Modern lifestyle, growing industrial and economy thrive on energy which is getting expensive over time. Natural convection based systems are getting attraction for their promising heat transfer efficiency and zero energy utilization. Refrigerants having ozone depletion potential (ODP) and high Global Warming Potential (GWP) have been banned or under time bared permission under the Montreal (1987) and Kyoto (1997) protocols. To retrofit and modify existing cooling and heating systems, various natural and synthetic refrigerants are being investigated worldwide. We have devolved a Refrigerant Parametric Quantification (RPQ) method for the choice of optimal refrigerant for thermosyphon driven solar water heaters. A set of 29 refrigerants are simulated Using REFPROP under various temperature and pressure conditions. The optimal parameters of thermosyphon system are identified from governing equations, international environmental and safety protocols. The proposed RPQ method shows most appropriate refrigerant for given temperature range. In the second part, a glass evacuated tube collector is designed; fabricated and tested employing the best refrigerant emerged from a simulation study.