In the present work the thermal behavior of a typical solar still is examined experimentally and theoretically and its basic characteristics are analyzed. It is concluded from a large number of experimental measurements that the operation of such stills during daytime is characterized by three phases: (1) starting, (2) pseudo steady-state, and (3) saturation. A mathematical equation is formed for the instantaneous simulation of the solar still during the pseudo steady-state phase. From this the equation in its integrated form determines satisfactorily the daily output by the daily solar radiation (solar energy input), the average ambient air temperature during the day, T-a(av), and the temperature of the water in the basin at the beginning of the day, T-win as parameters. This method Is called the ''input-output'' method.
