This paper evaluated energy saving, such as reduction of energy, cost and $CO_2$ emission, by comparing the performance of solar desiccant air-conditioning system with the performance of heat pump air-conditioning system. The performances of both air-conditioning systems placed on office building are analyzed. The meteorological data of Naha city in Okinawa prefecture are used in our simulation works. Empirical results indicated that solar desiccant air-conditioning system can reduce energy, $CO_2$, and cost, significantly. Moreover, it was found that solar desiccant air-conditioning system is effective to reduce peak power and achieve power leveling.