Experimental data and an analysis are presented for a solar water heater of the integrated collector-storage variety. The novelty of the particular unit investigated lies in the fact that its design incorporates a device that automatically lowers the heat-loss coefficient of the system during night time. This overcomes one of the basic problems associated with solar water heaters of this kind. The analytical apparatus employed For the study is our previously-published MUE method but with the incorporation of some newly suggested experimental details that lead to improved accuracy.