Single pane glazed flat solar heaters found rather wide application for domestic hot water supply. However, the practice of use shows some discrepancy between estimated and observed data: in one case the temperature of hot water does not conform to the expected pattern, in other case doubts arise about correctness of their sizes, in some cases the quantity of heated water is not enough, etc. Because of such divergence of opinions, it is becoming necessary to think about correctness of existing methods of sizing and designing of flat plate solar water heaters. This article attempts to develop a new structure and improved method for calculation and design of flat plate solar water heaters. Analysis proved relatively higher energy and cost effectiveness of the new structure of flat plate solar water heater.