This paper describes field experiments and numerical simulations on hybrid utilization of renewable energy and fuel cells for a residential energy system. It presents results of empirical testing and evaluation of hybrid utilization involving solar energy. First, field experiments were conducted on an electric power and domestic hot water supply system that uses both solar energy and fuel cells. The system achieved a large amount of reduction in primary energy consumption compared with conventional systems. Secondly, a simulation was performed on the optimum scale and effect of introduction of the system. The simulation results proposed the optimum capacities of the solar energy utilization and fuel cells to minimize primary energy consumption of the system. (C) 2011 Elsevier B.V. All rights reserved.