In this paper, a two-step mathematical model is developed to simulate the solar-thermal energy conversion and consequent change in the properties of working fluid in an unconventional solar thermal water pump with flat plate collector. The available thermal energy to the working fluid for the incident solar radiation is calculated in the first step of the model. A single component boiling model is used in the second step to calculate the working fluid's boil-off rate and therefore number of moles of working fluid in each phase (liquid and vapour phases) at an instant. As the accurate thermodynamic property estimation is essential, Peng-Robinson equation of state is used for this purpose. An experimental set-up of an unconventional solar thermal water pump with pentane as working fluid is used to validate the model. The experimentally obtained temperature and pressure of working fluid in the relevant tanks under no-pumping but the energy conversion during solar day are compared with the model predictions. A close agreement between the experimental and simulated values are seen. The reasons for the deviations of experiment and model predicted values are elucidated. The relevance of the developed two step model for evaluating the performance of the pump is presented. (C) 2018 Elsevier Ltd. All rights reserved.