Sorption systems traditionally fall into two categories: closed (for chilling and heat pumping) and open (for dehumidification). Recent work has explored the possibility of semi-open systems, which can perform heat pumping or chilling while utilizing ambient humidity as the working fluid of the cycle, and are capable of being driven by solar, waste, or combustion heat sources. The efficiencies of closed and open systems are well characterized and can be accurately determined from four temperatures (one for each of the main components—desorber, absorber, condenser and evaporator). In this work, the performance potential of semi-open systems is explored by adapting expressions for the efficiency of closed and open systems to the novel semi-open systems. A key new parameter is introduced, which involves both the ambient dry bulb and ambient dew point temperature, since both are critical to semi-open absorber operation. The dew point temperature is necessary to capture the absorption performance, while the dry bulb temperature is needed to calculate sensible heat transfer with surrounding air.