Carnot cycle, therefore, it has almost the highest efficiency in most of the power generation device. Fuel cell as the motor of building cogeneration system is the integration of high efficiency energy conversion type with high efficiency energy utilization method. In this paper, a Solid oxide fuel cell (SOFC)-based Building Combined Cooling, Heating and Power (BCHP) system is simulated, and it can meet the basically electrical and heating (cooling) load of the end users. The peak power load could be supplied by power grid, and the peak heating (cooling) load could be supplied by the backup equipments. In this paper, the restricted nonlinear optimization model is built in order to solve the economic load balancing problem of the system, and the sequential quadratic programming (SQP) is used to successfully solve the optimal equipment capacity for the production costs minimization on the equality constraint of customer’s heat and power requirement, and the other inequality constraint of equipment capacity. The computation results show that it is a good method to size SOFC power generation capacity meet the electrical base load of a building. The model can be used to provide guidance in the SOFC based BCHP system design and optimization.Copyright © 2009 by ASME