With the increasing penetration of variable renewable energy, integrated energy systems have gained a lot of attention due to their ability to flexibly integrate various energy sources and meet various types of energy demand. In order to cope with the challenges caused by the uncertainty of equipment efficiency, the off-design characteristics of the energy conversion equipment are introduced into the system optimization model for study. To this aim, a two-stage optimization framework was used to optimize the integrated energy system. When off-design characteristics are considered, the optimal capacities of the energy conversion equipment tend to decrease, the energy storage capacities tend to increase, and the total annual cost of system would increase. Regarding the system operation, the effects of off-design characteristics on the operation of the gas turbine and electric energy storage are most significant for all equipment. In addition, the effects of the gas turbine and heat pump on the system cost are higher for energy conversion equipment.