An analysis is developed for the investigation of the transmittance-absorptance product (tau alpha), and directional characteristics of plastic-film water-bag solar water-heaters, which are important far longterm system performance investigations. The analysis allows a complete comparative investigation of the effects of a large number of parameters, like water layer thickness, bottom solar absorptance, radiation transmission conditions and extinction properties of semi-transparent films, on the transmittance-absorptance product. Finally the comparative effect of diffuse and specular reflection and transmission assumption on the derived results is investigated. It is concluded that the use of low iron content glass panes may possibly lead to (tau alpha) figures far in excess of 0.8 for new systems.