In the present paper, an attempt has been made to develop a computer model based on transient analysis of the greenhouse. The model predicts room air temperature, storage water temperature and the thermal energy storage effect of a water mass in a low cost, passive greenhouse. Analytical expressions, based on an energy balance for each component, have been derived in terms of climatic as well as design parameters. Numerical computations have been done on typical days for the months from December 1999 to June 2000 at New Delhi. It has been observed that (i) there is a significant thermal energy storage effect of the water mass on room temperature and (ii) TLL, which is found to decrease with an increase in the mass of storage water, varies with month of year. An experimental validation of the developed model has also been demonstrated. The predicted room and water temperature show fair agreement with experimental values.