The thermal performance of a two-accumulator, self-pumping solar water heating system is characterized in a daily simulation. The passive vapor transport system operates in cycles, alternating between three phases. During the run phase, liquid refrigerant is gravity fed to the collector from an elevated accumulator tank. The evaporated refrigerant moves downward to the heat exchanger and the condensate is stored in a lower accumulator. In the pressurizing phase, vapor travels directly to the lower accumulator until the pressure differential between the upper and lower tanks is sufficient to return the condensate to the upper accumulator during the pump phase. Three isothermal closed-system thermodynamic models characterize the operational phases of the system. The applicable conservation of mass and energy equations of each model are combined in the numerical simulation. Instantaneous temperature and heat transfer rates as well as integrated energy quantities and thermal efficiencies are compared to experimental values. The qualitative behavior of the analytical model agrees with that of the physical system. Multiplying thermal loss coefficient by 2.5 and adjusting the theoretical solar model to correspond with measured insolation forces quantitative agreement of overall daily performance. The simulation reveals the impact of the duration of the pressurizing and pumping phases on overall performance. With the current design, self-pumping drops daily efficiency (heat across the condenser/incident insolation) to 27 percent from 33 percent predicted for an identical system operated with a mechanical pump.