Electrical energy is mostly converted from thermal energy drown from the thermal or nuclear power plant for large scale and Diesel generator (DG) sets are used as a backup device. In these, we mostly used water as a working substance and cooling agent for maintaining the operating temperature of the devices. Experimental and theoretical evaluation of three identical reduced scales long still prototypes covered with the different metallic sheets (viz. GI, Cu, and Al) are reported in this manuscript. Hot wastewater at 70±1 ºC is fed to the distiller unit as a discharge of the thermal unit is ranges between 65 to 75 ºC. The theoretical result obtained from the regression analysis gives good agreement with the result obtained from the experimentation. Long still covered with the Cu sheet material is gives better performance as compared with the others (viz. 55.03 and 34.69% higher than the GI and Al sheet cover). It has been observed that the still covered with the Cu sheet gives better distillate yield as compared with the still covered with the Al or GI sheet material.